AlessiaCoccato_kaft_v3.pdf 1 10/05/2017 10:03:51 The non-destructive examination of paintings, pigments, and their degradation Application of Raman and X-ray fluorescence spectroscopies to materials

Application of Raman and X-ray fluorescence spectroscopies to Cultural Heritage materials The non-destructive examination of paintings, pigments, and their degradation

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Alessia Coccato

Thesis submitted in fulfillment of the requirements for the degree of Doctor of Alessia Coccato

Supervisor Prof. dr. Peter Vandenabeele Department of Archaeology – Ghent University Co-supervisor Prof. dr. Danilo Bersani Department of Physics and Earth Sciences – University of Parma

Dean Prof. dr. Marc Boone Rector Prof. dr. Anne De Paepe Thesis submitted in fulfillment of the requirements for the degree of Doctor of Archaeology

2017

Thanks to my supervisor, Prof. dr. Peter Vandenabeele, for the opportunity and for the support through the years. I do appreciate (now) all the challenges you proposed me, and all the times you had to push me to do more. It hasn’t always been easy, or smooth, but I have made it this far. Thanks for having trusted me. Thanks to Prof. dr. Danilo Bersani, for having first planted the Raman-seed almost 10 years ago, when I first started studying conservation science. Thanks for the feedback, for the scientific cooperations and for the support. Thanks to Prof. dr. Luc Moens, for the kind curiosity on whatever new project I was working on. Thanks for the great effort to make the Raman Group projects advance, and for being always positive. I would like to acknowledge the financial support of the European FP7 MEMORI project, which allowed me to take part to the fruitful discussions of the consortium, even though I joined in the final stages of the project. Special thanks to the colleagues at English Heritage, Paul Lankester and David Thickett, for the input and feedback regarding pigment sensitivity and degradation. Next to this, the GOA project of Ghent University “Archaeometrical research of the Ghent Altarpiece” is greatly acknowledged. Thanks to this project, I had the chance to admire the outer wing panels closely during the conservation treatment, and to appreciate on the one hand Van Eyck’s mastery, and on the other, the incredible work of the conservation team. I would like to thank the conservators from KIK-IRPA, especially Bart and Hélène, for the help during our measurement sessions, and in the interpretation of our results. Thanks to dr. Jana Sanyova (KIK-IRPA, Brussels, Belgium), for having shared her expertise, and for having welcomed us in the institute, to learn more about the study of paint samples. Thanks as well for the cooperation on the study of cross sections of green paint layers of the Ghent Altarpiece, and for being part of my Doctoral Advisory Committee.

i Thanks to Prof. dr. Wim De Clercq, for the advice and feedback kindly provided as member of my Doctoral Advisory Committee, and as a jury member for my PhD. Thanks to Prof. dr. Maximiliaan Martens, also member of my Doctoral Advisory Committee. Thanks for all the information on the history of the Ghent Altarpiece. Thanks for your commitment to the GOA project as well (including transport of people and instrumentation to the )! Thanks for the art historical input on my thesis. Thanks to the other jury members, Prof.dr. Jean Bourgeois, dr. Kepa Castro, dr. Claudia Conti, Prof. dr. Philippe Crombé, Prof. dr. José Mirão, for having taken the time to read my work, and to give me valuable feedback on the work done so far. Thanks to Prof. dr. Jan Jehlička (Charles University, Prague, Czech Republic) for the geological insight on carbons for the paper on carbon-based black pigments, and for the feedback on the interpretation of Raman spectra. Thanks to Alexia Coudray (KIK-IRPA, Brussels, Belgium) for the measurements and photos of the Ghent Altarpiece cross section and copper resinate samples. Thanks to dr. Bart Vekemans and Prof. dr. Laszlo Vincze, for having shared with me their knowledge about X-rays. Thanks for the training sessions (hXRF, TXRF, data processing, scientific writing), for the detailed feedback, and encouragement. Thanks to the colleagues at Universities of Catania and Salento, and of the Soprintendenza dei Beni Culturali: Prof. dr. Germana Barone, Prof. dr. Paolo Mazzoleni, dr. Simona Raneri, Prof. dr. Nicola Francesco Neri, Prof. dr. Giuseppe Agostino, for the chance to perform in situ Raman analyses in Sala Vaccarini, for making the logistic arrangements, to Davide Manzini for providing the BWTek instrument and for agreeing on joining the measurement campaign on scaffoldings, and to everybody for the nice cooperation which resulted in a scientific paper. Thanks to Prof. dr. Simona Quartieri, dr. Giuseppe Sabatino, Luca Grioli (University of Messina), to the director of Museo Regionale Interdisciplinare di Messina, dr. Caterina Di Giacomo, and the staff members of the museum, for the opportunity to study Caravaggio and Caravaggeschi paintings, and for the effort to let us work in the best possible conditions, notwithstanding an unforeseen blackout on the day of the measurements. Thanks to the colleagues of University of Évora, for having allowed the measurement campaign in Funchal. Thanks to Prof. dr. Antonio Candeias and Prof. dr. José Mirão for the logistic support on site, and for having shared with us issues about their results, which led to the micro-Raman analysis of cross sections of Funchal’s Altarpiece in Ghent, for confirming and cross-checking results from other techniques. Thanks to the conservators on site, and for the staff of the Direcção Regional dos Assuntos Culturais in Madeira, for following us

ii during the in situ campaign on the Altarpiece, and for welcoming us in the Museu Quinta das Cruzes, allowing us to measure their of glyptics. Thanks to the KongoKing/BantUGent research groups, for the fruitful cross• disciplinary cooperation on glass beads. Special thanks to Prof. dr. Koen Bostoen and to dr. Bernard Clist, for giving input and feedback on the archaeological samples, and to Charlotte Verhaeghe for having reached out to our research group for support (in the form of chemical analyses of her samples) for her master thesis. Thanks to dr. Karlis Karklins for having shared with us his extensive knowledge on European trade beads, and to the colleagues in Évora (Department of Chemistry, Department of Geosciences, Hercules laboratory) for the analyses that supported a detailed characterization of the composition of beads. Thanks to the Raman spectroscopy research group and to Archaeometry Ghent, and it’s not only work-related, to Anastasia, Debbie, Jolien, Mafalda, Possum and Sylvia. Thanks for the logistic and bureaucratic help when I arrived, when I moved and when I had no clue on what was going on. Thanks for all the trainings and sessions with new instruments, softwares, procedures. Thanks for all the science-related discussions. Thanks to Sylvia for the support in the lab, especially for the TXRF part. Special thanks to Debbie, we shared a lot during the last 4 years, as life happened while we were in the office. Thanks for all the translations, and for the nice atmosphere in the office, in the black tent at MSK, during conferences and beyond. Thanks for listening to me, and thanks for trusting me. Obrigada to Mafalda for having listened to my rambles (mostly weather-related) and to my explanations (even taking notes!), for the trips to Delhaize and for making my desk look even messier than what it is… Thanks to the fellow PhDs, postdocs, guests, staff, and professors whom I met in S12. Special thanks to Alice (and Simone), Asha, Farzin, Jan, Lara, Rosie, Yulia; to Sylvia and Kris for the organization of many fun activities. Thanks to Philip for the qwerty keyboard, and for all the help with computer-stuff. Thanks to Carlo, Winfried, Marianela, Giovanni. Thanks to the students, professors and staff at Archaeology as well, for the support and kindness during my (rare, very short) visits in UFO. Heel veel dank aan de vrienden die ik gedurende de Nederlandse lessen leerde kennen: Emiljano, Nuria, Pablo and Fien, Yulia and Jens, Marco jr, Marco- Fatma-Delia-Dafne. Thanks for having made the process of learning Dutch a bit more fun, and thanks for all the food and drinks and laughter and fun! You made me appreciate Ghent even more. It was lovely! Thanks to Geraldine and Bart for reminding me to breathe. Grazie a Elisa e Valeria per le lunghe chiacchierate in italiano, davanti a un’ottima birra belga: combinazione perfetta! Grazie alle mie compagne di Universita’ a Parma, soprattutto Erica e Vero.

iii Grazie ai miei adorati fernesi, per farmi sentire ogni volta di nuovo cciovane! Siamo cresciuti parecchio in questi anni, ma fortunatamente certe cose non cambiano mai. Grazie ai miei compagni del liceo (soprattutto Irene, Maria, Mattero, Michele, Silvia, Zampo), grazie per i tentativi di organizzare rimpatriate, anche se gli impegni si mettono in mezzo. Continueremo a provarci, e prima poi ci riusciremo! Grazie alla Luni per l’affetto incondizionato. Grazie agli zii e cugini vicini e lontani, per farmi sapere regolarmente che mi pensano, e per la sana curiosita’ sulla mia vita in Belgio. Grazie a Suzy e Joe, per l’entusiasmo e l’affetto che riescono a trasmettermi, anche con un oceano di mezzo! Grazie a Kissi e Aldo per la dolcezza con cui mi accolgono ogni volta. Grazie alla ziaBa e alla Titti, che mi viziano e coccolano in ogni modo. Vi voglio un sacco di bene. Grazie a papa’ e mamma, per avermi dato davvero radici e ali, per avermi fatto credere in me stessa e per avermi supportato in ogni decisione. Grazie a papa’ per avermi permesso di vivere con leggerezza, nonostante tutto. Un grazie specialissimo alla Nai, che in tutta questa storia non ha mai smesso di spronarmi a fare il mio meglio, ricordandomi di prendermi cura di me, anche da lontano. Grazie per non esserti mai tirata indietro di fronte a nessuna sfida. Ain’t no mountain high enough! Thanks to Martino (my love, not the sandwich – which I do not really appreciate), for being always there notwithstanding the physical distance. Thanks for all the kilometers by car, bus, train, plane, for all the planning and for all the countdowns. Sorry for being sad and grumpy whenever it is time to say goodbye, and for being annoying when we are together. Hopefully we won’t need to say goodbye that often anymore in the future, but I guess I will still be annoying... Thanks for the incredible patience when I am unbearable. Thanks for always trying to let me see things from another perspective, although, you know, I can be stubborn. Thanks for bringing light, lightness and love whenever I need it the most. I am curious about the future, which right now looks like a giant question mark, but we are together, and that sounds like the best part of it! E per finire, grazie a me stessa per aver tenuto duro in questi quattro anni a 1000 km da casa. Sono stati anni intensi, dal punto di vista personale, umano, scientifico. Ho affrontato momenti bui, ma le persone fantastiche che mi circondano non hanno lasciato che mi perdessi d’animo. Poi io ci ho messo del mio, e qualche soddisfazione me la sono tolta. Ho imparato parecchie cose, ma soprattutto la danza della margherita, cantata da Van de Sfroos: impara a cürvass senza s'cepàss

iv ATR Attenuated Total Reflection

AXES Antwerp X-ray analysis, Electrochemistry and Speciation

AXIL Analysis of X-ray Spectra by Iterative Least Squares

BSU Basic Structural Units

CCD Charge-coupled Device

ED-XRF Energy-Dispersive X-ray Fluorescence

ED Electron Diffraction

FORS Fiber-optic reflectance spectroscopy

FP7 7th Framework Programme

FTIR Fourier-transform infrared

FWHM Full-Width at Half Maximum

GOA Geconcerteerde Onderzoeksactie – Concerted Research Action

HXRF Handheld X-ray Fluorescence

IR Infrared

IS Internal standard

KIK-IRPA Koninklijk Instituut voor het Kunstpatrimonium – Institut royal du Patrimoine artistique – Royal Institute for Cultural Heritage

LA-ICP-MS Laser-ablation inductively-coupled plasma mass spectrometry

v laser light amplification by stimulated emission of radiation

LIBS Laser-Induced Breakdown Spectroscopy

LLD Lower Limit of Detection

LOAEL Lowest Observable Adverse Effect Limit

MA-XRF Macro-X-ray Fluorescence

MEMORI Measurement, Effect Assessment and Mitigation of Pollutant Impact on Movable Cultural Assets. Innovative Research for Market Transfer

MOLAB Mobile Laboratory

MSK Museum voor Schone Kunsten

NAA Neutron Activation Analysis

Nd:YAG Neodymium-doped yttrium aluminium garnet

NIR Near Infrared

NIST SRM National Institute of Standards and Technology Standard Reference Material

NOAEL No Observable Adverse Effect Limit

PIXE Particle-Induced X-ray Emission

RH Relative humidity

ROI Region of Interest

SDD Silicon Drift Detector

SEM-EDS Scanning Electron Microscope - Energy dispersive system

SEM-EDX Scanning Electron Microscope - Energy dispersive X-ray spectroscopy

SERS Surface-Enhanced Raman Spectroscopy

SIMS Secondary Ions Mass Spectroscopy

SOP Standard Operating Procedure

SORS Spatially-Offset Raman Spectroscopy

SR Synchrotron Radiation vi TEM Transmission Electron Microscope

TXRF Total-Reflection X-ray Fluorescence

USB Universal Serial Bus

UV Ultraviolet

WD-XRF Wavelength-Dispersive X-ray Fluorescence

XANES X-ray Absorption Near-Edge Spectroscopy

XAS X-ray Absorption Spectroscopy

XPS X-ray Photoelectron Spectroscopy

XRD X-ray Diffraction

XRF X-ray Fluorescence

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Table 1.1: Broad topics and specific questions that can be of interest when studying a work of art as a material entity...... 3 Table 1.2: Example of a check-list for in situ measurements. In this specific case, it was a portable Raman campaign in Sicily, including a flight transfer that limited the amount of transportable items in size and weight. In bold are indicated items whose availability on site was granted by the collaborators. More details on the use of portable Raman spectrometers can be found in Sub•chapter 4.4...... 12 Table 3.1: List of some modern pigments encountered in the overpaints on the green vest of the Cumaean Sybil (outer wings of the Ghent Altarpiece), including the key elements and production date (Bell et al. 1997, Burgio and Clark 2001, Eastaugh et al. 2008)...... 83 Table 3.2: List of the most common ancient pigments in use in Europe during the Middle Ages, including the key elements and notes regarding their use (Klockenkämper et al. 1993, Moens et al. 1994, 1995, Bell et al. 1997, Burgio and Clark 2001, Eastaugh et al. 2008). Elements in italic are not detectable by means of XRF...... 84 Table 3.3: Effect of chamber replacement on the intensity of some elements on cleaned sample carriers (five repetitions, 1000 seconds), for 4 and 2h total cleaning time...... 104 Table 4.1: Overview of green pigments and degradation compounds of interest for archaeometry...... 133 Table 4.2: Forms of carbon according to (Winter 1983, van Krevelen 1984, Marshall and Fairbridge 1999) and JJ...... 161 Table 4.3: Positions of band maxima after baseline correction, observations on the fluorescence background and presence of additional

ix bands in the spectra of the studied pigments. In bold are the data related to the 785 nm excitation, in italic to 532 nm. *: after linear baseline correction...... 182/183 Table 4.4: Concentrations of acetic acid (as acetic acid solution dilution, vapour phase concentration in mg/m3 and ppm), and corresponding doses used in this study...... 188 Table 4.5: Overview of pigments sensitivity after the first tests with high concentrations of acetic acid (above ca. 150 ppm acetic acid corresponding to doses above 1000 ppm * days) ...... 189 Table 4.6: Summary of the proposed reaction paths and thresholds for the studied pigments (observed LOAELs and calculated NOAELs). For , the LOAEL was obtained in the first set of experiments (high concentrations/doses of acetic acid); for the lead pigments in the second, with lower doses...... 193

Appendices ...... Table A.1, Part 1/5: Summary of the observed discolourations on traditional inorganic mediaeval pigments. Pigments sensitivity to RH (relative humidity) and pollutants. If the pigment is reported in literature to be stable to a specific parameter, the symbol “–ˮ is used. If the corresponding cell is left blank, no information could be retrieved from published works...... 258 Table A.1, Part 2/5: Summary of the observed discolourations on traditional inorganic mediaeval pigments. Pigments sensitivity to RH (relative humidity), pollutants and oxidizing agents. If the pigment is reported in literature to be stable to a specific parameter, the symbol “–ˮ is used. If the corresponding cell is left blank, no information could be retrieved from published works...... 260 Table A.1, Part 3/5: Summary of the observed discolourations on traditional inorganic mediaeval pigments. Pigments sensitivity to alkali and acids. If the pigment is reported in literature to be stable to a specific parameter, the symbol “–ˮ is used. If the corresponding cell is left blank, no information could be retrieved from published works...... 262 Table A.1, Part 4/5: Summary of the observed discolourations on traditional inorganic mediaeval pigments. Pigments sensitivity to biological attack, and effect of material selection. If the pigment is reported in literature to be stable to a specific parameter, the symbol “–ˮ is used. If the corresponding cell is left blank, no information could be retrieved from published works...... 264

x Table A.1, Part 5/5: Summary of the observed discolourations on traditional inorganic mediaeval pigments. Pigments sensitivity to light, chemical analyses and high temperature. If the pigment is reported in literature to be stable to a specific parameter, the symbol “–ˮ is used. If the corresponding cell is left blank, no information could be retrieved from published works...... 266 Table B.1, Part 1/3: Analysis of Kremer pigment particles sampled with a cotton swab, processed by Qcalc and SinerX. Results of pigment analysis (white and yellow/orange). The values are calculated based on a given concentration of Fe = 1 ppm and are expressed in percentage of detected elements. The standard

deviation sj is calculated on three measurements per sample. The main element(s) for each pigment are marked in bold...... 269 Table B.1, Part 2/3: Analysis of Kremer pigment particles sampled with a cotton swab, processed by Qcalc and SinerX. Results of pigment analysis (red and blue). The values are calculated based on a given concentration of Fe = 1 ppm and are expressed in percentage of detected elements. The standard

deviation sj is calculated on three measurements per sample. The main element(s) for each pigment are marked in bold...... 270 Table B.1, Part 3/3: Analysis of Kremer pigment particles sampled with a cotton swab, processed by Qcalc and SinerX. Results of pigment analysis (green and black). The values are calculated based on a given concentration of Fe = 1 ppm and are expressed in percentage of detected elements. The standard

deviation sj is calculated on three measurements per sample. The main element(s) for each pigment are marked in bold...... 271 Table C.1, Part 1/3: Overview of the studied Cu-containing samples listed according to their commercial identification. and measurement conditions are given (number of points x seconds, accumulations), for both used lasers. Finally, the identified materials on the basis of the recorded vibrational signature are listed...... 273 Table C.1, Part 2/3: Overview of the studied Fe-containing samples listed according to their commercial identification. Provenance and measurement conditions are given (number of points x seconds, accumulations), for both used lasers. Finally, the identified materials on the basis of the recorded vibrational signature are listed...... 274

xi Table C.1, Part 3/3: Overview of the studied synthetic samples listed according to their commercial identification. Provenance and measurement conditions are given (number of points x seconds, accumulations), for both used lasers. Finally, the identified materials on the basis of the recorded vibrational signature are listed...... 275 Table C.2, Part 1/3: Overview of the Raman band positions of the identified Cu-containing green materials of interest for Cultural Heritage, with both the used laser wavelengths. The relative intensity values are given according to (Vandenabeele and Moens 2012). .. 276 Table C.2, Part 2/3: Overview of the Raman band positions of the identified Fe-containing green materials of interest for Cultural Heritage, with both the used laser wavelengths. As explained in Paragraph 4.3.2, no intensity values are given for the green earths...... 277 Table C.2, Part 3/3: Overview of the Raman band positions of all the identified synthetic green materials of interest for Cultural Heritage, with both the used laser wavelengths. The relative intensity values are given according to (Vandenabeele and Moens 2012)...... 279 Table D. 1: Raman bands of carbonaceous materials...... 281 Table D. 2: Carbon containing black Kremer pigments studied in Paragraph 4.3.2. Literature is not always consistent about materials’ composition: here are briefly reported the descriptions found on Kremer’s website, references [K] and (Winter 1983) [W]...... 282

xii Figure 1.1: Examples of the different categories of spectroscopic instrumentation, according to the degree of mobility. Transportable: Bruker Senterra Raman instrument and EDAX Eagle III XRF spectrometer; Mobile: Mobile Raman instrument (MArtA) and Bruker ARTAX (Image from: www.bruker.com); Portable: Enwave EZ-Raman-I-Dual; Handheld: Olympus InnovX Delta XRF spectrometer; Palm: SciAps ReporteR Raman instrument (Image from: www.SciAps.com). Image after (Vandenabeele and Donais 2016)...... 11 Figure 1.2: Flowchart illustrating the options for analysis based on the possibility of sampling and of moving the object under study away from its location. For the definitions of destructive, invasive and portable refer to Sub-chapter 1.5...... 15 Figure 2.1: Structure of an Early Netherlandish painting. Stratigraphy based on a cross section from the Ghent Altarpiece, panel IV – Deity Enthroned, red mantle (Coremans 1953, p. 73)...... 54 Figure 3.1: Radiograph of the hand of Röntgen's wife. In this image the ring and bones appear darker than the flesh...... 72 Figure 3.2: The photoelectric absorption process and emission of characteristic X-rays. First, the atom is ionized upon irradiation with appropriate radiation (E = hν). The X-ray emission that follows electronic rearrangement is schematized by illustrating two possible options allowed by the selection rules (Kα and Kβ lines). Image after (Kalnicky and Singhvi 2001)...... 73 Figure 3.3: Scheme of an ED-XRF instrument. Image after (Kalnicky and Singhvi 2001)...... 74 Figure 3.4: The handheld Olympus InnovX Delta XRF spectrometer...... 78 Figure 3.5: a) The InnovX Delta hXRF instrument and its in-house developed holder; b) the holder is attached on cross-mounted sliding elements fixed to a tripod; c) safety measures following the Standard Operating Procedure of the instrument (Lycke and Monsieurs 2015): USB connection to operate the instrument from a distance, and cones to delimit the safe area. Photo

xiii UGent (Sint-Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW)...... 79 Figure 3.6: The InnovX Delta hXRF being used to analyse the outer wings (panels and frames) of the Ghent Altarpiece at the MSK (Ghent, Belgium). a) and b) phases of the positioning, c) and d) measurement of the panel Erytrean Sybil and city view. Photo UGent (Sint-Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW)...... 80 Figure 3.7: Detail of the painting Cumaean Sibyl and interior during the conservation treatment. Photo KIK-IRPA (Sint-Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW)...... 82 Figure 3.8: Comparison of archival photographs of the painting Cumaean Sybil. a) 1902 (Photo RKD, Friedländer ), b) 1937 (Photo RKD, Van der Veken archive), c) 2012 – before conservation treatment. Photo KIK-IRPA (Sint-Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW)...... 85 Figure 3.9: HXRF spectra collected on points indicated by the conservators (different shades of green). In all the points, due to the penetrating power of X-rays, Cu, Sn and Pb were identified, coming from the original underlying layers. The signals of Ti/Ba• L, Cr and Zn are clearly visible in all the measured areas. These spectra allowed the first identification of overpaints based on modern pigments on the outer wings of the Ghent Altarpiece. Photo KIK-IRPA (Sint-Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW)...... 87 Figure 3.10: Elemental distribution images as obtained by MA-XRF scannings. a) Picture of the Cumaean Sybil painting during overpaint removal Photo KIK-IRPA (Sint-Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW); b) image of Cu; c) image of Pb-L; d) image of Ti; e) image of Cr; f) image of Ba; g) image of Fe; h) image of Zn. Images AXES Research Group, University of Antwerp (Sint-Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW).)...... 88 Figure 3.11: Scheme of TXRF measurement geometry. The incident X-ray beam is totally reflected by the optically flat sample carrier. The detector for the secondary fluorescence emitted by the sample is placed above the sample carrier...... 91 Figure 3.12: Sampling with a cotton swab performed by a conservator of the Departamento de Conservação e Restauro of Universidade Nova de Lisboa on the Altarpiece of the Cathedral of Funchal, Portugal, during the conservation treatment of the painting (October 2013). Photo UGent (Raman Spectroscopy Research group)...... 92 Figure 3.13: a): Pigment particles collected between the cotton fibers of a dry cotton swab gently rubbed on an exposed paint layer. b): the cotton swab is tapped gently onto the support for the analysis. Photo UGent (Raman Spectroscopy Research group)...... 93 Figure 3.14: G.N.R. TX2000 total-reflection X-ray fluorescence spectrometer. Photo UGent (Raman Spectroscopy Research group)...... 99

xiv Figure 3.15: Dots of commercial eyeliner positioned at 0, 2 and 4 mm (left to right) from the centre of the support...... 100 Figure 3.16: Effect of duration of cleaning time. Observed intensity of residual elements after cleaning for different times (step 3 + step 5 = 24, 4 or 2 hours, see Paragraph 3.4.2), measured 1000 seconds, as a function of cleaning time, versus the atomic number. The standard deviation sj is shown for each cleaned batch, measured five times. All the measurements were performed before chamber replacement. One standard deviation sj is indicated...... 102 Figure 3.17: Study of the effect of surfactant (Triton). Observed intensity of residual elements after cleaning for 2 hours (step 3 + step 5), and of a blank solution of surfactant (Triton). One standard deviation sj for five repeated measurements of 1000s is given. .... 103 Figure 3.18: Study of the effect of surfactant (Triton). Observed spectral signal of residual elements after cleaning for 2 hours (step 3 + step 5, Paragraph 3.4.2), and of a blank solution of surfactant (Triton). A clean sample carrier (full line), a cleaned sample carrier showing Fe contamination (dashed line), and a blank (Triton, dotted line) are shown...... 104 Figure 3.19: Calculated LLD (ppb), based on the equation in the text, for 1000s measurement time. The given data correspond to the calculated LLD from a 1 ppm multielement standard solution (full circles), and to the calculated LLD for the studied NIST 1640a water sample (empty circles). For the multielement standard elements not shown (L-lines), the LLD are: Cd = 52 ppb, Ba = 24 ppb, Tl = 5 ppb, Pb = 4 ppb, Bi = 4 ppb. For the NIST SRM 1640a elements not shown (L-lines), the LLD are: Ba = 36 ppb, U = 2 ppb...... 105 Figure 3.20: Calculated relative sensitivities (to Fe and Ga, full and empty symbols, respectively) for elements of interest present in a 10 ppm multielement standard solution (1000s measurement time). A: K-lines, B: L-lines...... 106 Figure 3.21: Comparison of elemental concentration ratios to Fe for NIST 1640a water sample, using Fe as an internal standard. The calculated elemental ratios via both methods are compared to the reference material data. One standard deviation sj is indicated...... 107 Figure 3.22: Study of the effect of distance from the centre and angle. Fe/Mn intensity ratio of commercial eyeliner when measuring ca. 1 mm diameter dots at angles of 0, 90, 180, 270 ° (from a reference position) as a function of distance from the centre of the sample carrier. The standard deviation sj was obtained by three repeated measurements of the same sample carrier...... 108 Figure 4.1: C.V. Raman and the spectrograph used to collect the first Raman spectrum, photographed at the Indian Association for the Cultivation of Science, where he carried out his experiments on

xv the scattering of light. Image from Indian Association for the Cultivation of Science...... 120 Figure 4.2: Energy level diagram explaining the Rayleigh effect, and the Raman effect. Image after (Baker et al. 2011)...... 122 Figure 4.3: Environmental light shielding in practice. In the case of in situ analyses, a variety of possibilities are available: a) shielding the windows with blinds or with black paper (right side of the picture), b) using black cloth and some supports to enclose the sample, in this case a manuscript, and the probehead with its holder; c) positioning under a black tent-like structure in the MSK in Ghent, Belgium; d) use of a sliding black plastic tube with a foam rim to prevent any damage to the painted surface (used for panel paintings); e) use of a copper tube of appropriate length, lined with tape to reduce damage (for wall paintings). The used laboratory instrument (f), equipped with sliding elements to surround the objective lenses and sample stage. Photos UGent (Raman Spectroscopy Research Group). . 126 Figure 4.4: General scheme of a Raman spectrometer. 1: Optics to focus the laser beam on the sample, neutral density filters, microscope system; 2: optics to collect the scattered light, filters. Image after (Vandenabeele 2013)...... 129 Figure 4.5: Raman spectra of selected Cu-containing carbonates and silicates (532 nm). a) Malachite (Kremer 10300); b) chrysocolla (Kremer 1035); c) subtraction of the spectrum of malachite from that of chrysocolla. The bands of the latter are more clearly visible...... 135 Figure 4.6: Raman spectra of selected Cu-containing phosphates and sulphates. a) Libethenite (Libethen, Slovakia; 532 nm); b) (Baccu Locci, Sardinia, Italy; 532 nm); c) (Voltaggio, NW Italy; 785 nm); d) Langite (Voltaggio, NW Italy; 532 nm); e) (Morocco; 532 nm). Spectra c and d were recorded on a sample described as langite...... 139 Figure 4.7: Raman spectra of selected Cu-containing mixed sulphates. a) Serpierite (Corchia, NW Italy; 532 nm); b) connellite (Monte Fucinaia, central Western Italy; 532 nm); c) chalcophyllite (Cap Garonne, France; 532 nm); d) spangolite (Monte Fucinaia, central Western Italy; 532 nm)...... 141 Figure 4.8: Raman spectra of selected Cu-chlorides. a, b, c) Atacamite (Monte Fucinaia, central Western Italy; 532 nm); d) clinoatacamite (Piombino, central Western Italy; 532 nm). The spectrum d was collected on a sample labeled as paratacamite. For more details about the nomenclature issue, see the text...... 143 Figure 4.9: Raman spectra of green earth pigments. a) green earth from Bavaria, identified as glauconite (Kremer 11100, 532 nm); b) celadonite from Cyprus (Kremer 17410, 532 nm); c) green earth from France (Kremer 40830, 532 nm); d) PG 7 (532 nm)...... 145 Figure 4.10: Raman spectra of Cu-containing artificial green pigments. a) Verdigris (Kremer 44450, 532 nm); b) Cu resinate Kremer 1220

xvi (532 nm) 3x60s; c) Egyptian green (Kremer 11100, 532 nm); d) Tridymite SiO2 R090042; e) Co-green (Kremer 4410, 532 nm). .... 148 Figure 4.11: Cr-containing green pigments. a) Chrome oxide green Cr2O3 (Kremer 44204); b, c, d) Viridian Cr2O3·nH2O (Kremer 4425). It seems that the pigment Viridian is a mixture of anhydrous and hydrated Cr2O3. The term “Viridian” refers to the hydrated form (spectrum d)...... 152 Figure 4.12: Raman spectra of synthetic organic green pigments. a) PG 36 (532 nm); b) PG 36 (785 nm); c) reference spectrum of PG 36 (785 nm (Fremout and Saverwyns 2012)); d) PG 8 (785 nm)...... 154 Figure 4.13: a) detail of the painting Archangel and Prophet Zacharias. The sample was collected during the campaign of 1951-1952 on the green cover; b) microphotograph of the studied cross section. Photos KIK-IRPA (Sint-Baafscathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW)...... 155 Figure 4.14: Raman spectra of a Ghent Altarpiece cross section. a) C10.020, layer 5 (532 nm) 10x150s; b) C10.020, layer 5 (514 nm) 10x40s; c) Cu resinate (Kremer 1220, 514 nm); d) Cu resinate Kremer 1220 (532 nm) 3*60x100s. The bands marked with dashed lines correspond to lead tin yellow type I (195 and 128 cm-1) ...... 156 Figure 4.15: a) Ordered graphite; b) graphite from metamorphic rocks; c) black earth, 785 nm laser, 4mW, 30x10s; d) black earth 532 nm laser, 1.45mW, 50x10s. The selected spectra show some examples of carbonaceous materials Raman spectra. Different measurement conditions were required to obtain those signatures, which correspond to ordered structures (a and b, graphites), and to less ordered materials (c and d, black earth). Also, the effect of the laser wavelength on the band positions and relative intensities is visible in black earth Raman spectra (c and d)...... 164 Figure 4.16: Raman spectra of graphite pigment, obtained with different laser wavelengths. a) Graphite pigment 785 nm laser, 7.4mW, 40x10s; b) graphite pigment (baseline corrected, with deconvolution) 785 nm laser, 7.4mW, 40x10s; c) graphite pigment 532 nm laser, 1.4mW, 40x10s; d) graphite pigment (baseline corrected, with deconvolution) 532 nm laser, 1.4mW, 40x10s...... 170 Figure 4.17: Raman spectrum of black earth pigment. a) black earth 532 nm laser, 1.4mW, 50x10s; b) black earth (baseline corrected) 532 nm laser, 1.4mW, 50x10s, Raman spectrum of black earth pigment; c) black earth 532 nm laser, 1.4mW, 50x10s, (zoom in); d) Calcite R050009 (RRUFF database); e) R050125 (RRUFF database); f) Anatase R070582 (RRUFF database)...... 172 Figure 4.18: Ordered carbons Raman spectra collected with the red laser: a) graphite 785 nm laser, 7.4mW, 40x10s; b) shungite 785 nm laser, 15mW, 40x10s; c) black chalk 785 nm laser, 4mW, 30x10s...... 173 Figure 4.19: Raman spectra of coke pigments: a) ivory black pieces 785 nm laser, 4mW, 30x10 s; b) ivory black 785 nm laser, 4mW, 30x10s; c) ivory black 532 nm laser, 1.4mW, 80x10s...... 174

xvii Figure 4.20: Raman spectra of charcoal. a) wood charcoal 785 nm laser, 15mW, 40x10s; b) wood charcoal (baseline correction) 785 nm laser, 15mW, 40x10s; c) wood charcoal (baseline correction, with deconvolution) 785 nm laser, 15mW, 40x10s...... 175 Figure 4.21: Flame carbons: fruitstone soots Raman spectra, collected with the red laser: a) peach black 785 nm laser, 15.4 mW, 35x10s; b) cherry black 785 nm laser, 15.4 mW, 40x10s; c) grape black 785 nm laser, 15.4 mW, 20x20s; d) grape black (baseline corrected, with deconvolution) 785 nm laser, 15.4 mW, 20x20s. .. 176 Figure 4.22: Raman spectra of flame carbons: bistre and furnace black, both green laser excited. a) Bistre 532 nm laser, 1.4mW, 100x10s; b) bistre (baseline corrected, with deconvolution) 532 nm laser, 1.4mW, 100x10s; c) furnace black 532 nm laser, 1.4mW, 300x10s; d) furnace black (baseline corrected, with deconvolution) 532 nm laser, 1.4mW, 300x10s...... 178 Figure 4.23: Raman spectra of humic earths/bituminous materials all collected with the green laser excitation. a) Van Dyck brown 532 nm laser, 1.4mW, 30x10s; b) Cassel earth 532 nm laser, 0.17mW, 30x10s; c) asphaltum 532 nm laser, 1.4mW, 60x10s; d) atramentum 532 nm laser, 1.4mW, 30x10s; e) Van Dyck brown 532 nm laser, 1.4mW, 30x10s (zoom in)...... 178 Figure 4.24: Other carbon-based black pigments Raman spectra: Sepia. a) Sepia average of 3 measurements 532 nm laser, 1.4mW, 10•40x10s; b) Sepia average of 3 measurements 785 nm laser, 15.4mW, 5-25x20-35s...... 180 Figure 4.25: Weekly Raman monitoring of the pigment lead tin yellow type I exposed to low concentrations of acetic acid. Each spectrum is the average of a 10x10 point mapping (5*30s), and are labeled with the corresponding dose, as ppm * days. For each graph, the spectra are recorded weekly, from 0 to 5 weeks of exposure, from top to bottom...... 190 Figure 4.26: Selection of red lead spectra, zoomed in in the region of the main band of lead acetate (1000-800 cm-1) showing the first appearance of lead acetate, and the different parameters related to detectable degradation...... 191 Figure 4.27: Correlation of the band intensity ratio of the bands of lead acetate and lead tin yellow type I versus the calculated dose for a selection of acetic acid concentrations...... 192 Figure 4.28: Schematic of the portable instrument EZRAMAN-I-DUAL by TSI Inc., Irvine, USA. Image after (Lauwers 2017)...... 195 Figure 4.29: The articulating arm. a) Positioning of the probehead by using the articulated arm. Photo UGent (Sint-Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW); b) detail of the probehead mounted; c) Raman spectroscopic study of a supposedly Clara Peeters painting. Due to its size, the analyses were performed by using the mobile instrument available in the laboratory. Photos UGent (Raman Spectroscopy Research Group)...... 196

xviii Figure 4.30: The tripod mount. a) and b) scheme and picture of the tripod setup, c) fine positioning (focussing) by means of the micrometric screw. Photo UGent (Sint-Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW). d): the setup being used at the Museo Regionale di Messina, Sicily, Italy. Photo UGent (Raman Spectroscopy Research Group)...... 197 Figure 4.31: Examples of holding the probehead by hand. a) and b): Measurements on the Altarpiece in Funchal, Madeira, Portugal; c) and d): measurements on the painted vault of Sala Vaccarini, Catania, Sicily. Photos UGent (Raman Spectroscopy Research Group)...... 198 Figure 4.32: The Altarpiece in Funchal’s Cathedral (Madeira, Portugal). Photo Direcção Regional dos Assuntos Culturais da Madeira...... 199 Figure 4.33: Pentecost painting, a) before treatment. Photo Direcção Regional dos Assuntos Culturais da Madeira; b) after cleaning. Photo UGent (Raman Spectroscopy Research Group)...... 201 Figure 4.34: Analysed points on the painting “Pentecost”. Raman spectra of white and yellow shades. a) White veil of Virgin Mary. λ=785nm, STD lens, 10x5s acquisition time, 35mW, ext. power supply, baseline corrected; b) white dove. λ=785nm, STD lens, 4x3s acquisition time, 35mW, ext. power supply, baseline corrected; c) halo of the dove. λ=785nm, STD lens, 10x2s acquisition time, 35mW, ext. power supply, baseline corrected, average. Reference spectra (dotted lines): d) lead white (2PbCO3·Pb(OH)2); e) anhydrite (CaSO4); f) lead tin yellow type I (Pb2SnO4)...... 202 Figure 4.35: Analysed points on the painting “Pentecost”. Raman spectra of orange and skin tone. a) Orangish shirt of man in background. λ=785nm, STD lens, 10x2s acquisition time, 35mW, ext. power supply, baseline corrected; b) skin tone of Virgin Mary. λ=785nm, STD lens, 4x10s acquisition time, 35mW, ext. power supply, baseline corrected, average. Reference spectra (dotted lines): c) lead white (2PbCO3·Pb(OH)2); d) anhydrite (CaSO4); e) vermillion (HgS)...... 203 Figure 4.36: Analysed points on the painting “Pentecost” and “Nativity”. Raman spectra of green and blue shades. a) Greenish highlight in green vest. λ=785nm, STD lens, 20x2s acquisition time, 35mW, ext. power supply, baseline corrected; b) bluish sleeve. λ=785nm, STD lens, 10x5s acquisition time, 35mW, ext. power supply, baseline corrected. Reference spectra (dotted lines): c) carbon black (C); d) lead white (2PbCO3·Pb(OH)2); e) blue wing of angel in Nativity scene (painting F) λ=785nm, STD lens, 6x10s acquisition time, 35mW, ext. power supply, baseline corrected. Reference spectra (dotted lines): f) azurite Cu3(CO3)2(OH)2...... 204 Figure 4.37: Micro-Raman study of cotton swab sample (blue mantle of Virgin Mary). a) Black particle in dark blue area. λ=532nm, STD lens, 60-2640 cm-1, 3-5 cm-1, 50x1000aperture, 20× objective,

xix 60x5s acquisition time, 6.9 mW, average, baseline corrected. Reference spectra (dotted lines): b) carbon black (C); c) cotton. d) Bright blue particle in light blue area. λ=532nm, STD lens, 50-1550 cm-1, 3-5 cm-1, 50x1000aperture, 50× objective, 10x10s acquisition time, 0.8 mW, average, baseline corrected. Reference spectra (dotted lines): e) azurite Cu3(CO3)2(OH)2...... 205 Figure 5.1: Overview of the Altarpiece (outer wings panels) after the conservation treatment. Photo Dominique Provost - Lukas - Art in Flanders VZW...... 229 Figure 5.2: Scheme of the planks and the frames of the outer wings of the Ghent altarpiece. Drawing taken from (Coremans 1953, p. 12). .. 230 Figure 5.3: Detail of the applied polychromy. Each stone-like section is decorated with painted gemstones in alternating colours (pink/yellow/green). Photo UGent (Sint-Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW)...... 231 Figure 5.4: a) The painting Saint John the Evangelist with its frame before the conservation treatment. Photo KIK-IRPA (Sint•Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW). b) Detail of the bottom right corner of the frame during the conservation treatment. Photo UGent (Sint•Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW)...... 232 Figure 5.5: Detail of the area investigated by means of Raman spectroscopy. Photo UGent (Sint-Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW)...... 233 Figure 5.6: Dinolite picture taken during positioning for in situ Raman analysis. The area under study is one of the lines marking the stones. The low-power laser can be seen shining on the white detail (brushstroke thickness ca. 2 mm). Photo UGent (Sint•Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW)...... 234 Figure 5.7: HXRF spectra of the stone-like imitation. Comparison of the original (full line) and restored (dotted line) parts: in the first the signals of Ca, Fe and Pb are present, and related to the wood (in grey). The signal of Ag is also visible. In the latter Mn, Cu, Zn and Sn signals are evident, which confirms the presence of a cheaper metallic-looking layer...... 235 Figure 5.8: hXRF spectra of the pink gemstones. Comparison of the original (full line) and restored (dotted line) parts: in the first silver is visible, in the latter Mn, Cu, Zn and Sn signals are evident, which confirms the presence of a cheaper metallic-looking layer. In both areas the lines of mercury are detected...... 236 Figure 5.9: Raman spectra on the pink gemstones in the original part of the frame of John the Evangelist. The environmental signal, and the reference spectra of lead white and vermillion are given for comparison. a) spectrum (λ=785nm, 30x15s, STD lens, 0.06mW, External power source); b) Lead white (2PbCO3·Pb(OH)2); c) Vermillion (HgS); d) Environmental signal...... 237

xx Figure 5.10: Raman spectra on the pink gemstones in the restored part of the frame of John the Evangelist. The environmental signal, and the reference spectra of chalk are given for comparison. a) spectrum λ=785nm, 30x10s, STD lens, 0.06mW, External power source; b) Chalk (CaCO3); c) Environmental signal...... 238

xxi

Acknowledgments i List of Abbreviations v List of Tables ix List of Figures xiii Chapter 1: Introduction ...... 1

1.1 Paintings are made of matter ...... 2 1.2 The characterization of materials in Cultural Heritage ...... 3 1.3 Scientific examination of paintings, archaeometry, conservation science: a non-disciplinary approach ...... 5 1.4 The interdisciplinary study of the Ghent Altarpiece ...... 6 1.5 Invasive, destructive, direct analysis: some definitions ...... 8 1.6 Answering the questions: a flow chart ...... 13 Outline of the thesis ...... 16 References ...... 17

Chapter 2: Pigments and paintings ...... 21

2.1 On the stability of mediaeval inorganic pigments: a literature review of the effect of climate, material selection, biological activity, analysis and conservation treatments ...... 22 2.1.1 Low atomic number (Z) elements ...... 25 2.1.2 ...... 2 2.1.3 Iron ...... 29 2.1.4 Cobalt ...... 32 2.1.5 Copper ...... 33 2.1.6 Arsenic ...... 40

xxiii 2.1.7 Mercury ...... 42 2.1.8 Lead ...... 44 Conclusions ...... 51

2.2 Early Netherlandish painting technique ...... 52 References ...... 55

Chapter 3: X-ray fluorescence in archaeometry ...... 71

3.1 X-ray fluorescence ...... 72 3.2 XRF in the archaeometrical study of pigments and paintings ...... 75 3.3 Handheld X-ray fluorescence ...... 77 3.3.1 Non-invasive approach: Overpaints on the Ghent Altarpiece ...... 80 Results and discussion ...... 86 Conclusions ...... 89

3.4 Total-reflection X-ray fluorescence ...... 90 3.4.1 Non-destructive approach: TXRF study of pigment particles ...... 92 3.4.2 Pigment particles analysis with a total reflection X-ray fluorescence spectrometer: study of influence of instrumental parameters ...... 96 Experimental and Methods ...... 98 Results and Discussion ...... 101 Conclusions ...... 110

3.5 Advantages and limitations of XRF techniques in the study of Cultural Heritage objects ...... 110 References ...... 112

Chapter 4: Raman spectroscopy in archaeometry ...... 119

4.1 The Raman effect ...... 120 4.2 Raman spectroscopy in archaeometrical research ...... 123 4.3 Micro-Raman spectrometer ...... 129 4.3.1 Raman spectroscopy of green minerals and reaction products with an application in Cultural Heritage research ...... 131 Experimental ...... 132 Results and discussion ...... 134 Conclusions ...... 157 4.3.2 Raman Spectroscopy for the investigation of Carbon-based black pigments ...... 159 Experimental ...... 166 Results and discussion ...... 168 Conclusions ...... 183

xxiv 4.3.3 ...... Pigments degradation: acetic acid pigments degradation studied with Raman spectroscopy ...... 185 Experimental ...... 186 Results and discussion ...... 188 Conclusions ...... 194 4.4 Portable Raman spectrometer...... 195 4.4.1 In situ Raman spectroscopic study of Funchal’s Altarpiece ...... 199 Experimental ...... 200 Results and discussion ...... 202 4.5 Advantages and limitations of Raman spectroscopy in the study of Cultural Heritage objects ...... 206 References ...... 207

Chapter 5: Combination of spectroscopic techniques for the in situ study of painted objects ...... 227

5.1 Raman and XRF spectroscopies in archaeometrical research: in situ multi-technique study of the frames of the Ghent Altarpiece ...... 228 Experimental ...... 231 Results and discussion ...... 233 Conclusions ...... 237 References ...... 239

Chapter 6: Summary and conclusions ...... 243 Chapter 7: Samenvatting en conclusie ...... 249 APPENDIX A ...... 258 APPENDIX B ...... 269 APPENDIX C ...... 273 APPENDIX D ...... 281 Publications and activities ...... 285

xxv xxvi Paintings represent a huge part of our Cultural Heritage (Dawson 2007). They are complex polychrome objects, which are often regarded mainly as pictures, two•dimensional entities which carry a practical, symbolic, and aesthetic value (Brandi 1963, van Asperen de Boer 1975, Stoner 1985, Berrie et al. 2003, Jones 2004, Leona and Van Duyne 2009, Heritage and Golfomitsou 2015). Such pictures were made by artists and artisans thanks to their skilful use of materials and techniques available at that time and in that place. This means that the images we admire in , or that are reproduced in and websites, are actually material entities, made of substances expertly handled to produce an object in response to a need. This is also true for any material culture entity as defined in archaeology, which carries information on the technical skills, craftsmanship, cultural values and exchanges in past societies (Ehrenreich 1995, Jones 2004, Adriaens 2005, Leona and Van Duyne 2009, Price and Burton 2011). The visual appearance is based on materials which are subjected to ageing and interaction with the surrounding environment. The study of the original and aged/degraded constituents can shed light on the creative process and material history of objects belonging to our Cultural Heritage, as well as support preservation and conservation treatments. In this thesis, the characterization and identification of inorganic materials of relevance for Cultural Heritage studies is pursued by means of a non-destructive approach. This is applied both on raw materials (i.e. pigments) and on artifacts (i.e. paintings). Moreover, an attempt is made to interpret the results with regards to an interdisciplinary comprehension of the object, including its creative process, material history, present condition, and preservation for the future. In the next Sub-chapters of this introduction, some concepts will be given on the reasons and aim of the analytical study of our Cultural Heritage (Sub•chapters 1.1 and 1.2). Then, some comments on interdisciplinarity will be

1 discussed (Sub-chapters 1.3 and 1.4). Finally, the non-destructive approach will be described (Sub-chapter 1.5) and put into context with aspects typical of the study of Cultural Heritage materials (Sub-chapter 1.6).

When we acknowledge that traditional works of art are made of tangible matter (Price and Burton 2011), we can start considering polychrome objects from a broader perspective, not only seeing their visual appearance, but their material constitution as well (Stoner 1985). This concept is at the basis of the present thesis. In fact, the appropriate preparation of each and every single component, and the combination of such materials into the final object is an intriguing topic and a powerful vehicle to understand and appreciate our past (Leona and Van Duyne 2009). Such material connotation of works of art, although partial and incomplete with respect to the general understanding of the object, is fundamental in appreciating its immaterial values (Leona and Van Duyne 2009). In fact, on the one hand, the iconographic build-up of the image corresponds to the commissioner’s view of the world, to display of power, to religious or secular meanings (Menu 1990). On the other hand, the choice of materials used to achieve such manifold purposes is determined by tradition, by alchemical practices, by the artist’s preferences, idiosyncrasies, experimentations and skills, and by the wealth of the commissioner, as well as it is subject to the assimilation of foreign techniques (Menu 1990, Ehrenreich 1995, Jones 2004, Guineau 2005, Leona and Van Duyne 2009, Huang et al. 2012, Heritage and Golfomitsou 2015). Art history and material science happen to be more interconnected than one would expect, so that the material study of works of art can contribute to the understanding of the creative process (van Asperen de Boer 1975, Dawson 2007, Leona and Van Duyne 2009), to the assessment of the risk of degradation, and to the understanding of alteration processes and preservation state (Dik et al. 2002). This thesis focusses on the material aspects of paintings which can be considered relevant in answering questions about the materiality of a work of art, and that might have an impact on the general interpretation of the object. The link between material aspects of a work of art with its actual overall complexity will be described in the next Sub-chapter.

2 A list of research areas of interest for art historians and conservators, based on the materials present, but in a broad perspective regarding the overall study, documentation, conservation and restoration of paintings is listed in the first column of Table 1.1 (Leute 1987, Clark 1995, Price and Burton 2011, Edwards and Vandenabeele 2012). The second column, on the other hand, reports some specific questions that can help in obtaining information on various aspects of the masterpiece (van Asperen de Boer 1975, Dawson 2007, Leona and Van Duyne 2009). All these questions are related to the identification and characterization of the materials actually present in the object. The answers to these questions are informative, sometimes decisive, but not sufficient for the attribution of a painting to a specific workshop/artist. In fact, stylistical considerations, archival search, etc., support the art historical interpretation of masterpieces (van Asperen de Boer 1975).

Table 1.1: Broad topics and specific questions that can be of interest when studying a work of art as a material entity.

Is it possible to obtain a date by dating techniques? CHRONOLOGY Are the materials coherent with the suggested timeframe? Are there materials with known production/discontinuation dates?

Are there materials that allow dating (see Chronology)? AUTHENTICATION Is the palette consistent with the artist’s? Are there clear indications of forgery?

Which painting technique was used? CREATIVE Did the painter change his mind during the creation of the painting? PROCESS/ Are there expensive materials? MATERIAL SELECTION Are there materials of specific provenance, suggesting trading, or local supplying?

Did the painting undergo restoration/overpainting treatments? MATERIAL Was it exposed to harmful conditions, like high humidity - floods, rain - or high HISTORY temperature - fires -, etc.?

Is it discoloured? Are degradation/alteration processes taking place? PRESERVATION Are there sensitive materials? Is there a risk to observe alterations if the painting is not taken care of?

3 Each work of art that we observe today contains a variety of materials. Part of these materials correspond to the ones used originally, and are not chemically altered (e.g. the support, stable pigments). Some other materials might have changed, either following the natural ageing processes (e.g. verdigris in oil; binders that have dried/polymerized) or as a result of undesired reactions. These latter reactions might be related to the creative process, if the painter used unsuitable materials (like for example the use of azurite in fresco and of smalt in oil, or the mixture of verdigris and orpiment, etc.), or to the interaction with the environment (discolouration of ochres according to humidity and temperature, sulphation of wall paintings exposed to SO2/SO3 pollutants, etc.). It appears then that the correct identification of the present materials is necessary, on one hand, to understand the creative process of the object and the original materials, and on the other, to detect and clarify the ongoing alteration processes. A summary of mediaeval inorganic pigments and their stability will be provided in Chapter 2. The consequences of such characterization are reflected in a deeper understanding of the painting practices, on the development of digital methods to reconstruct the lost polychromy (Dik et al. 2002), and on the intervention to limit/stop/prevent the progress of degradation. Due to the material complexity of the objects under investigation, it is important to define the target of the analysis, in order to answer the question (van Asperen de Boer 1975). For example, in the case of chronological issues: dating a wooden support does not necessarily provide the same information as the study of the paint layers in terms of binders and pigments, as the panel could have been recycled. If there is no agreement between support and painting materials, authenticity is in doubt. Also, in the case of modern pigments, production dates are available, and the comparison between those dates with the proposed timeframe can highlight later interventions, or forgeries (see paragraph 3.3.1). All the questions listed in Table 1.1 can be answered by identifying the present materials, and subsequently by understanding the chemical and physical processes, and by interpreting the results in relation with art history and technical art history. This field of interdisciplinary research on works of art can be described as “scientific examination of paintings” (van Asperen de Boer 1975), “archaeometry” (with a broader interest in objects from our past (Leute 1987, Ehrenreich 1995), “the measurement of things old” (Menu 1990)), “conservation/” (Berrie et al. 2003, Whitmore 2005, Leona and Van Duyne 2009, Heritage and Golfomitsou 2015), “archaeological chemistry” (Price and Burton 2011). This field of research exploring the boundaries between art and science will be discussed in the next Sub-chapter.

4 As seen in the previous Sub-chapters, and from the questions listed in Table 1.1, the answer to those research questions involves a variety of disciplines such as materials science, technical art history, art history, and conservation. Also, often the final aim is preservation of the object for the future by means of a conservation treatment. All the mentioned disciplines are interconnected, and necessary to comprehend each masterpiece, and eventually to define the optimal conservation strategy (Leona and Van Duyne 2009, Heritage and Golfomitsou 2015). It seems worth to provide some definitions at this point, and realize that: “[…] modern society increasingly demands application-oriented knowledge, and the usability of scientific knowledge generally requires the combination and integration of knowledge form various scientific disciplines”. (Van Den Besselaar and Heimeriks 2001) In this case, the application is the preservation of our Cultural Heritage by means of a conservation treatment. The necessary knowledge can be referred to as “non-disciplinary” -in contrast with “disciplinary”, where questions, methods, and approach are shared. For the purpose of this thesis, it will be nuanced following (Van Den Besselaar and Heimeriks 2001):  multidisciplinary: each discipline approaches the problem using its own methodology and tools, and the results are not integrated;  interdisciplinary: an autonomous theoretical, conceptual and methodological approach is developed, which allows for a coherent integration of the results;  transdisciplinary: the approach is explicitly formulated and is built on common theoretical understandings, including terminology and methodologies. It seems that the field of research explored in this thesis can be described as interdisciplinary, meaning that the communication is intense both inside the single specialties involved, and across the different communities. On the critical note side, often the communication between the scientist, the art historian, and the conservator is not articulated in terms that are understandable to every party, hampering a fruitful cooperation. The scientific details on the chemical and physical processes involved during the analysis may seem not that relevant in obtaining an answer to an archaeological/art historical question (materials identification vs provenance studies, for example (Tate 1986)), while a detailed explanation of the general and comprehensive framework

5 of the problem under investigation can help in pinpointing the most appropriate analytical approach. The effort required for reciprocal understanding is massive from every side, as it is the only way to achieve meaningful results through such an interdisciplinary attitude (Snow 1959, van Asperen de Boer 1975, Ehrenreich 1995, Huang et al. 2012). On the other hand, such a complex approach is the only way to address the research questions of Table 1.1 (the “application-oriented knowledge” mentioned above), while taking into account some specific aspects of the objects under investigation. First, it is of primary importance to understand what sort of information is needed, from the chemical point of view, to answer the question. Secondly, one needs to know which available techniques can actually provide that information. Finally, given the uniqueness and irreplaceability of Cultural Heritage materials (Leona and Van Duyne 2009), the analysis of choice should not alter the object in a visible way (Lahanier et al. 1986, Dawson 2007, Huang et al. 2012; Sub-chapter 1.5). The next Sub-chapter will provide some details on a concrete application of such a complex approach to a masterpiece of the Flemish Primitives.

The realization that the conservation of works of art requires an interdisciplinary approach was at the basis of the pioneering experience of the urgent restoration carried out on the Ghent Altarpiece in the early 1950s. The polyptych, completed in 1432 by Jan van Eyck was finally reunited after World War II, after centuries of unfortunate events, following local and European history ((Coremans 1953), see Sub-chapters 3.3 and 5.2). The examination of the panels after their return to Belgium revealed a very precarious conservation state of the varnishes and paint layers, which showed flaking and detachments. “L’Agneau Mystique était malade. Un traitement lui fut appliqué.”, Coremans wrote in the introduction to the book “L’Agneau Mystique au laboratoire” (Coremans 1953, p.7). That specific treatment resulted from the first case of a modern, interdisciplinary conservation approach. In fact, Paul Coremans, PhD in Analytical Chemistry at the Université Libre de Bruxelles (1932), lecturer in Art Techniques at the Higher Institute for Art History and Archaeology at Ghent State University (1948), head of the centrales iconographiques d’art national et le laboratoire central des musées de Belgique (1948, since 1957 Koninklijk Instituut voor het Kunstpatrimonium-Institut Royal du Patrimoine Artistique, KIK-IRPA),

6 and founder in 1949 of the Centre national de recherches “Primitifs flamands”, gathered an interdisciplinary team involving conservators, as well as academics in the fields of art history and archaeology (Coremans 1953). Moreover, an international expert commission was asked to intervene in the decision-making process, to ensure the best practice in the delicate conservation of the masterpiece (Coremans 1953). Almost 60 years later, however, it appeared that the Ghent Altarpiece was in need of a deeper conservation treatment, and the very same strategy was followed. The conservation treatment that begun in 2012 is in fact led by KIK•IRPA, providing expertise knowledge in conservation, documentation and chemical analyses on samples, but involves as well other Belgian institutions such as Ghent University (through the Concerted Research Action (GOA) project “Archaeometrical study of the Ghent Altarpiece”) and University of Antwerp (Antwerp X-ray analysis, Electrochemistry and Speciation (AXES) Research group). Ghent University supports the conservators form the art historical perspective, as well as by in situ chemical characterization via point analysis, and microscope observation of the Altarpiece under restoration, while University of Antwerp provided in situ chemical imaging techniques. Also in this case, an international experts commission is regularly informed and consulted on the decisions to take in the process (Martens 2015). Such an interdisciplinary group offers the chance of a deep comprehension of the masterpiece under study, providing valuable indications for its conservation and information for better understanding its material history. Furthermore, from the archaeometrical point of view, it shows the challenging nature of analysis of works of art, which needs to find a balance between the type of information needed and the choice of the analytical technique, keeping into account ethical issues regarding the safeguard of the object, as well as the safety of the operators, the practical aspects of the analysis (set-up, environmental conditions, etc.), and aspects related to data interpretation (availability of reference databases, etc.). Finally, it is important to mention that, in the last 60 years, technological improvements and an increasing interest in the scientific examination of works of art changed deeply the role and impact of materials characterization in conservation science (Leute 1987, Price and Burton 2011). In the next Sub•chapter, details are given regarding definitions and some practical aspects of analysis with respect to the investigation of works of art.

7 In the past, there was limited access to analytical techniques for the study of works of art. The most advanced approaches encompassed X-ray radiography, infrared photography and reflectography, UV fluorescence observation, and sampling for analysis in the laboratory (Rorimer 1931, Lahanier et al. 1986, Leute 1987, Menu 1990, Dawson 2007, Guerra 2008). This latter phase (i.e. sampling) was recognized to be crucial to identify the pigments (Plesters 1956, McCrone 1994). Progress in techniques and instrument design, and the development of archaeometry and conservation science allowed for a huge improvement in the approach to paintings, and works of art in general, by combining techniques and methods from chemistry, physics, earth and life sciences with approaches from humanities (Vandenabeele and Donais 2016). Moreover, the increasing availability of mobile instruments suitable for in situ archaeometrical research is promising and tantalizing for people in both, humanities and material sciences (Price and Burton 2011, Vandenabeele and Donais 2016). An additional point to consider, however, is related to ethical aspects of performing analyses for the purpose of good practice in conservation. In the 1920s (Stoner 2003), when the most advanced analytical techniques available were X-ray radiography and UV fluorescence observations, the study of paintings was considered sacrilegious1. This contempt was accompanied by extensive retouching of polychrome works of art, in a general underestimation of the material aspects of paintings. However, the development of analytical possibilities and their application to works of art allowed the individuation of restoration interventions (Rorimer 1931), as well as to understand and disclose master’s painting technique (Desneux 1958, Effmann 2006). It appears that an analytical attitude, coupled with technical developments, allowed clarifying a variety of aspects of the savoir faire of masters. In the case of Van Eyck, for example, extensive investigations regarded the study of underdrawings (Desneux 1958), and the chemical characterization of the paint medium, after many efforts were made to understand its composition based on empirical and theoretical considerations (Effmann 2006). In both cases, the results of analyses provided the experts with scientific evidence, which needed to be incorporated in the theoretical knowledge of Van Eyck’s modus operandi.

1 “There was contempt for concern about condition. That was as naughty as to inquire about the digestive system of an opera singer. […] a picture […] was something of beauty and your sensitivities for the quality of the beauty were the important matter.” (Stoner 2003, p. 43)

8 This example highlights the dynamic character of professional ethics in conservation studies, which requires the inclusion of new findings into the recognised good practice (Martens 2015). Establishing a correspondence between the type of object, the information needed (Table 1.1), and the analytical technique(s) which can provide it requires a specific training in both technical art history and analytical chemistry, as well as some ethical considerations. In fact, every object requires a tailored approach (van Asperen de Boer 1975, Lahanier et al. 1986, Adriaens 2005). Additionally, from the point of view of conservators, safeguarding the integrity of the object under investigation is of primary importance. The ethical issues related to the integrity of the masterpiece under investigation might conflict with the need to answer questions by means of chemical analyses. Also, it is important to consider that sampling, on the other hand, might minimize the risks related to the handling of the whole object (e.g. transportation (Clak 1995)). Ethical issues are as well related to the conservation practice (Brandi 1963, Martens 2015). In any case, ethics needs to be dynamic, adapt to specific conditions and circumstances, as well as to the research question that needs to be solved. Therefore, the next paragraphs will feature the description of some relevant aspects of the analytical approach significant to the purpose of this thesis. First, “invasive” refers to any approach requiring a modification of the whole object, which may not be visible by naked eye, therefore not necessarily hampering the overall appearance of the artefact. The nuances here range from non-invasive, to minimally- (or micro-) invasive, to invasive. Many analytical techniques now in use in archaeometry are non-invasive, as they do not even require a direct contact with the object (e.g. infrared reflectography, X•ray radiography, infrared reflectance spectroscopy, in situ Raman spectroscopy, in situ X-ray fluorescence analysis, etc. (van Asperen de Boer 1975, Adriaens 2005, Dawson 2007, Vandenabeele and Donais 2016)). On the other hand, the collection of representative microscopic samples for stratigraphic analysis, or for the identification of pigments, binders and organic colourants, as well as cotton swab sampling from unvarnished layers for pigment analysis, and laser ablation techniques are to be considered minimally-, or micro•invasive. The size of the collected sample/sampled area is of few tenths of a millimetre, therefore not visible by naked eye, unless under close examination of the surface. However, a stratigraphic sample offers the major advantage of clarifying the layer build-up of the painting (Plesters 1956, Lahanier et al. 1986, Tate 1986, McCrone 1994). Moreover, some techniques require larger amounts of material in order to be able to obtain reliable results, such as for petrographic analysis of ceramics (by means of thin sections), for radiocarbon dating, for digestion of samples for

9 techniques such as mass spectrometries, etc. Finally, a technique which might be considered invasive, but for a different reason, is neutron activation analysis (NAA): in fact, the object is made radioactive by exposure to radiation, and time needs to pass before it can be safely handled, or displayed, again (Price and Burton 2011, Bonneau et al. 2014). Secondly, the definition of “destructive” requires some explanation. This term, in fact, is related to methods which either alter the object (destructive strictu sensu), or consume the collected sample so that it cannot be further analysed (sample-consuming) (Tate 1986, Price and Burton 2011, Vandenabeele and Donais 2016). This connotation is slightly different from the more intuitive interpretation, where “non-destructive” equals “no sampling” (Lahanier et al. 1986, Vandenabeele and Donais 2016). Some of the traditional micro-chemical tests performed on cross sections are sample-consuming, as they alter the materials permanently (Plesters 1956, van Asperen de Boer 1975, McCrone 1994). The cross section can be polished again to expose a non-treated surface, but repeating this polishing might end up in a complete destruction of the sub•millimetric sample. Chromatographic techniques, and all the other techniques which require a sample in a liquid (or gas) form, as well as laser based techniques (as LA-ICP-MS and LIBS, for example) are micro•destructive and sample-consuming. Synchrotron-radiation-based techniques are non-destructive, but they require either a sample or the displacement of the whole object to the facility (Harbottle et al. 1986). Thirdly, it is important to discuss direct analysis. In cases when sampling is not allowed, and/or the whole object cannot be moved from its location, a different approach is required (Lahanier et al. 1986, Vandenabeele and Donais 2016). In fact, every manipulation of the Cultural Heritage object, including transport to a different location for the purpose of analysis, or for other reasons, potentially increases the risk of damage, and might not be recommended (Edwards and Chalmers 2005). Moreover, the costs of safe transportation and insurance might make this option not feasible. However, the technological development of the last decades allowed the progress in the design of robust, compact sized, reliable instruments that can be utilized outside of the lab. Here the distinction encompasses laboratory equipment, mobile, and portable instruments (Lauwers et al. 2014, Vandenabeele et al. 2014, Vandenabeele and Donais 2016, Lauwers 2017). In principle, most laboratory instruments are technically transportable, with a car or a van, although they are not designed to be moved frequently. On the other hand, mobile instruments are designed for mobility (Vandenabeele and Donais 2016, Lauwers 2017). Contrarily to laboratory instruments, mobile instruments are ready to be used without time-consuming starting-up (alignment, calibration, etc.)

10 procedures. The category of mobile instrumentation encompasses mobile instruments strictu sensu (check-in luggage size, max 30 kg), portable instruments (hand luggage size, ca. 20 kg), handheld instruments (max 5 kg) and palm instruments (Lauwers et al. 2014, Vandenabeele et al. 2014, Vandenabeele and Donais 2016), see Figure 1.1. Although many museums now host state-of-the-art laboratories supporting the conservation practice of Cultural Heritage objects (Bryce and Tate 1980, Menu 1990), not all of them can afford such laboratories. Moreover, not all the works of art are kept inside museums! A temporary installation of instruments in the vicinity of the objects to be investigated can take place, and is being widely exploited by museums and other institutions. The application of portable instruments is widely represented in archaeometrical literature (Lahanier et al. 1986, Vandenabeele et al. 2007, 2014, Miliani et al. 2010, Colomban 2012, Alfeld and Broekaert 2013, Lauwers et al. 2014, Van De Voorde et al. 2014, Barbi et al. 2014, Brunetti et al. 2016, Conti et al. 2016, Vandenabeele and Donais 2016). Finally, the case of MOLAB (MObile LABoratory) perfectly represents the use

Transportable

Mobile

Portable

Handheld

Palm

Figure 1.1: Examples of the different categories of spectroscopic instrumentation, according to the degree of mobility. Transportable: Bruker Senterra Raman instrument and EDAX Eagle III XRF spectrometer; Mobile: Mobile Raman instrument (MArtA) and Bruker ARTAX (Image from: www.bruker.com); Portable: Enwave EZ-Raman-I-Dual; Handheld: Olympus InnovX Delta XRF spectrometer; Palm: SciAps ReporteR Raman instrument (Image from: www.SciAps.com). Image after (Vandenabeele and Donais 2016).

11 of transportable and mobile instruments, providing the installation of mobile facilities for in situ measurements (Miliani et al. 2010, Brunetti et al. 2016). So far, some features of the analytical techniques to be used for the analysis of Cultural Heritage objects were discussed. In the final part of this Sub•chapter some observations on the practical aspects of in situ analysis will be tackled. In fact, it seems relevant to remark that performing analysis outside of a dedicated laboratory poses some challenges, although in certain cases this might be the only way to obtain the chemical characterization of the materials.

Table 1.2: Example of a check-list for in situ measurements. In this specific case, it was a portable Raman campaign in Sicily, including a flight transfer that limited the amount of transportable items in size and weight. In bold are indicated items whose availability on site was granted by the collaborators. More details on the use of portable Raman spectrometers can be found in Sub-chapter 4.4.

Where? MESSINA CATANIA Caravaggio Sala Vaccarini What? Canvas paintings, varnished Wall paintings on the vault Points of interest at ca. 2m height Scaffolding When? 28/4 29/4 (morning) Instrument Enwave Enwave Table Tripod Setup Cross Handheld probehead (light-blockers) Holder Ladder(s) Battery? (ca. 6 + 6 hours) Battery? (ca. 6 + 6 hours) Power supply? Otherwise: extension cable + power Otherwise: extension cable + power adaptor adaptor Not complete Working in contact with the surface, Darkness of the room Black cloth? no interference is expected Counterweight, screws and screwdrivers Camera + charger Varia Calibration products Logbooks DinoLite?

On the one hand, in general, the quality of the results provided by laboratory instruments is better than that of portable/mobile devices (Vandenabeele and Donais 2016). On the other hand, the acquisition of data through mobile instruments might not be trivial, although the instrument itself is ready to use. Analyses might be required in sites without electricity, which calls for battery•operated instruments; or at considerable heights from the ground, requiring the need for ladders or scaffoldings, and the option to transport and install, and operate, all the devices from a safe and stable location. Moreover, in

12 some cases the environment might interfere with the analysis, which requires the foresight of adequate protections (e.g. light for Raman spectroscopic analyses). These aspects need to be known (or at least estimated) in advance, and a checklist of all potential difficulties should be compiled, and tackled one by one. An example of this preparatory stage used during this thesis is exemplified in Table 1.2. This phase is fundamental to ensure the optimal conditions for in situ analysis, either in a museum a few km from the laboratory, either on scaffoldings inside an historical library in Catania, Italy (Barone et al. 2016), either in prehistoric hunters-gatherers shelters in the Argentinian steppe (Rousaki et al. 2017). To conclude, the analytical approach (invasive/destructive), the instrumental characteristics (transportable/mobile), and the issues (in situ) highlighted in this Sub-chapter are relevant on their own from the experimental point of view. All these aspects need to be considered together with the art historical research questions. In the next Sub-chapter, some indications are given regarding the selection of the most appropriate analytical techniques to maximize the equilibrium between information and risk of damage for the object (Vandenabeele and Donais 2016). The discussion on the analytical techniques used in this research project and their capabilities and limitations in providing useful information to tackle the non•destructive characterization of materials and their degradation will be given in Chapters 3, 4 and 5.

The questions highlighted in Table 1.1 can be easily answered based on the identification of materials, on condition that an appropriate analytical methodology is followed. Also, the balance between the integrity of the work of art and the need for analyses needs to be considered. A (simplified) flow chart is here proposed to try to give some guidelines for the analysis of works of art and archaeological objects (Figure 1.2). This might seem straightforward and simple, but each specific case requires some sort of adaptation according to many factors, mainly the safeguard of the integrity of the masterpiece, ethical issues, and the availability and characteristics of analytical techniques. In general, a good starting point to analyse a work of art is looking at it, possibly in situ (van Asperen de Boer 1975). A careful observation by the naked eye can already provide some insight in the material composition of an object. The naked eye can be aided with a microscope, to observe details at a smaller

13 scale. Moreover, visible light sources can be used to investigate surface roughness and morphology (raking light observation/photography). Then, different regions of the electromagnetic spectrum can be used to investigate different aspects of the same object: infrared light (IR photography and IR reflectography (van Asperen de Boer 1968) can be used to investigate the underdrawing, while UV light can be diagnostic and useful in the study of varnish layers and restorations (Rorimer 1931)). Moreover, X-ray radiography provides information on differences in density and on the presence of heavier elements, while neutron activation analysis (NAA) provides the spatial distribution of elements (van Asperen de Boer 1975). All the information coming from imaging techniques, or techniques of visualization (van Asperen de Boer 1975, Tate 1986, Edwards and Vandenabeele 2012) are normally helpful in defining the areas of interest to be further studied. Limitations might apply on the range of techniques that can be applied at this stage: in fact, safety reasons related to the use of X-rays and of radioactive sources for NAA, or practical aspects (presence of glass enclosures for IR•methods; need for complete darkness for UV-fluorescence observations, etc.) might limit the applicability of the abovementioned techniques of visualization. Traditionally, the second step after imaging techniques would have been the collection of minute (stratigraphic) samples in order to clarify, under the microscope and by using micro-chemical tests, the nature of the pigments and binders in each paint layer (Plesters 1956). However, according to the research question, sampling (i.e. altering irreversibly the painted surface) might even become unnecessary. This would be the case if the question regards only the uppermost layers, which can be investigated by surface techniques. If, contrarily to this, a sample would actually help in solving the issue, permission needs to be granted. sometimes allow the collection of material from objects under their responsibility, but this is not always the case (Edwards and Chalmers 2005, Price and Burton 2011). If a sample is taken, it is recommended to first perform all the non-destructive analyses of the case, and eventually proceed with the destructive step, in order to optimize the consumption of the sample to gain as much information as possible. If, on the contrary, there is no possibility to remove material from the object, and depending on the research question, other options might be available in the form of direct analysis (Adriaens 2005). However, this might mean that some additional actions need to be taken, regarding the possibility of moving a precious and sometimes very fragile object from its usual location to an analytical laboratory to be studied in detail. In this case, the size of the object limits the analytical possibilities, depending on the dimensions of the measurement chamber of the laboratory instrument.

14 Figure 1.2: Flowchart illustrating the options for analysis based on the possibility of sampling and of moving the object under study away from its location. For the definitions of destructive, invasive and portable refer to Sub-chapter 1.5.

The alternative solution to “oversized” objects actually corresponds to the approach proposed to study paintings and items which cannot be moved (for physical impossibility - wall paintings, for safety reasons, for insurance and shipment costs, etc. (Guerra 2008)). The development of mobile instruments provides the archaeometrists, conservators, and art historians with powerful tools to pursue the scientific examination of Cultural Heritage by means of direct analysis (Vandenabeele and Donais 2016). Again, the research question, the safeguard of the artefact and operators’ safety, the costs related to the analysis vs. those of safe transportation of the object to a research facility, have to influence the choice of the technique most suitable to solve the current issue, but many techniques are nowadays available for in situ studies of materials from our Cultural Heritage. To conclude, the “Re-think strategy!” box (Figure 1.2, bottom right) might sound like a failure of the decision-making process. However, as there are many steps, the change in strategy might occur at a variety of levels, including the possibility of a sample (ethical), the cooperation with colleagues and companies to

15 make available alternative instrumentation (practical), in terms of mobility or measurement chamber size. Moreover, as already mentioned at the beginning of this Sub•chapter, the approach proposed in this flow chart is not “black or white”, and solutions need to be elaborated according to each and every specific situation. Finally, it seems important to mention the importance of comparing and contrasting the results of different analytical methods, for a deeper understanding of the materials and on-going processes (Sub-chapter 5.1). A multi-technique study is constantly recommended, although not always possible. To summarize, and to link this general introduction to the content of this thesis, the starting point for each analysis should be a specific research question, which, in this case, concerns the characterization and identification of pigments and of degradation processes. Based on this information, a deeper understanding of the object can be achieved, by making the interdisciplinary connection between material and immaterial aspects. The outline of this thesis, based on these concepts, is given in the following.

So far, in Chapter 1, a few concepts at the basis of this work have been highlighted. First of all, the material composition of polychrome objects belonging to Cultural Heritage was pointed out, and the link between materials, the creative process and conservation strategies explained in Sub-chapters 1.1 and 1.2. The advantages of applying analytical techniques to Cultural Heritage materials, in order to achieve a complete and detailed comprehension of the object, were discussed in Sub-chapters 1.2 and 1.3, and an example of such interdisciplinary approach to works of art was given in Sub-chapter 1.4. The two final Sub•chapters of this introduction (1.5 and 1.6) regard specific aspects of paintings investigation, including the application of non-invasive and non-destructive approaches in the framework of characterization of materials. Next, Chapter 2 reviews literature on the most common inorganic mediaeval pigments and their degradation processes, as a starting point for the application of a minimally-invasive (most of the time non-invasive at all) analytical approach combined with the use of non-destructive techniques for this purpose. The techniques used in this thesis are described in Chapters 3 and 4. Chapter 3 describes the used X-ray based techniques and their application of to the study of pigments by in situ analysis of the outer wings of the Ghent

16 Altarpiece (Sub-chapter 3.3), and in samples (Sub•chapter 3.4), while Chapter 4 concerns non-destructive Raman spectroscopy and its application to characterize pigments and Cultural Heritage materials. Micro-Raman spectroscopy (Chapter 4) is used to characterize selected classes of pigments (green compounds: Sub-chapter 4.1, carbon blacks: Sub•chapter 4.2), and pigment degradation in museum-like environments (Sub•chapter 4.3)). Direct Raman analyses on a mediaeval polyptych in Funchal, Portugal, are reported in Sub-chapter 4.4. Furthermore, Chapter 5 will provide some insight in the non-destructive multi•technique characterization of materials on the frames of the Ghent Altarpiece. Special attention is given to the practical aspects of in situ analysis (Table 1.2, Paragraphs 3.3.1, 4.4.1, and Chapter 5). The final chapters (Chapter 6 and Chapter 7) will provide some concluding remarks and future perspectives, in English and in Dutch, respectively.

Adriaens, A. 2005. Non-destructive analysis and testing of museum objects: an overview of 5 years of research. Spectrochimica Acta Part B: Atomic Spectroscopy 60:1503•1516. Alfeld, M., and J. Broekaert. 2013. Mobile depth profiling and sub-surface imaging techniques for historical paintings-A review. Spectrochimica Acta Part B: Atomic Spectroscopy 88:211-230. van Asperen de Boer, J. R. J. 1975. An introduction to the scientific examination of paintings. Netherlands Yearbook for History of Art / Nederlands Kunsthistorisch Jaarboek Online 26:1-40. van Asperen de Boer, J. R. J. 1968. Infrared Reflectography: a Method for the Examination of Paintings. Applied Optics 7:1711–1714. Barbi, N., R. Alberti, L. Bombelli, and T. Frizzi. 2014. A Non-Invasive Portable XRF System for Cultural Heritage Analyses. Microscopy and Microanalysis 20:2020-2021. Barone, G., D. Bersani, A. Coccato, D. Lauwers, P. Mazzoleni, S. Raneri, P. Vandenabeele, D. Manzini, G. Agostino, and N. F. Neri. 2016. Nondestructive Raman investigation on wall paintings at Sala Vaccarini in Catania (Sicily). Applied Physics A 122:838. Berrie, B., E. R. De La Rie, R. Hoffmann, F. H. Rhodes, J. Tomlinson, T. Wiesel, and J. Winter. 2003. Scientific Examination of Art: Modern Techniques in Conservation and Analysis. Page (Sackler NAS Colloquium) Scientific Examination of Art: Modern Techniques in Conservation and Analysis (2005). National Academies Press, Washington, D.C., USA. Bonneau, A., J.-F. Moreau, R. G. V. Hancock, and K. Karklins. 2014. Archaeometrical analysis of glass beads - potentials, limitations and results. BEADS: Journal of the Society of Bead Researchers 26:35-46.

17 Brandi, C. 1963. Teoria del Restauro. Einaudi, Torino, Italy. Brunetti, B., C. Miliani, F. Rosi, and B. Doherty. 2016. Non-invasive Investigations of Paintings by Portable Instrumentation: The MOLAB Experience. Topics in Current Chemistry 374:10. Bryce, T., and J. Tate. 1980. The role of the national museum conservation laboratory. Pages 4-7. The laboratories of the National Museum of antiquities of Scotland. National museum of antiquities of scotland, Edinburgh, UK. Clark, R. J. . 1995. Pigment identification on medieval manuscripts by Raman microscopy. Journal of Molecular Structure 347:417–427. Colomban, P. 2012. The on-site/remote Raman analysis with mobile instruments: a review of drawbacks and success in Cultural Heritage studies and other associated fields. Journal of Raman Spectroscopy 43:1529-1535. Conti, C., A. Botteon, M. Bertasa, C. Colombo, M. Realini, and D. Sali. 2016. Portable Sequentially Shifted Excitation Raman spectroscopy as an innovative tool for in situ chemical interrogation of painted surfaces. Analyst 141:4599-4607. Coremans, P. 1953. Les Primitifs Flamands. III. Contribution a l’etude des Primitifs Flamands 2. L’Agneau Mystique au Laboratoire. De Sikkel, Antwerp, Belgium. Dawson, T. L. 2007. Examination, conservation and restoration of painted art. Coloration Technology 123:281-292. Desneux, J. 1958. Underdrawings and pentimenti in the pictures of Jan Van Eyck. The Art Bulletin 40:13–21. Dik, J., M. den Leeuw, W. Verbakel, P. Peschar, R. Schillemans, and H. Schenk. 2002. The digital reconstruction of a smalt discoloured painting by Hendrick Ter Brugghen. Zeitschrift filer Kunsttechnologie und Konservierung 16:130-146. Edwards, H. G. M., and J. Chalmers, editors. 2005. Raman Spectroscopy in Archaeology and Art History. Royal Society of Chemistry, Cambridge, UK. Edwards, H., and P. Vandenabeele, editors. 2012. Analytical Archaeometry. Royal Society of Chemistry, Cambridge, UK. Effmann, E. 2006. Theories about the Eyckian painting medium from the late-eighteenth to the mid-twentieth centuries. Studies in Conservation 51:17–26. Ehrenreich, R. M. 1995. Archaeometry into archaeology. Journal of Archaeological Method and Theory 2:1-6. Guerra, M. F. 2008. Archeometry and museums: fifty years of curiosity and wonder. Archaeometry 50:951-967. Guineau, B. 2005. Glossaire des matériaux de la couleur et des termes techniques employés dans les recettes de couleurs anciennes. Brepols. Turnhout, Belgium. Harbottle, G., B. M. Gordon, and K. W. Jones. 1986. Use of synchrotron radiation in archaeometry. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 14:116-122. Heritage, A., and S. Golfomitsou. 2015. Conservation science: Reflections and future perspectives. Studies in Conservation 60:2-6. Huang, J.-F. 2012. Data and interpretation: enhancing conservation of art and Cultural Heritage through collaboration between scientist, conservator, and art historian. IOP Conference Series: Materials Science and Engineering 37:12003. Jones, A. 2004. Archaeometry and materiality: materials-based analysis in theory and practice. Archaeometry 46:327-338. Lahanier, C., F. D. Preusser, and L. Van Zelst. 1986. Study and conservation of museum objects: Use of classical analytical techniques. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 14:1-9. Lauwers, D., A. Hutado, V. Tanevska, L. Moens, D. Bersani, and P. Vandenabeele. 2014. Characterisation of a portable Ramanspectrometer for in situ analysis of art

18 objects. Spectrochimica Acta Part A-Molecular and Biomolecular Spectroscopy 118:294-301. Lauwers, D. 2017. Characterisation and optimisation of mobile Raman spectroscopy for art analysis. Ghent University. PhD thesis. Leona, M., and R. Van Duyne. 2009. Chemistry and Materials Research at the Interface Between Science and Art: Report of a Workshop Cosponsored by the National Science Foundation and the Andrew W. Mellon Foundation, July 6-7, 2009, Arlington, USA. Leute, U. 1987. Archaeometry : an introduction to physical methods in archaeology and the history of art. VCH Verlagsgesellschaft mbH, Weinheim, Germany. Martens, M. P. J. 2015. “Leave it or take it away”: ethical considerations on the removal of overpaintings. The case of the Ghent Altarpiece. CeROArt. Conservation, exposition, Restauration d’Objets d’Art. 10:1-8. McCrone, W. C. 1994. Polarized Light Microscopy in Conservation: A Personal Perspective. Journal of the American Institute for Conservation 33:101-114. Menu, M. 1990. IBA in the museum: Why AGLAE. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 45:597•603. Miliani, C., F. Rosi, B. G. Brunetti, and A. Sgamellotti. 2010. In situ Noninvasive Study of Artworks: The MOLAB Multitechnique Approach. Accounts of Chemical Research 43:728-738. Plesters, J. 1956. Cross-sections and chemical analysis of paint samples. Studies in Conservation 2:110-157. Price, T. D., and J. H. Burton, editors. 2011. An introduction to archaeological chemistry. Springer Science & Business Media, New York, USA. Rorimer, J. 1931. Ultra-violet rays and their use in the examination of works of art. Metropolitan Museum of Art, New York, USA. Rousaki, A., C. Vazquez, V. Aldazabal, C. Bellelli, M. Carballido Calatayud, A. Hajduk, E. Vargas, O. Palacios, P. Vandenabeele, and L. Moens. 2017. The first use of portable Raman instrumentation for the in situ study of Prehistoric rock paintings in Patagonian sites. Journal of Raman Spectroscopy (in press). Snow, C. P. 1959. The Two Cultures. Cambridge University Press, London, UK. Stoner, J. H. 1985. Ascertaining the artist’s intent through discussion, documentation and careful observation. The International Journal of Museum Management and Curatorship 4:87-92. Stoner, J. H. 2003. Changing approaches in Art Conservation: 1925 to the present. Pages 40–57. In National Academy of Sciences, editor. Scientific Examination of Art: Modern Techniques in Conservation and Analysis - Proceedings of the National Academy of Sciences. The National Academies Press, Washington, D.C., USA. Tate, J. 1986. Some problems in analysing museum material by nondestructive surface sensitive techniques. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 14:20-23. Vandenabeele, P., K. Castro, M. Hargreaves, L. Moens, J. M. Madariaga, and H. G. M. Edwards. 2007. Comparative study of mobile Raman instrumentation for art analysis. Analytica chimica acta 588:108-116. Vandenabeele, P., H. G. M. Edwards, and J. Jehlička. 2014. The role of mobile instrumentation in novel applications of Raman spectroscopy: archaeometry, geosciences, and forensics. Chemical Society Reviews 43:2628-2649. Vandenabeele, P., and M. K. Donais. 2016. Mobile Spectroscopic Instrumentation in Archaeometry Research. Applied spectroscopy 70:27-41. Van De Voorde, L., J. Van Pevenage, K. De Langhe, R. De Wolf, B. Vekemans, L. Vincze, P. Vandenabeele, and M. P. J. Martens. 2014. Non-destructive in situ study

19 of “Mad Meg” by Pieter Bruegel the Elder using mobile X-ray fluorescence, X-ray diffraction and Raman spectrometers. Spectrochimica Acta - Part B Atomic Spectroscopy 97:1-6. Van Den Besselaar, P., and G. Heimeriks. 2001. Disciplinary, Multidisciplinary, Interdisciplinary: Concepts and Indicators. Pages 705–716. In M. Davis and C. S. Wilson, editors.The 8th conference on Scientometrics and Informetrics – ISSI2001. Bibliometric & Informetric Research Group (BIRG), Sydney, Australia. Whitmore, P. M. 2005. Conservation Science Research: Activities, Needs, and Funding Opportunities ; a Report to the National Science Foundation. Virginia, USA.

20 In this Chapter, the technological aspects of painting-making will be tackled. In fact, this thesis focusses on the study of the inorganic pigments for the study of panel paintings, for better understanding the polychrome object as a whole. However, it is important to be aware of the variety of binding media that can be used, which affects the actual process of making a work of art, the choice of materials, as well as its ageing process, its preservation and conservation. The creative process of a painting requires many materials, from support, to pigments and binders, to metal leaves and varnishes (Pliny the Elder: Naturalis Historia, Theophilus: De diversis artibus, Cennini: Il libro dell’arte, Thompson 1956, Dawson 2007). From the practical point of view, pigments are used for their optical properties to render polychromies, while a binder allows the formation of a cohesive film and an appropriate and adhesion of the pigment particles to the support. It defines the painting technique (Maltese 1991). The first part of this Chapter (Sub-chapter 2.1) is dedicated to a literature review of common inorganic mediaeval pigments in use in Europe. Particular attention is paid to their stability and sensitivity to degradation. The published results of investigations of works of art are combined, when possible, with specific studies on pigments reactivity performed in the lab using powders or mock-ups. Attention is given to the evolution of the interpretation of specific processes as a result of scientific advancement in understanding the ongoing chemical reactions. This review is not limited to the application of a specific analytical technique for detecting and understanding degradation processes; on the other hand, it focusses on the so far identified alterations, their impact on the polychromy appearance, and their causes (see Table A.1) A precise identification of degradation processes, which in the case of pigments mainly appear as discolourations, is informative on:

21  The creative process and material selection made by artists and artisans;  The material history of an object;  The decision-making process for conservation treatments. Then, the build-up of panel paintings by the so-called Flemish Primitives (Sub•chapter 2.2) is described. The painting technique is based on siccative oils. However, it is important to note that a variety of materials showing film-forming properties was employed (bone marrow, plant sap, urine, blood, milk and derivates, gums, egg, oils, resins, etc. (Maltese 1991, Casadio et al. 2004, Smith and Clark 2004)). The different binders can be classified as hydrophilic (polysaccharidic - gums, proteinaceous - milk, egg white), hydrophobic (fatty • siccative oils), or emulsions of the two (egg yolk, whole egg, fat tempera). They could be used alone or mixed among themselves (fat tempera: egg + oil) and with other additives to improve their optical and physical properties. In case of wall paintings, moreover, the possibility of a fresco technique needs to be mentioned, where pigments are dispersed in water, and the binding agent is provided by the wet plaster. A mixed technique (fresco/secco) was often used to apply pigments sensitive to to the alkaline conditions given by the lime binder (Cennini: Il libro dell’arte, Rowland and Howe 2001), and for fine details (Maltese 1991, Casadio et al. 2004).

Colour is one of the most important properties of objects, in archaeology and art history. Pigments, used to achieve chromatic effects, are in theory stable to environmental conditions and are inert to the binder in which they are used, to guarantee good optical performances through time. However, this appears not always to be the case. Discolouration and degradation of pigments significantly alter the legibility of polychrome works of art, so that the development of monitoring methods to ensure the preservation of Cultural Heritage objects is of primary importance. The first part of this Chapter is based on the paper “On the stability of mediaeval inorganic pigments: a literature review of the effect of climate, material selection, biological activity, analysis and conservation treatments” by A. Coccato, L. Moens, and P. Vandenabeele, published in Heritage Science (Coccato et al. 2017), and

22 focusses on the inorganic pigments in use during the Middle Ages in Europe (Cennini: Il libro dell’arte, Bell et al. 1997, Guineau 2005, Eastaugh et al. 2008), with specific interest on their sensitivity to degradation. It was first presented as a poster at the 4th International Congress Chemistry for Cultural Heritage, Brussels, Belgium (Coccato et al. 2016a). In the framework of the European FP7 MEMORI project (Measurement, Effect Assessment and Mitigation of Pollutant Impact on Movable Cultural Assets. Innovative Research for Market Transfer), Ghent University was given the task to collect information on pigments’ stability, and to study the degradation of pigments in museum environments, specifically by assessing pigment degradation upon acetic acid exposure (Grøntoft and Dahlin 2012, Grøntoft et al. 2016, De Laet et al. 2013). This literature review is to be considered part of the development of the MEMORI-dosimeter, which evaluates the impact of climate (relative humidity, temperature, illumination, etc., including volatile organic compounds) on moveable objects, and informs the end-user on the air quality, and therefore on the safety of the objects inside display-cases, storage facilities and exhibition rooms (Grøntoft and Dahlin 2012, Grøntoft et al. 2016, Lankester et al. 2013). It is important to note that all the materials (and not only the pigments) present in Cultural Heritage objects are subject to ageing, and have complex interactions among each other and with the environment. The study of pigment degradation is clearly a relevant topic in conservation science, as the knowledge of the undesirable alteration processes can provide insight into the manufacturing process and material history of the object, and influence preservation strategies and conservation treatments. The list of inorganic pigments available and used in Europe during the Middle Ages is quite limited (Theophilus: De diversis artibus, Cennini: Il libro dell’arte, Thompson 1956, Bell et al. 1997, Guineau 2005). The topic of material selection was already considered in artistic treatises, on the basis of empirical knowledge, as it was the specific aim of painters to produce durable masterpieces (Cennini: Il libro dell’arte, Theophilus: De diversis artibus). Centuries of practical knowledge allowed artists and artisans to select materials which were not susceptible to fading or discolouration, and advice on unstable pigment mixtures1 (therefore not recommended), or on the use of specific pigments with selected binders, are common in artistic literature and treatises (see, for example, (Cennini: Il libro dell’arte)). Artists and artisans were anyway constantly experimenting, and the material selection was performed according to their savoir faire and to practical

1 E.g. “Orpiment will ly faire on any culler, except verdigris, but no culler can ly faire on him, he kills them all.” Cornelius Jansen, 17th century, quoted in (Thompson 1956, p. 178).

23 considerations (Guineau 2005, Pastoureau 2007), often using unstable mixtures during the actual production of the object, which nowadays result in hardly legible works of art. Once the polychrome object is exposed to the environment, factors such as relative humidity (RH; stable, fluctuating), temperature, illumination conditions, inorganic pollutants, and volatile organic compounds need to be taken into account. Also, the material history of the polychrome objects, being subjected to the environment, including seasonal climatic changes and extreme conditions such as in fires and floods, to biological activity, and to pollution related to the industrial revolution (sulphur and nitrogen compounds SO2, SO3, NOx (Haneef et al. 1992, Saunders 2000, Srivastava et al. 2004, Alebic-Juretic and Sekuli-Cikovic 2009)), is bound to provoke alterations on those materials sensitive to climatic factors, to biological activity, and to anthropogenic pollutants. Incautious human interventions (including restorations) might also have a negative impact on the object. More often, the causes of degradation are complex and involve a variety of parameters, from the intrinsic properties of materials used as pigments (instability to specific factors, presence of impurities from inaccurate purifying or manufacturing, etc.), their interaction with the binders (acidity of the oil, alkalinity in fresco) and when mixed (blackening of lead white when mixed with verdigris or arsenic pigments); the exposure to environmental conditions (RH, temperature, illumination conditions, pollution, etc.) can moreover facilitate or cause degradation processes. Finally, human intervention (display in museums and exhibitions, storage in deposits, conservation treatments, scientific analysis, etc.) also needs to be taken into account for an efficient preservation of our Cultural Heritage. Moreover, in another part of this thesis (Paragraph 4.3.3), original results of pigment exposure to volatile organic compounds (i.e. acetic acid) are provided in detail. It is important to mention that all these factors often act in synergy. The pigments considered here are whites (lead white and calcium carbonate), yellows (ochres, orpiment, massicot, lead tin yellows), orange-reds (ochres, realgar, vermillion, litharge, red lead), blues (ultramarine, blue ochre, smalt, azurite, Egyptian blue), greens (green earths; malachite, verdigris and other Cu containing materials), brown (umbers) and black (carbon black).

Pigments as Naples yellow (Pb2Sb2O7/Pb3(SbO4)2), lead chromate yellows

(PbCrO4/PbCr1•xSxO4), Prussian blue (KFe[Fe(CN)6]·xH2O/Fe4[Fe(CN)6]3·xH2O), zinc white (ZnO), etc., although known to degrade, are not included in this review, as they were not available during the Middle Ages (Eastaugh et al. 2008). As it concerns organic colourants, although well known to discolour upon exposure to environmental conditions, including relative humidity, light and pollutants (see for example Whitmore and Cass 1989, Salmon and Cass 1993,

24 Williams et al. 1993, Saunders and Kirby 1994; and references therein), they were not considered here. In fact, they require a separate review, as the variety of compounds, including the development of synthetic colourants chemistry, and of application (with different mordants; as lakes in paintings; for textile dying, etc.) demands. Moreover, the detection and identification of such organic materials often require invasive and destructive methods (see Sub-chapter 1.5: sampling; separation techniques, mass spectrometries, surface-enhanced Raman spectroscopy (Casadio et al. 2010)). For the purpose of this review, mainly chemical and archaeometrical literature was considered, so that an explanation for the ongoing degradation processes of inorganic pigments could be provided as revealed by advanced analytical techniques. Finally, it seems worth to mention that the understanding, monitoring and characterization of each degradation process needs to be performed according to the chemical and physical reactions causing discolouration. Specific aspects need to be monitored in each case, like the replacement/substitution/migration of elements/ions, the changes happening regarding ion coordination and oxidation state of metals, the formation of salts, etc. Also, simulations in the laboratory (preparation of mock-ups, accelerated ageing, etc.) help in the study of pigment degradation, although the comparison with real case studies need to be performed with care, as the simplified experiment might not straightforwardly apply to a masterpiece created centuries ago. The following is articulated in sections corresponding to the main element present in the pigment, according to increasing atomic number. When possible, a general introduction to the sensitivity of the class of pigments is given, with more details for each single pigment. On the other hand, a colour-based overview of pigments and of their observed alterations, and the main factors involved in the process (climate, material selection, biological activity, analysis and conservation treatments) is given in APPENDIX A, Table A.1.

Carbon blacks (disordered C) are worldwide commonly used in arts and crafts throughout the centuries (Berrie 2007, Eastaugh et al. 2008, Coccato et al. 2015; paragraph 4.3.2). They are of natural or artificial origin, and the microscopical structure of the pigment is related to its origin and manufacture process (Winter 1983, Coccato et al. 2015). They are stable to light and humidity, but burn at high

25 temperatures. Their curing properties in oil are poor, often requiring the addition of siccatives (van Loon and Boon 2005, Berrie 2007). Evidence of degradation is observed upon exposure to oxidizing agents, and especially when impurities are present (e.g. presence of residual salts/uncarbonized moieties in carbonaceous pigments applied by using the fresco technique) (Marzetti and Scirpa 2000, van Loon and Boon 2005, Berrie 2007). Whitening is also reported for bone black (amorphous carbon with apatite

Ca3(PO4)2) in oil binding medium, possibly related to the photodegradation of the aromatic structure, probably catalysed by lead (present as a siccative, or pigment) (van Loon and Boon 2004). In another study, by the same authors, a variety of issues was identified related to the causes of discolouration of carbon blacks, such as the formation of lead carboxylates from the reaction of the siccative with the binder, or the alteration of lakes present in the mixture, or the degradation of the aromatic carbon compounds. The latter process is related to both the manufacturing process of the carbon-based pigment, and the presence of siccatives (van Loon and Boon 2005). Upon infrared laser irradiation, the colour of mock-ups prepared with organic and inorganic binders changed, showing evidence of darkening (Sansonetti and Realini 2000), while a 248 nm (UV) laser induced no changes (Bordalo et al. 2012).

The term lazurite refers to the blue mineral of the sodalite group

(Na8[Al6Si6O24]Sn), lapis lazuli to the rock from which it is extracted. Notwithstanding its enormous price related to its rarity and colour, natural ultramarine was extensively used throughout history, as a decorative stone (Roy 1993, Eastaugh et al. 2008), as a colourant for ceramics (Clark et al. 1997a, 1997b, Colomban 2003), and in paintings and manuscripts both in Asia and Europe starting from the 6th century (Clark and Gibbs 1998a), as well as in pre•Columbian cultures (Osticioli et al. 2009). In European contexts, this extremely valuable material was mined in present•day Afghanistan and traded via Venice (Roy 1993, Eastaugh et al. 2008), and it was reserved to specific iconographic elements of the composition, such as Christ and the Virgin’s blue cloaks (Coremans 1953, Plesters 1966, Roy 1993, Eastaugh et al. 2008). It was used both in tempera and oil, to produce either mixtures (with crimson or a white pigment for example) or pure layers, of different opacity according to the binder (e.g. oil based blue glazes painted over a cheaper blue layer (Coremans 1953, Eastaugh et al. 2008)). The high price and marvellous colour somehow promoted the development of adulterations, and finally the synthesis of artificial ultramarine in 1828 by Guimet, and in the same year, but with a different process, by Gmelin (Roy 1993, Berke 2007, Eastaugh et al. 2008).

26 Natural ultramarine is stable to light, including lasers (Sansonetti and Realini 2000, Raimondi et al. 2013), heat, acids and alkalis (Church 1915, Roy 1993, Del Federico et al. 2006). No oxalates were formed upon exposure of the pigment to oxalic acid (Zoppi et al. 2010). It was however observed that in presence of pollution (SO2/SO3), the pigment discolours only when liquid water (e.g. condensation) is present (Saunders 2000). The sulphur present in the crystalline matrix of lazurite does not affect pigments that are otherwise sensitive to hydrogen sulphide (H2S), such as lead white (Roy 1993). No alteration could be observed on decorative plasterworks in the Alhambra (Granada, Spain) (Dominguez-Vidal et al. 2012), neither on manuscripts (Clark and Gibbs 1998a). However, a greyish alteration of the paint surface (“ultramarine sickness”) can be sometimes detected, but it seems that many factors can cause discolouration of ultramarine paint layers, such as the oil degradation in presence of humidity (Roy 1993), or the discolouration of smalt in case of mixtures (Plesters 1969, Roy 1993). It is reported in literature that ultramarine sickness is related to an acidic attack of the pigment by pollutants, biological metabolites, or even acidity of the binder (Wyld et al. 1980, Leo 2000). This hypothesis seems to be confirmed by the fact that, in oil medium, when the basic pigment lead white was added to the mixture (to enhance curing, increase opacity, and adjust the shade), no visible alteration can be detected, the lead white somehow protecting the blue pigment (Wyld et al. 1980). The zeolitic structure of the mineral allows for ion exchanges (Roy 1993), and - for trapping volatile molecules, such as the S 3 chromophore (Nassau 1987), and

CO2 whose presence was recently related to natural ultramarine from Afghanistan (Miliani et al. 2008). Artificial ultramarine discolouration studies revealed a variation in the aluminium coordination, resulting in the opening of the cage and release of the chromophore (Del Federico et al. 2006, 2007, Cato et al. 2016). Such a cage-opening process can be initiated either by high temperatures, acids (Del Federico et al. 2006, 2007, Miliani et al. 2008) or alkalis (Del Federico et al. 2006). Moreover, as known for sulphur-saturated zeolites (Dudzik and Preston 1968), it appears that ultramarine can have a catalytic effect on the binder breakdown, and that such property can be related to a pigment pre-treatment (heating), which can explain the inconsistent appearance of degradation (Keune et al. 2016b).

Calcium containing pigments are of mineral, biogenic, fossil or artificial origin, mainly corresponding to groups of carbonates, sulphates, phosphates, or

27 fluorides (Eastaugh et al. 2008). A variety of Ca, CaMg carbonates, and sulphates was used as white pigment (chalk, bianco di San Giovanni: CaCO3; huntite:

CaMg3(CO3)4 and dolomite: CaMg(CO3)2; : CaSO4∙2H2O), as well as mixed with glue to prepare ground layers in northern and southern Europe, respectively (Gettens et al. 1974, Roy 1993, Heywood 2001, Ambers 2004, Eastaugh et al. 2008), and to prepare parchment for manuscripts (Deneckere et al. 2011). The calcination of bones gives a white pigment mainly containing apatite Ca3(PO4)2 (Eastaugh et al. 2008). Calcium compounds were as well used as extenders (for example with lead white, to achieve transparency effects), and for lakes, as chemical or physical supports (Roy 1993, Noble and Loon 2005). The purple mineral fluorite (CaF) was also used as a pigment, mainly in 15th-16th century central European artworks (Spring 2000, Eastaugh et al. 2008, Stege 2016).

Calcite (CaCO3) can be considered stable under normal circumstances. It decomposes to lime (CaO) and CO2 when heated, and it is dissolved by acids with release of CO2 and formation of the corresponding salt. SO2 pollution is responsible for the formation of gypsum (CaSO4∙2H2O), which is soluble and produces mechanical stress, as its volume is larger than that of the starting material (Gettens et al. 1974, Roy 1993). Gypsum is a common degradation product in wall paintings and manuscripts (Deneckere et al. 2011). Being alkaline, these pigments are not suitable for mixing with pigments such as verdigris (Thompson 1956), but they are stable to sulphide containing materials (pollutants or pigments) (Gettens et al. 1974). The lists of occurrences reported by (Gettens et al. 1974) and (Roy 1993) mainly refer to calcite-containing ground layers, although the use as a pigment is ascertained. Calcium carbonate pigments are not suitable for use in oil medium, as they don’t have hiding power (Gettens et al. 1974), but they are used in mixtures with other white pigments to adjust the shade and hue, as well as in wall paintings (Guineau 2005). In proteinaceous binder, interactions are reported between pigment and medium (Duce et al. 2013).

Huntite and dolomite, Mg-rich carbonates having formula CaMg3(CO3)4 and

CaMg(CO3)2, respectively, are also identified as white pigments in rock art paintings. As many other carbonatic materials, they are converted to Ca-oxalates

(weddellite CaC2O4∙2H2O and whewellite CaC2O4∙H2O) in presence of oxalic acid, with the excess magnesium being included in dolomite and magnesite MgCO3 (which can be considered as intermediate degradation products, sensitive to oxalic acid), in the very soluble magnesium oxalates MgC2O4∙xH2O, and finally lixiviated (Ford et al. 1994). The attack from polluted water can as well form

28 gypsum CaSO4∙2H2O and the extremely soluble magnesium sulphate MgSO4, and calcite. The adsorption of water negatively affects the stability of paint layers, as swelling occurs (Ford et al. 1994).

The use of a buon fresco technique implies the use of Ca(OH)2 as a binder, which recrystallizes into calcium carbonate, surrounding the pigments and binding them to the wall surface. The formed calcite is subjected to degradation processes involving acids, from biological activity or binders degradation (when details are added a secco) (Lepot et al. 2006, Zoppi et al. 2010). Moreover, carboxylic acids from organic binding media are expected (Boon and Ferreira 2006). However, degradation compounds were not always detected in samples (Mazzeo et al. 2008). On the other hand, Ca-carboxylates were successfully identified at the boundary between the ground layer and the paint in egg-tempera samples (Salvadó et al. 2009, Sotiropoulou et al. 2016). They were as well detected on artworks (in the laboratory and in situ), especially underneath the varnish and on top of the ground layers (Monico et al. 2013, Dominguez-Vidal et al. 2014), and on manuscript samples. There, a correlation between calcium oxalates and the loss of the proteinaceous signal in green areas was highlighted, so that the binder degradation is catalysed by Cu-ions, present in the green pigment, and oxalic acid is released (Melo et al. 2016, Sotiropoulou et al. 2016). Chalk is lightfast (Gettens et al. 1974), and gypsum does not show discolouration upon Nd:YAG laser irradiation (Sansonetti and Realini 2000). Ion beam analysis proved to be potentially harmful to chalk, as discolouration was observed (Zucchiatti and Agulló-Lopez 2012).

Ochres are natural products related to rock weathering, whose predominant phases are phyllosilicates (clays). The presence of less than 2 % chromophores, such as iron oxides and oxi-hydroxides, the red haematite (α-Fe2O3) and yellow goethite and lepidocrocite (α-FeOOH and γ-FeOOH, respectively), is sufficient to impart a deep colour to the rock (Berrie 2007, Eastaugh et al. 2008). Other minerals are commonly present and can help to ascertain provenance and paragenesis, such as quartz (SiO2), feldspars ((Na,K)AlSi3O8), and clays

(complex hydrosilicates), gypsum (CaSO4∙2H2O), ferrihydrite Fe10O14(OH)2, maghemite (γ-Fe2O3) and magnetite (Fe3O4), iron sulphates (jarosite group, 3+ KFe 3(SO4)2(OH)6), etc. (Berrie 2007, Eastaugh et al. 2008). If manganese oxides

(MnO, MnO2, Mn3O4) are present together with the iron oxides, the shade of the

29 ochre is darker and suitable for producing various shades of brown (Siennas and umbers), and for shadow rendering (Berrie 2007, Eastaugh et al. 2008). The shade of iron oxides and oxi-hydroxides can be adjusted by roasting processes, which produce darker shades (yellow to red-brown; red to purple-dark red) (Berrie 2007, Eastaugh et al. 2008). In fact, haematite, the anhydrous oxide, is the most stable, being the end product of heating goethite and lepidocrocite

(150-400 °C); and maghemite (γ-Fe2O3) can also be produced if organic matter is present (Berrie 2007). The transformation of magnetite (Fe3O4) to haematite

(α­Fe2O3) is also reported (de Faria et al. 1997, Shebanova and Lazor 2003). These materials are very stable and maintain their colour when ground to powders, moreover their occurrences are numerous: diffusion, stability, lightfastness and inertness make their use as pigments straightforward (Saunders and Kirby 2004, Berrie 2007, Eastaugh et al. 2008). In oil layers, Fe-ions promote photo-oxidative reactions, while Mn-ions act as a siccative (Berrie 2007). In proteinaceous binder, interactions are reported between pigment and medium (Duce et al. 2013). In rabbit skin glue, haematite shows a high degree of interaction with the binder, making the paint layer more stable than for other pigments (Ghezzi et al. 2015). The instability to light reported for some umbers (Brommelle 1964) seems to be attributable to the presence of tarry materials and other impurities (Berrie 2007). As for the red and yellow ochres, burning produces a darker shade (burnt umber, burnt Sienna) (Eastaugh et al. 2008). Ochres are stable to light, moisture, alkali, and diluted acids, and are inert in mixtures. Their stability to acids is testified upon oxalic acid exposure, which causes only the formation of Ca-oxalates, from the other minerals present in the ochre (Luxán and Dorrego 1999, Zoppi et al. 2010). They are, however, sensitive to high temperatures, such as fires (Berrie 2007, Sotiropoulou et al. 2008), or local heating effects related to the use of lasers for cleaning (Athanassiou et al. 2000, Zafiropulos et al. 2003, Bordalo et al. 2012) or for spectroscopical analysis (i.e. Raman spectroscopy, (de Faria et al. 1997, Walter et al. 2001, Shebanova and Lazor 2003, De Santis et al. 2007)). A UV (248 nm) laser showed darkening of yellow ochre and raw Sienna, as a result of dehydration, conversion to haematite and modification of the manganese phases present (Athanassiou et al. 2000), but low fluences of the same laser caused little modification of this ochre (Bordalo et al. 2012). High fluence of 355 nm, and 633 nm laser, caused the conversion of yellow ochre to haematite (De Santis et al. 2007, Raimondi et al. 2013). The effect of NIR laser irradiation (1064 nm) was as well investigated, showing significant discolouration of both yellow and red ochres when mixed with gypsum (no organic binder) (Sansonetti

30 and Realini 2000), and an increase of haematite content after irradiation of ochres (Zafiropulos et al. 2003). Moreover, some issues are encountered on wall paintings. In fact, coquimbite/paracoquimbite Fe2(SO4)3∙9(H2O) were identified in Pompeii, together with magnetite Fe3O4 and gypsum CaSO4∙2H2O, as a result of the degradation of the fresco paint layer due to SO2/SO3 pollution (Maguregui et al. 2010). On wall paintings in the Tournai Cathedral, on the other hand, anhydrous haematite was found to be converted to iron oxi-hydroxides due to the humid conditions, which resulted in a visible discolouration of some red areas (Lepot et al. 2006).

3+ The silicates glauconite ((K,Na)(Fe ,Al,Mg)2 (Si,Al)4O10(OH)2) and celadonite, 3+ 2+ (K[(Al,Fe ),(Fe ,Mg)] (AlSi3,Si4)O10(OH)2) give the colour to green earths, and can be found in outcrops all over the world (Feller 1987, Eastaugh et al. 2008, Coccato et al. 2016b). After crushing and grinding, the material is ready for use as a pigment, showing good stability and lightfastness in all media, although in oil the hiding power is relatively poor (Feller 1987, Eastaugh et al. 2008). Both these characteristics supported the wide use of such pigments. Green earths are soluble in both acids and alkalis, and they turn brown upon heating, as divalent iron is oxidised to its trivalent counterpart (Feller 1987). They were as well used as lake substrates (Feller 1987). Green earth oil layers exposed to humidity and heat showed discolouration, pinpointing differences based on the binder composition, and ageing conditions (Genty-Vincent et al. 2015).

∙

Vivianite is a hydrated iron phosphate. Its blue colour arises from intervalency charge transfer between Fe2+/Fe3+ (Nassau 1987, Eastaugh et al. 2008), as a consequence of oxidation of some of the Fe2+ in colourless vivianite (Noble and Loon 2005). Occurrences of vivianite deposits and of its use as pigment are listed in literature (Wallert et al. 1995, Richter 2004, Noble and Loon 2005, Eastaugh et al. 2008, van Loon 2008, Čermáková et al. 2014, Spring 2016). As a pigment, it only shows medium stability (Noble and Loon 2005, Spring and Keith 2009). The discolouration of this blue monoclinic phosphate to a greenish hue is attributed either to unbalanced Fe2+/Fe3+ ratio, or to its oxidation to green triclinic metavivianite Fe3(PO4)2(OH)2∙6H2O (Eastaugh et al. 2008, Bärbel 2009), and finally to yellowish brown amorphous santabarbaraite Fe3(PO4)2(OH)3∙5H2O (Frost et al. 2002c, Pratesi et al. 2003, Bärbel 2009,ermáková Č et al. 2014). Oxidation is expected to be faster in air, and to slow down once the pigment is embedded in a binder (Noble and Loon 2005). On top of these oxidation reactions, heat related

31 damages can be observed starting already at 70 °C, causing colour changes in both pure vivianite (Frost et al. 2003) and oil paint layers containing it (Čermáková et al. 2015). The degradation seems to preferentially affect larger grains (Čermáková et al. 2015).

∙

Smalt is a synthetic cobalt-doped potash glass. More details on smalt fabrication and (early) occurrences are given elsewhere (Mühlethaler and Thissen 1969, Roy 1993, Billinge et al. 1997, Altavilla and Ciliberto 2004, Santopadre and Verità 2006, Eastaugh et al. 2008). It was not only used as a pigment, but as an additive to improve oil curing as well (Mühlethaler and Thissen 1969, Plesters 1969, Noble and Loon 2005). Coarse grinding of the blue glass was required in order to obtain a satisfactory colour, which seems to have an impact on the degree of alteration (Plesters 1969, Boon et al. 2001). The discolouration of smalt can be already observed in 25-50 years, and it can be related to various phaenomena: on the one hand, the similarity of optical properties (refraction index) of the glass and of the oil, on the other the instability of potassium glass, Co- and K-ions being the two species considered as responsible (Plesters 1969, Roy 1993, Eastaugh et al. 2008, Robinet et al. 2011). In fact, smalt use is recommended in wall paintings (secco technique) and in aqueous media only (Mühlethaler and Thissen 1969). Environmental moisture proved harmful to oil paint layers, to potash glasses (Boon et al. 2001), and to wall paintings (Gil et al. 2011). The first theories on the cause of smalt discolouration in oil assume the role of Co2+ as a catalyst in the binder’s oxidation, with the formation of an organometallic compound at the grain boundary (Plesters 1969, Giovanoli and Mühlethaler 1970). Moreover, a change in cobalt coordination occurs and it is attributed to the oxidation of cobalt (Giovanoli and Mühlethaler 1970). Later studies (Boon et al. 2001, Daniilia and Minopoulou 2009, Cianchetta et al. 2012), however, demonstrated that the distribution of potassium in degraded particles of smalt is not homogeneous, while cobalt is confined to the particles, and that K-ions are leached out of the glass affecting its K/Co ratio, the pH, and the Co(II) coordination, which result in colour change. As a consequence of K+ leaching, the coordination of Co2+ changes from tetrahedral to octahedral (X-ray absorption spectroscopy studies (XAS) (Robinet et al. 2011, Cianchetta et al. 2012), vibrational spectroscopies and elemental analysis (Robinet et al. 2013)), causing the colour loss. Simultaneously, the glass network is modified as the Q3

32 component related to alkali content decreases, and hydration is observed (Robinet et al. 2013). The change in pH might additionally damage the glass network. The leached K+ ions interact with the aged oil, increasing its water sensitivity, forming soaps and causing blanching (Wyld et al. 1980, Boon et al. 2001, Noble and Loon 2005, Spring et al. 2005, Robinet et al. 2013). The paint composition might affect the discolouration, by providing chemical species to buffer the potassium leaching. If calcium is present in the glass network, the pigment appears to be less sensitive (Boon et al. 2001, Noble and Loon 2005, Spring et al. 2005, Robinet et al. 2011). When smalt is mixed with lead white in oil, lower degrees of alterations are observed, probably because lead soaps are favoured compared to the potassium ones, so that leaching of alkali is not as relevant as in pure smalt paint layers (Plesters 1969, Spring et al. 2005, Robinet et al. 2011, Pottasch et al. 2016). The discolouration of smalt and the rough surface of degraded particles significantly alter the appearance of oil paintings where it was used for the sky, or other parts intended to be bright blue (Plesters 1969, Boon et al. 2001, Dik et al. 2002, Spring et al. 2005, Franceschi et al. 2010, Cianchetta et al. 2012, Van De Voorde et al. 2014, Panighello et al. 2016). Smalt egg tempera samples proved sensitive to environmental conditions in museums, so that such a paint layer might be successfully used as a dosimeter to evaluate the air quality in terms of conservation issues (Odlyha et al. 2000a). In glue binder, the role of humidity and airborne pollutants in accelerating glassy pigments degradation was demonstrated, where leaching of both potassium and cobalt ions occurred. No evident effect of SO2 and NOx synergy was observed though (Altavilla and Ciliberto 2004). In fresco wall paintings, smalt is expected to deteriorate due to the very alkaline conditions, to the presence of liquid water (including condensation, capillary rise and infiltrations), to the small particle size increasing the surface reaction, and to the possible contamination by pollutants. Again, leaching of alkali is observed, and in some strongly degraded smalt particles showing cracks, cobalt and other divalent ions are leached as well, probably due to aggressive environmental conditions (humidity, basic pH) (Spring et al. 2005, Santopadre and Verità 2006, Gil et al. 2011). On top of ions lixiviation and weathering of the glass, examples of heat degraded smalt are reported in wall paintings affected by fire (Doménech•Carbó et al. 2012).

It was recently observed that historical copper-based pigments are not only limited to malachite, azurite, verdigris and copper resinate. In fact a variety of

33 salts (organic acids salts such as copper citrate (Eremin et al. 2008), silicates, phosphates, sulphates, chlorides, etc.) were as well used as pigments (Guineau 2005, Eastaugh et al. 2008, Coccato et al. 2016b, Bersani and Lottici 2016; paragraph 4.3.1), and should not be regarded anymore as degradation products only. Moreover, the situation is complicated by inconsistent nomenclature use in artistic literature (Eastaugh et al. 2008, Coccato et al. 2016b). It is well known that malachite and azurite are not stable in fresco, and that they tend to discolour in oil (Thompson 1956). Studies on metallic copper exposed to a museum-like environment demonstrated its sensitivity to organic acids, including from the binder, so that Cu(II) compounds are always formed (Robinet and Corbeil 2003, De Laet et al. 2013, Tétreault et al. 2013, Anselmi et al. 2016). More details on the reactivity of copper salts in acidic conditions are given for each pigment in the next sections. Finally, it is important to mention that copper pigments negatively affect cellulosic materials, as the release of Cu2+ is involved in the degradation of those supports (Roy 1993).

∙

Azurite (2CuCO3∙Cu(OH)2) was the most diffused blue pigment during Middle Ages, and before (Roy 1993, Eastaugh et al. 2008). It appears to be stable to light and atmosphere, and shows good performances both in oil and tempera mediums (Gettens and FitzHugh 1966, Odlyha et al. 2000b), although its poor hiding power in oil is reported in literature (Taft and Mayer 2001). Degradation of azurite in frescoes seems related to pH and grain size (Mattei et al. 2008, Gil et al. 2011).

Azurite degrades to green compounds: malachite (CuCO3∙Cu(OH)2) and paratacamite/atacamite (Cu2Cl(OH)3) are some examples (Gettens and Stout 1958, Gettens and FitzHugh 1966, Dei et al. 1998, Bärbel 2009, Odlyha et al. 2000b, Saunders and Kirby 2004, Berke 2007, Cavallo 2009, Lluveras et al. 2010). Humidity and chloride ions from various sources cause the formation of black copper oxides (CuO) and green chlorides (nantokite CuCl, paratacamite/atacamite or botallackite Cu2Cl(OH)3 (Gettens and Stout 1958, Bärbel 2009, Vandenabeele et al. 2005, Svarcová et al. 2009, Lluveras et al. 2010, Doménech-Carbó et al. 2012, Pereira•Pardo et al. 2016)). Azurite degrades to black tenorite CuO when exposed to heat in presence of alkali (Roy 1993, Taft and Mayer 2001, Vandenabeele et al. 2005, Mattei et al. 2008, Sotiropoulou et al. 2008, Kotulanová et al. 2009b, Gil et al. 2011, Doménech- Carbó et al. 2012, Špec et al. 2014), while cold alkaline conditions might not affect it (Gettens and FitzHugh 1966), or cause conversion to malachite (Svarcová et al.

2009), or the formation of tenorite via formation of copper hydroxide Cu(OH)2 (Wyld et al. 1980, Saunders and Kirby 2004, Mattei et al. 2008).

34 On the other hand, it is decomposed by acids, such as oxalic acid to form oxalates (CuC2O4∙nH2O, mooloite) (Mazzeo et al. 2008, Lluveras et al. 2010, Zoppi et al. 2010). It has been reported that the combination of oxalic acid and chlorides in wall paintings results in Cu-hydroxychlorides Cu2Cl(OH)3 and Ca•oxalates (Dominguez-Vidal et al. 2014); no Cu-oxalates were observed in azurite paint layers (Monico et al. 2013). On the other hand, oxalates attributable to the biodegradation of an organic binder were found in both a gypsum preparation (weddellite CaC2O4∙2H2O and whewellite CaC2O4∙H2O) and in the overlying azurite-containing paint layer (Lluveras et al. 2010). Cu-carboxylates are rarely formed, as lead white, often mixed with azurite, is more reactive to acids; moreover, as azurite is preferred in a tempera medium, carboxylic acids are less present (Salvadó et al. 2009). In proteinaceous binder, other interactions are reported (Duce et al. 2013).

Verdigris xCu(CH3COO2)∙yCu(OH)2∙zH2O might be formed when high relative humidity interacts with an azurite containing paint layer, degrading the oil binder (Saunders and Kirby 2004). Also, the yellowing of the binder/varnish can alter the blue shade to a greenish one in oil paintings (Gettens and FitzHugh 1966). Small particles are reported to have less colouring power and to be more sensitive to chemical interaction (Wyld et al. 1980), and to dissolve in organic binding media (Gettens and FitzHugh 1966). In egg tempera, azurite is not affected by light, while thermal ageing, high relative humidity, and pollutants affect the paint film (oxidative processes) (Odlyha et al. 2000b). Also, the copper interacts with the proteinaceous moieties to form metalloproteins (Odlyha et al. 2000b).

Azurite turns bluish-black when exposed to H2S vapours (Smith and Clark 2002), especially in frescoes (Gettens and FitzHugh 1966), as covellite (CuS) is formed. The effect of laser irradiation on azurite is the formation of black CuO, which depends on the particle size and therefore on the temperature increase (Odlyha et al. 2000b, Frost et al. 2002a, Mattei et al. 2008). Selective biological activity is observed towards lead pigments (Smith and Clark 2002).

∙

Malachite (CuCO3∙Cu(OH)2) is more stable than azurite (2CuCO3∙Cu(OH)2) and verdigris (xCu(CH3COO2)∙yCu(OH)2∙zH2O), therefore showing less or slower reactivity towards many factors (Kotulanová et al. 2009b, Svarcová et al. 2009). It is known to be permanent in all binding media, lightfast and alkali proof. Its deep colour is obtained by coarse grinding, and as a consequence of having relatively low , it shows better performances in tempera than in oil (Thompson 1956, Gettens and FitzHugh 1974).

35 Due to its chemical composition, malachite is subject to interactions with acids, bases, humidity, temperature and circulating ions. In presence of humidity, malachite stains can be observed, which are actually caused by proteinaceous binders degradation (Saunders and Kirby 2004). Malachite particles in the form of spherulites were identified in brownish tempera layers, the discolouration being caused by the pigment-binder reaction (Bomford and Kirby 1978). Moreover, ions such as Cl- present in the mortar, sand or in the bricks, can react with the basic carbonate to form copper hydroxychlorides ((Cu2Cl(OH)3) atacamite, clinoatacamite, paratacamite, botallackite) (Gettens and FitzHugh 1974, Scott et al. 2003, Vandenabeele et al. 2005, Castro et al. 2008, Bärbel 2009, Kotulanová et al. 2009b, Svarcová et al. 2009) and the copper chloride nantokite CuCl (Doménech-Carbó et al. 2012). Sulphate ions are also likely to be present in wall paintings (Bärbel 2009), especially from the degradation of calcite to gypsum

(SO2/SO3 pollution), or from gypsum preparation layers (Lepot et al. 2006).

Copper basic sulphates (CuSO4∙yCu(OH)2∙zH2O: brochantite, langite, , antlerite) are formed according to pH, humidity and temperature (Pérez-Alonso et al. 2006, Castro et al. 2008, Bärbel 2009, Svarcová et al. 2009). Copper chlorides and sulphates can further react to form other salts (Castro et al. 2008). In tempera medium, it seems that the carbonate function is affected, more than in fatty media, and more when no varnish is present to protect the paint layer from ageing (Špec et al. 2014). In oil or oil/resin medium, it can be converted to copper resinates (Gettens and FitzHugh 1974) and other organometallic compounds (Wyld and Plesters 1977, Braham et al. 1978). Copper carboxylates, originating from the binding media interacting with the pigment’s cation, were observed for both tempera and oil binders (Ioakimoglou et al. 1999, Mazzeo et al. 2008, Gunn et al. 2013). Acidic conditions are commonly found in, and surrounding, polychrome objects, as a result of biological activity on the object (Lepot et al. 2006) or of degradation of the organic binders (e.g. oxalic acid (Pérez-Alonso et al. 2006, Nevin et al. 2008)). Diluted acids decompose malachite (for example acetic, hydrochloric, nitric acid (Gettens and FitzHugh 1974)), causing the release of Cu2+ ions, and the formation of the most stable phase according to pH and other present ions. The copper oxalate mooloite was studied (Zoppi et al. 2010) and identified in various works of art (Lepot et al. 2006, Nevin et al. 2008). The release of acetic and formic acid by the organic building materials of the display cases has been ascertained (Padfield et al. 1982, Laine 2005, Gibson and Watt 2010). Bluish Cu-acetates, corresponding to the well-known pigment verdigris, were identified on pure pigment powders exposed to acetic acid atmospheres (De Laet et al. 2013).

36 H2S vapours cause darkening by formation of the bluish sulphide covellite CuS, which is formed on the pure malachite pigment (Gettens and FitzHugh 1974, Weeks 1998), although in paint layers this has not yet been identified (Gettens and FitzHugh 1974, Smith and Clark 2002). On the other hand, it seems that Cu•ions act as a biocide towards microorganisms that produce sulphuric acid, leading to a selective H2S attack on non-copper pigments (Smith and Clark 2002). In fresco wall paintings, malachite can discolour due to the alkaline pH of the lime binders, especially when the particle size is small. The exposure to high temperature (fires) can lead to the formation of black copper oxides (Weeks 1998, Vandenabeele et al. 2005, Eastaugh et al. 2008, Gil et al. 2011, Doménech•Carbó et al. 2012, Valadas et al. 2013), especially in frescoes due to alkaline conditions (Gettens and FitzHugh 1974). On exposure to laser light, pure malachite darkens (Weeks 1998), as a consequence of reduction of Cu2+ to form dark cuprite Cu2O and black tenorite CuO (Chappé et al. 2003, Oujja et al. 2013). The same reactions take place when heating malachite above ca. 360 °C (Frost et al. 2002a).

∙ ∙

Verdigris is currently used to indicate a variety of blue-green compounds resulting from the exposure of copper to various acidic media (vinegar, for example) (Chaplin et al. 2006, Eastaugh et al. 2008). The instability of verdigris is well known (Thompson 1956). Improved performances seem related to oil, oil/resin rich paint layers, or to the selection of the neutral respect to the basic form (less reactive towards the acidity of the oil), or by adding yellow glazes on top of it (Woudhuysen-Keller 1995, Eastaugh et al. 2008). Verdigris is less stable than malachite and azurite to salt solutions, forming Cu•salts when the appropriate anions are present, such as nitrates, sulphates, chlorides (Kotulanová et al. 2009b, Svarcová et al. 2009). It promotes oil curing and it retards its oxidation (Roy 1993). Copper acetates and resinates catalyse the oxidation process of oil films and influence the curing of the oil (Ioakimoglou et al. 1999, Boyatzis and Ioakimoglou 2002, Cartechini et al. 2008, Gunn et al. 2013). Verdigris (neutral and basic), as well as the copper carbonate pigments azurite and malachite, shows the tendency to react with the oil binding media to form copper soaps (Wyld et al. 1980, Luxán and Dorrego 1999, Dawson 2007), to brown and darken, as Cu+ is formed by ambient light absorption (photoreduction), and oxygen promotes the formation of brown peroxide species (Santoro et al. 2014). Cu-salts of organic acidic compounds are formed in verdigris-containing paint layers (or at the interface with the varnish), as fatty and resin acids can extract copper ions from the verdigris pigment (Cartechini et al. 2008, Gunn et al. 2013). It seems important to mention that such reactions are equilibria, and therefore the

37 addition of overpaint, or the removal of paint or varnish layers during restoration, might affect the chemical stability of the aged layer (Gunn et al. 2013). In tempera, verdigris seems less prone to discolouration, but still reactions of Cu-extraction can take place (Gunn et al. 2013, Santoro et al. 2014). Copper acetates are very soluble in acidic conditions, and copper oxalates are formed in presence of oxalic acid from various sources (Zoppi et al. 2010). In alkaline conditions, blue copper hydroxides are formed (Roy 1993). When exposed to

H2S, the pure pigment turns dark, as covellite is formed; however, this compound has not been identified yet in paint layers (Roy 1993, Smith and Clark 2002). Verdigris should not be mixed with orpiment or lead white, as darkening occurs (Thompson 1956). Verdigris is not sensitive to lasers (Pouli and Emmony 2000, Pouli et al. 2001, Chappé et al. 2003, Oujja et al. 2013), but it darkens during particle-induced X-ray emission (PIXE) analysis (Calligaro et al. 2015). It is involved in cellulosic materials degradation, especially for critic conditions of illumination and SO2 concentrations (Roy 1993).

Copper resinate was used as a glaze, prepared by mixing Cu-acetates (verdigris) with oil and/or resins. In this process, some of the acetates remain unreacted (Cartechini et al. 2008). When mixed with oil, the coordination of the copper ion changes (Altavilla and Ciliberto 2006, Cartechini et al. 2008). Copper carboxylates are formed from the oil fraction of the binder, while metalloproteins may appear if egg tempera was used (Cartechini et al. 2008). It darkens with light exposure (Braham et al. 1978, Franceschi et al. 2010) and in alkaline conditions (Braham et al. 1978). Also, being an artificial pigment, its production process might affect its stability, if reactive moieties are not carefully removed (Franceschi et al. 2010). The lipidic fraction of the paint layer undergoes photo-oxidation, with Cu-ions acting as a catalyst: local molecular arrangement changes are likely to be involved in discoloration (Roy 1993, Cartechini et al. 2008). Upon prolonged exposure to X•rays, Cu2+ is reduced to Cu+ (Altavilla and Ciliberto 2006).

The artificial pigment Egyptian blue, also known as Pompeian blue (a silica-rich glass, whose colour is related to the presence of the mineral cuprorivaite,

CaCuSi4O10 (Pagès-Camagna et al. 1999, Bianchetti et al. 2000)) is a very stable pigment (to acids, alkali, light) (Berke 2007). It is related to Egyptian green, whose colour is due to the presence of Cu-doped (max 2 %) parawollastonite (CaSiO3),

38 which does not show evident degradation issues (it transforms into the polymorph pseudowollastonite above 1150 °C.) (Pagès-Camagna and Colinart 2003). Egyptian blue requires coarse grinding to maintain its colour, showing poor hiding power (Daniels et al. 2004). Some discolouration can anyway be observed on Egyptian blue, towards greenish or black compounds. It is reported to degrade to a green coloured material as Cu2+ ions are leached and react with Cl- ions from the glass itself, or from circulating solutions to form carbonates and chlorides (Schiegl et al. 1989, Lau et al. 2008, Perez-Rodriguez et al. 2015). Degraded Egyptian blue was found to contain chlorides (atacamite and eriochalcite CuCl2∙2H2O) (Lau et al. 2008). Moreover, the yellowing/darkening of the organic materials (varnishes, gums) can alter the appearance of this blue pigment, and pigment-binder interactions could promote selective darkening, thanks to the release of copper ions from the pigment itself, possibly in relation to pH, impurities and gum type (Daniels et al. 2004). No evidence of black tenorite CuO formation was found (Davies 2001, Daniels et al. 2004).

The polymorphs of copper hydroxychlorides (Cu2Cl(OH)3) pose relevant nomenclature issues, especially as regards paratacamite which is often used as a synonym for clinoatacamite, even though the two minerals have different formulas

((Cu,M)2Cl(OH)3) and Cu2Cl(OH)3, respectively, where M = Zn, Ni) (Eastaugh et al. 2008, Bertolotti et al. 2012). Copper chlorides were identified as pigments in objects from different cultures (Gettens and Stout 1958, Billinge et al. 1997, Eastaugh et al. 2008, Chaplin et al. 2010, Naumova and Pisareva 2013), and recently atacamite was found in colonial art (Tomasini et al. 2012), in China (Yong 2012) and in Alhambra’s plasterwork (Dominguez-Vidal et al. 2014). As a pigment, it can be natural or synthetic, and the particles’ morphology and composition can clarify its origin (Yong 2012, Naumova and Pisareva 2013). It is expected to form as a degradation product of Cu-containing pigments, or artefacts such as bronzes, in presence of humidity and chloride anions (Dei et al. 1998, Pérez-Rodrı́guez et al. 1998, Frost et al. 2002b, Vandenabeele et al. 2005, Lau et al. 2008, Svarcová et al. 2009, Bertolotti et al. 2012). It is however not stable in acidic conditions (oxalic acid, for example), which promotes the formation of mooloite (CuC2O4∙nH2O) (Castro et al. 2008). Also, among the polymorphs, different stabilities are observed, botallackite being the first formed and more unstable, while paratacamite seems the favoured species at ambient temperature (Pollard et al. 1989, Bärbel 2009). The polymorph paratacamite has so far been identified as degradation product on other Cu- containing pigments (Saunders and Kirby 2004, Pérez-Alonso et al. 2006, Cavallo 2009, Kotulanová et al. 2009b, Svarcová et al. 2009).

39 However, an interesting example is reported in literature: after the 1966 flood in Florence, green paratacamite was formed on azurite-containing wall paintings

(Dei et al. 1998). To treat sulphated walls, ammonium carbonate ((NH4)2CO3) and barium hydroxide (Ba(OH)2) were used. The green paratacamite alterations turned blue after the treatment, but not permanently. In fact, in the alkaline conditions given by the treatment, paratacamite is dissolved by the ammonium carbonate, and blue Cu(OH)2 formed. This hydroxide is not stable and it degrades either to black CuO or to green hydroxychlorides. This latter reaction is more probable due to the continuous ingress of Cl- ions via capillary rise in walls, thus generating more green products, making this treatment not suitable in presence of degraded azurite (Dei et al. 1998).

∙ ∙

Green copper sulphates are commonly found as degradation products on copper artefacts exposed to polluted environments (Martens et al. 2003, Hayez et al. 2004, Chiavari et al. 2007). Brochantite was identified as a pigment as well (Eastaugh et al. 2008, Valadas et al. 2015), but it can transform into the more stable polymorphs, antlerite, langite or posnjakite, according to the given conditions of relative humidity, inorganic pollutants (SO2/SO3) and biological activity (affecting the pH and the type of acids) (Pérez-Alonso et al. 2006, Castro et al. 2007, 2008). Posnjakite was also identified as a pigment (Naumova et al. 1990, Naumova and Pisareva 2013). Brochantite, as well as posnjakite and antlerite are also intermediate reaction products between copper carbonates and copper oxalates, and are therefore sensitive to oxalic acid (Castro et al. 2008).

Geologically, orpiment (As2S3) and realgar (As4S4) occur together, and are often associated with antimonates and other sulphides (Church 1915, Eastaugh et al. 2008). Orpiment and realgar were highly appreciated especially in Egypt and China for their rich yellow and red-orange shades (Schafer 1955, Corbeil and Helwig 1995, Burgio and Clark 2000, Rosalie David et al. 2001, Daniels and Leach 2004, Eastaugh et al. 2008), even though the pigments were considered “unpleasant” by many artists, and not recommended to use in combination with copper and lead based pigments, such as verdigris and lead white (Church 1915, Schafer 1955, Eastaugh et al. 2008). Orpiment and realgar exist as minerals, as well as synthetic pigments (Grundmann et al. 2009), and of the two arsenic sulphide pigments, the first one is the most stable. Realgar, being unstable, was less often reported in works of

40 art (FitzHugh 1997, Trentelman et al. 1996, Eastaugh et al. 2008, Nastova et al.

2012, 2013). It has a polymorphic photodegradation product, pararealgar As4S4. Permanence of arsenic sulphide pigments is problematic, especially on exposure to the green region of the visible spectrum (Daniels and Leach 2004, Keune et al. 2016a). Moreover, smaller particles are more readily degraded (Daniels and Leach 2004). The sensitivity to green light has to be taken into account when lasers need to be used, such as during Raman spectroscopic analyses (Muniz-Miranda et al. 1996, Trentelman et al. 1996, Correia et al. 2007). Arsenates (As(V)) are formed from orpiment, realgar and emerald green degradation: these ions are water soluble and migrate throughout the whole painting, accumulating at interfaces between layers, around Fe/Mn rich particles, and according to the local pH conditions in the paint layer. Due to this water-based transport, appropriate cleaning solvents must be selected, and relative humidity controlled (Keune et al. 2015, 2016a). It seems that polysaccharidic media, or egg yolk, negatively affect the stability of the arsenic sulphide pigments (Vermeulen et al. 2016). It needs to be pointed out that pararealgar was recognized as a mineral in 1980 (Corbeil and Helwig 1995), and that only recently the mechanisms of arsenic sulphides degradation was investigated (Daniels and Leach 2004, Keune et al. 2015). Care needs to be taken when considering published data, according to the year of publication and used techniques (compare for example (Corbeil and Helwig 1995) with (Daniels and Leach 2004)).

Permanence of orpiment to light was known to be poor, although some intact paint layers could be found on some illuminated manuscripts (FitzHugh 1997, Nastova et al. 2012, 2013). Coarser grain size seems to retard the colour change (Church 1915).

In water, upon heating, and in oxidizing conditions (ozone, NOx, for example) it decomposes to arsenic trioxide (As2O3), and hydrogen sulphide (H2S) is formed as well in presence of humidity (FitzHugh 1997, Saunders 2000). Relative humidity alone seems to have no effect, while degradation is observed when both humidity and light are present (Saunders and Kirby 2004). Light exposure causes discolouration (Church 1915, Scott et al. 2003, Correia et al. 2007, Cutts et al. 2010, Hradil et al. 2014). Orpiment darkens upon heating, it burns in air producing white arsenolite (As2O3) and volatile SO2, and it decomposes in water (the artificial product having smaller particles dissolves faster). It is soluble in many inorganic acids and in alkaline conditions as well, which make the pigment unsuitable for use on wet plaster (FitzHugh 1997, Kotulanová et al. 2009b). It is reported to be unstable in oil (Schafer 1955). When mixed with

41 lead and copper pigments, darkening occurs, for example the case of lead white is reported in literature (Church 1915, Thompson 1956, FitzHugh 1997).

Realgar, exposed to light, degrades (Church 1915). However, the formed yellow compound is not orpiment, as believed until 1980, but pararealgar (Roberts et al. 1980, Corbeil and Helwig 1995). Exposed to (green) light, both high- and low•temperature realgar transform into brittle, bright yellow pararealgar (Muniz•Miranda et al. 1996, Trentelman et al. 1996, Bersani and Lottici 2016), and finally to arsenic trioxide (As2O3) (Street and Munir 1970, Bersani and Lottici 2016, Keune et al. 2016a). The phaenomenon is complex and involves intermediate phases such as χ-realgar. Pararealgar is also to be considered an equilibrium phase (Douglass et al. 1992, Trentelman et al. 1996, Grundmann et al. 2009). Heating causes darkening of the colour of realgar, χ-realgar, as well as of pararealgar, which is not permanent: in fact the high-temperature polymorph converts to the low-temperature realgar with time (Douglass et al. 1992). As orpiment, it burns in air and shows similar (FitzHugh 1997). The role of the binder on realgar’s stability is not completely understood, yet it could protect the pigment from discolouration (Trentelman et al. 1996). Seen that the photodegradation of realgar yields a yellow material, which was long thought to be orpiment, but was finally identified as pararealgar (Corbeil and Helwig 1995), one might question the identification of orpiment in museum objects (Trentelman et al. 1996), as well as consider the possibility of pararealgar as a pigment (Clark and Gibbs 1997, 1998b, Vandenabeele et al. 2000, Rosalie David et al. 2001, Nastova et al. 2013, Bersani and Lottici 2016).

The deep red coloured cinnabar (HgS), as well as its synthetic counterpart vermillion (HgS), produced either by wet or dry processes (Feller 1967, Gettens et al. 1972), were extensively used (Eastaugh et al. 2008). Roman sources (Pliny the Elder: Naturalis Historia), however, seemed to use the term minium to identify this material, giving room to misinterpretations and nomenclature issues (Eastaugh et al. 2008). It was already known to Romans (Pliny the Elder: Naturalis Historia, Rowland and Howe 2001) that mercury sulphide darkens when exposed to light, although not systematically (Feller 1967, Gettens et al. 1972, McCormack 2000, Spring

42 and Grout 2002, Eastaugh et al. 2008, Nöller 2015). Moreover, HgS was not recommended for use in fresco, as humidity and pH affect its stability (Kotulanová et al. 2009b, Doménech-Carbó et al. 2012, Gutman et al. 2014), but still it was used on wall paintings (Gettens et al. 1972, Cotte et al. 2006, 2008, Maguregui et al. 2010, Gil et al. 2015). Paint layers of pure vermillion are very vulnerable (Spring and Grout 2002), while no darkening is observed when it is mixed with lead white (Gettens et al. 1972, Roy 1993, Burgio et al. 1999, Bordalo et al. 2012). An increased resistance to laser radiation is also observed for other mixtures, such as vermillion with chrome yellow, madder, Prussian blue, and bone black, although the single pigments’ behaviour is hardly predictable (Bordalo et al. 2012). Red lead, often mixed with vermillion to improve the curing of the oil binder, seems to have a protective effect on HgS (Roy 1993, Spring and Grout 2002). A series of reactions of vermillion with other pigments, different binders and substrates are reported in literature (Duce et al. 2012, Nöller 2015, Ghezzi et al. 2015). The presence of a varnish layer, and of organic binders, moreover, seems to protect the paint layer, by absorbing the incident light (more effectively when the varnish is thick and/or yellowed, and with different efficiency according to the composition, i.e. UV light is strongly absorbed) and by physically protecting the pigment grains from contact with chlorine-containing materials (disinfectants, dirt, etc.) (Spring and Grout 2002, Oujja et al. 2010). Sulphation of the red paint layers in Pompeian wall paintings was observed, and seems to be connected with the Cl-induced degradation of vermillion (Cotte et al. 2006, Bersani and Lottici 2016). The blackening of vermillion was often attributed to the formation of a black compound, supposedly metacinnabar (β­HgS) (Feller 1967), although no positive identification could be obtained (Nöller 2015). Binder type and pigment/binder ratio affect the pigment’s stability, and glazes seem to have a role in preserving the red hue of vermillion, by filtering some of the incident light and protecting the pigment from external sources of chlorine (Spring and Grout 2002). The same metacinnabar is hypothesized in wall paintings (Weeks 1998, Vandenabeele et al. 2005, Cotte et al. 2006), but only identified on laser irradiated vermillion (Zafiropulos et al. 2003, Chappé et al. 2003). Pure vermillion is sensitive to lasers of various wavelength and irradiation, so that the original structure is lost (Chappé et al. 2003, Oujja et al. 2010, 2013). Black β-HgS was identified by X-ray diffraction (XRD) on IR-laser damaged red α­HgS (Zafiropulos et al. 2003), and by

X-ray photoelectron spectroscopy (XPS) (Oujja et al. 2013). Dark Hg2S was detected as well, by X-ray photoelectron spectroscopy (XPS) by (Pouli et al. 0 2+ 2001). XRD and XPS studies suggest a photoreduction of HgS to Hg /Hg2 , instead of polymorph transformation (Pouli et al. 2001). Finally, the

43 photodegradation of vermillion was explained in terms of formation of metallic mercury and HgCl2, due to the halogen impurities present in the pigment, identified thanks to secondary-ion mass-spectrometry (SIMS) analysis (Keune and Boon 2005), and to X-ray spectroscopic analysis (Hogan and Pieve 2015). The role of Cl-ions, present as an impurity in the pigment, coming from marine aerosol, from the used binder or from pollution, in the degradation of vermillion has been ascertained thanks to the identification of chlorine species (McCormack 2000, Spring and Grout 2002, Keune and Boon 2005, Cotte et al. 2006, 2008,

Hogan and Pieve 2015, Radepont et al. 2015). Calomel (HgCl)2 was identified by portable and micro-Raman spectroscopy, while HgCl2 and metacinnabar were not (Spring and Grout 2002, Maguregui et al. 2010, Dominguez-Vidal et al. 2012, 2014). The photosensitivity of vermillion depends on the one hand on the presence of halogen impurities (Spring and Grout 2002, Keune and Boon 2005, Cotte et al. 2006), and on the other on the particles’ size and relative humidity (Saunders and Kirby 2004). The cause of the discolouration is, however, still under debate (elemental mercury or dark Hg-S-Cl species, as well as grey mercury chlorides) (Spring and Grout 2002, Keune and Boon 2005, Maguregui et al. 2010, Dominguez-Vidal et al. 2012). In tempera, vermillion is only slightly sensitive to 248 nm laser pulses, while in oil medium it shows the lowest discolouration threshold for that laser wavelength, compared to a selection of pigments (Gettens et al. 1972, Spring and Grout 2002, Castillejo et al. 2003, Bordalo et al. 2012). It is also reported that in oil medium the darkening is worse than in watercolour (Feller 1967). In this latter case, vermillion is reported to be sensitive to light, with oxygen and humidity accelerating the darkening (Brommelle 1964). No mercury oxalates were ever identified on vermillion paint layers (Salvadó et al. 2009).

Lead pigments are not suitable for use in fresco, but they were still used due to their good hiding properties and to the low prices compared to other pigments (Cennini: Il libro dell’arte, Petushkova and Lyalikova 1986, Giovannoni et al. 1990, Koller et al. 1990, Kotulanová et al. 2009b, Gutman et al. 2014). The degradation processes of the various Pb pigments yield a range of compounds which cause discolouration, strongly affecting the readability of the polychromy (Gutman et al. 2014). Lead pigments on ceramic artefacts are sensitive to sulphur containing pollutants, to acidic solutions (rain; CO2; microbial activity), to light and air: anglesite PbSO4, PbCO3, hydrocerussite 2PbCO3∙Pb(OH)2 and lead sulphide PbS are

44 formed (Pérez-Rodrı́guez et al. 1998). Anglesite, attributed to lead pigment degradation was identified on wall paintings as well (Barone et al. 2016). To laser irradiation, reduction of the lead compounds (lead white, massicot, red lead) occurs on the surface, so that the dark colour is attributed to formation of metallic lead, or Pb2O (Pouli et al. 2001). The subsequent oxidation of lead explains the reversibility of such a colour change (Pouli et al. 2001). The temperature increase related to the laser irradiation causes the formation of massicot on both minium and lead white (Pouli et al. 2001). Care needs to be taken when a laser source is used for analytical purposes, such as during Raman spectroscopic investigations, as the lead pigments and their degradation products, such as PbO2 and PbS, might be altered, affecting the interpretation of the degraded areas (Batonneau et al. 2000, Burgio et al. 2001).

All lead pigments turned black after exposure to H2S vapours (formation of , PbS), with increasing reactivity for basic moieties, such as Pb(OH)2 units in lead white (Smith and Clark 2002). It is reported that biological colonization on wall paintings can selectively affect lead(II) compounds, causing brown discolourations identified as PbO2 or PbS (Petushkova and Lyalikova 1986, Rosado et al. 2015), while copper pigments act as biocides (Koller et al. 1990, Smith and Clark 2002). Lead dioxide formation is favoured in humid alkaline conditions, such as in frescoes, when peroxides are produced, either by microorganisms or by photo•oxidation of organic materials (Giovannoni et al. 1990). When exposed to salt solutions, and light, the most stable phase for all the lead pigments is cerussite (Kotulanová et al. 2009b, 2009a, Gutman et al. 2014). In oil, lead compounds were not only used as pigments, but as siccatives as well (Tumosa and Mecklenburg 2005, Cotte et al. 2007). Moreover, lead soaps are formed in, or on the surface of, oil paint layers containing lead-based pigments (lead white, lead tin yellow type I, litharge, red lead) and are investigated as they affect the appearance of paintings (Plater et al. 2003, Higgitt et al. 2003, Robinet and Corbeil 2003, Keune and Boon 2004, 2007, Tumosa and Mecklenburg 2005, Erhardt et al. 2005, Boon and Ferreira 2006, Cotte et al. 2007, Dawson 2007, Mazzeo et al. 2008, Keune et al. 2011, Catalano et al. 2014, Vanmeert et al. 2015). Lead oxalates were observed on lead white treated with oxalic acid (Zoppi et al. 2010), and on real samples containing red lead and lead tin yellow (Salvadó et al. 2009). Plumbonacrite

3[2PbCO3∙Pb(OH)2]∙PbO was detected as well (Salvadó et al. 2009).

∙

The term “lead white” refers to a variety of white lead-based materials, of which the most common, and most important from the artistic point of view, is the basic

45 lead carbonate corresponding to the mineral hydrocerussite (2PbCO3∙Pb(OH)2) (Gettens et al. 1967, Eastaugh et al. 2008), possibly including cerussite as well, according to the fabrication process (Gettens et al. 1967, Roy 1993, Eastaugh et al. 2008). Considering it is a carbonate, it is relatively stable, although soluble in acids (Gettens et al. 1967, Roy 1993, Franceschi et al. 2010). Also, it shows catalytic effects (Cohen et al. 2000). Upon heating, massicot (orthorhombic PbO) is formed, then litharge

(tetragonal PbO) and finally red lead (Pb3O4) (Gettens et al. 1967). It was used in every medium. Black discolouration is observed on frescoes and watercolours (Roy 1993), and it is reported for manuscripts as well (Clark 2002, Melo et al. 2016).

The blackening is attributed to the formation of either PbO2, as a result of exposure to oxidizing agents or from microbiological activity, or PbS (Petushkova and Lyalikova 1986, Giovannoni et al. 1990, Koller et al. 1990, Roy 1993, Saunders 2000, Andalo et al. 2001, Franceschi et al. 2010, Gutman et al. 2014). The role of sulphur containing pigments on the blackening of lead white is not fully understood (Gettens et al. 1967, Clark and Gibbs 1998b), while atmospheric and biological sources of H2S strongly affect watercolours (Roy 1993, Clark and Gibbs 1998b, Clark 2002) and manuscripts (Clark and Gibbs 1997, 1998b, Burgio et al. 1999, Melo et al. 2016). If PbS is formed, a reconversion treatment (oxidation) can be applied to restore the white shades of degraded polichromy. However, the formed lead sulphate

(PbSO4) is not sufficiently stable to make the treatment recommended (Koller et al. 1990, Weeks 1998). Humidity seems to be involved in many degradation processes, especially in synergy with atmospheric pollutants such as SO2 and NOx (Pérez-Rodrı́guez et al. 1998, Saunders 2000, Saunders and Kirby 2004). The presence of sulphuric pollutants is responsible for the formation of lead sulphide, lead sulphates, and cerussite/hydrocerussite (Pérez-Rodrı́guez et al. 1998, Smith and Clark 2002). The sensitivity of lead white to laser light was studied extensively, especially with respect to laser cleaning and laser-based analysis of works of art, but the alteration process is not fully understood, the binding medium probably influencing the results (Burgio et al. 2001, Pouli et al. 2001, Smith et al. 2001, Cooper et al. 2002, Castillejo et al. 2003, Zafiropulos et al. 2003, Chappé et al. 2003). Analytical evidence of formation of metallic lead and lead(II) oxides upon IR laser irradiation is provided in literature (Cooper et al. 2002, Zafiropulos et al. 2003). A visible laser (632 nm) caused the appearance of a massicot-like alteration on lead white, possibly via a plattnerite (PbO2) intermediate, which is related to impurities in the lead white (De Santis et al. 2007). Low fluence UV•laser does not affect paint layers prepared with different binders (Raimondi et al. 2015).

46 Moreover, the exposure of the PbO degradation to environmental conditions is likely to yield a carbonate-hydroxide lead salt, making the degradation reversible to some extent, especially in absence of a binder (Cooper et al. 2002, Chappé et al. 2003, Bordalo et al. 2012). The influence of ion-beam-based analysis on polychrome objects can negatively affect lead white paint layers, as hydrocerussite is sensitive to particle•induced X-ray emission (PIXE) analyses (Calligaro et al. 2015). On wall paintings, the combined presence of humidity and salts can cause the formation of degradation products, sometimes coloured as in the case of NaPb2(CO3)2OH and PbO polymorphs, with cerussite being the most stable of them all (Kotulanová et al. 2009b, 2009a). Lead white tempera and oil layers show the formation of lead soaps, which negatively affect the appearance of the paintings (Luxán and Dorrego 1999, Mazzeo et al. 2008, Keune et al. 2011). Lead white tempera applied on fresco paintings yields PbO2, as the reaction takes place in alkaline environment (Giovannoni et al. 1990). Also, differences in naturally and artificially aged samples are observed, the latter being less degraded (Cohen et al. 2000). Lead oxalates were observed on lead white treated with oxalic acid (Zoppi et al. 2010), and on real samples (Salvadó et al. 2009); lead acetate was detected upon exposure to acetic acid

(De Laet et al. 2013). Plumbonacrite (3[2PbCO3∙Pb(OH)2]∙PbO) was detected as well (Salvadó et al. 2009).

In antique sources, the term litharge denotes a variety of compounds derived from lead oxidation, and other compounds. It is now recognized as the low•temperature polymorph of PbO, which is tetragonal (Eastaugh et al. 2008). As a pigment, it was identified on manuscripts and mural paintings (Eastaugh et al. 2008). It is less stable than massicot to water and to saline solutions commonly found in wall paintings (Kotulanová et al. 2009a). As a pigment, litharge is stable to infrared (1064 nm) and red (632 nm, 647 nm) lasers irradiation for Raman spectroscopic studies (Burgio et al. 2001, Smith et al. 2001). Using a 632 nm laser with a power above 9 mW (calculated fluence of approximately 45 kW/cm2) degradation is recorded (Burgio et al. 2001). For shorter wavelengths, and laser fluence at the sample above 2 kW/cm2, litharge degrades to massicot (Burgio et al. 2001). In oil and tempera binder, lead carboxylates are observed, showing stronger intensities compared to those formed from lead white (Mazzeo et al. 2008). This increased reactivity was probably well known to artists and artisans, as PbO was

47 often used as a siccative for binders and varnishes (Eastaugh et al. 2008). Litharge was as well used for the preparation of other pigments (Sotiropoulou et al. 2010). It degrades to cerussite and hydrocerussite, as well as to lead sulphate and phosphate, in archaeological sites attributed to the production of painting materials in Thera, Greece (Sotiropoulou et al. 2010).

Also in this case, as for litharge, nomenclature is complex and confusion may arise. Massicot corresponds to orthorhombic PbO, the high-temperature polymorph of this compound (Eastaugh et al. 2008). It is reported as a pigment in Egyptian artefacts, mural paintings and manuscripts (Eastaugh et al. 2008). On wall paintings, the presence of water and of bicarbonate ions from atmospheric CO2 dissolution causes the formation of hydrocerussite and cerussite, the latter being the most stable compound of the series, even when intermediate salts are formed according to the present anions in the saline solutions (Kotulanová et al. 2009b, 2009a). As for other lead containing paint layers, humidity causes first a darkening of the colour, and finally the formation of lead white (Saunders and Kirby 2004).

Upon exposure to H2S, black galena (PbS) is formed in a thin superficial layer (Smith and Clark 2002). Massicot is stable to laser irradiation, except to 10 mW of 514 nm laser, where a whitish degradation is observed (Burgio et al. 2001). The effect of infrared laser irradiation was investigated as well and it was found to cause reduction of the pigments, and discolouration due to the formation of metallic lead. This process is very intense for massicot, as the formed metallic particles are hardly subjected to re-oxidation, due to their bigger size (Pouli et al. 2001).

Red lead, or minium, has a long history of use in worldwide artistic and artisanal practices, although nomenclature issues are evident (Aze et al. 2006, 2008, Eastaugh et al. 2008). Its synthesis and relation with the other lead oxides is well described in literature (Aze et al. 2008). As a pigment, it is not stable in fresco, or in watercolour. It was anyway used, for its optical properties and cheap price, compared to other red pigments (Eastaugh et al. 2008, Pereira-Pardo et al. 2016). It is reportedly blackened on manuscripts and in glue tempera paintings exposed to light and humidity (Saunders and Kirby 2004, Eastaugh et al. 2008). On the other hand, it is reported that medium-rich proteinaceous red lead paints are stable to light, the binder protecting the pigment (Qingping et al. 1999). Moreover, the pigmented

48 layer has different properties than an unpigmented one (Duce et al. 2013). It is soluble in diluted acids (nitric, chloridric, oxalic, acetic), with formation of the corresponding salts, and when heated it transforms to litharge (Gettens and Stout 1966, Eastaugh et al. 2008).

It is sensitive to sulphur-containing compounds, as H2S and SO2 of atmospheric or biological origin, and pigments such as arsenic sulphides (AsxSy), vermillion

(HgS) and ultramarine (Na8[Al6Si6O24]Sn), especially in manuscripts, wall paintings, and in case the pigment is not mixed with a binder, which causes the formation of dark grey PbS (Smith and Clark 2002, Aze et al. 2008, Eastaugh et al. 2008, Bärbel 2009, Daniilia and Minopoulou 2009, Miguel et al. 2009, Melo et al. 2016) and of whitish PbSO4 (Smith et al. 2001, Aze et al. 2007, Sotiropoulou et al. 2008, Bärbel 2009, Dominguez-Vidal et al. 2014). This was known to artists and artisans, as demonstrated by the presence of a blank space between yellow orpiment and red lead in a 12th century illuminated manuscript, although mixtures of the two pigments do not behave consistently. The reaction between red lead and orpiment produces galena, which could further degrade into sulphates, and lead arsenate species (Miguel et al. 2009).

Mimetite Pb5(AsO4)3Cl was as well identified as a reaction product of minium and orpiment (Hradil et al. 2014). Moreover, it seems that minium promotes the blackening of lead white, when the pigments are mixed (Burgio et al. 1999). Some cases of red lead lightening are reported as well, and are related to the formation of lead sulphates and/or carbonates, according to the atmospheric pollutants and to the initial amount of PbO in red lead (Aze et al. 2006, 2008, Sotiropoulou et al. 2008, Bärbel 2009, Daniilia and Minopoulou 2009, Gutman et al. 2014), via the formation of plumbonacrite (3PbCO3·Pb(OH)2·PbO) (Vanmeert et al. 2015). The black discolouration of red lead due to light exposure was initially attributed to the formation of black PbO2, but this compound is not stable to light (Eastaugh et al. 2008), and it was rarely positively identified (Aze et al. 2008, Bärbel 2009, Daniilia and Minopoulou 2009, Pereira-Pardo et al. 2016), while mixed oxides are also hypothesized (Dominguez-Vidal et al. 2014). PbO2 could be a metabolite of micro•organisms such as fungi (Qingping et al. 1999, Rosado et al. 2015). The parameters responsible for the blackening of minium are identified as light (Cennini: Il libro dell’arte, Gettens and Stout 1966, Daniilia et al. 2000), including laser light (514 and 488 nm, (Burgio et al. 2001)), the pigment’s composition, and climate. These parameters all contribute to yield a grey discolouration at first, and finally a chocolate brown one (Gettens and Stout 1966). Red lead semiconductor properties are responsible for the reduction of Pb(IV) - to Pb(II), and the formation of PbO; the presence of bicarbonate ions (HCO3 ) promote then the formation of hydrocerussite and/or cerussite (Kotulanová et al.

49 2009b, 2009a, Ayalew et al. 2016). The PbO formed by laser irradiation can be re-oxidized to minium (Chappé et al. 2003). A role of Cl- ions is also hypothesized (Bärbel 2009, Daniilia and Minopoulou 2009, Benquerenca et al. 2009). The formation mechanism of black discolouration products (plattnerite (PbO2) and galena (PbS)) was recently investigated: no plattnerite could be detected in artificial ageing tests, while the natural ageing of fresco red lead samples yielded calcite CaCO3, minium Pb3O4, plattnerite PbO2, anglesite PbSO4 and gypsum CaSO4∙2H2O, with the minium grains being gradually converted to alteration products (plattnerite and anglesite) via a sulphation step followed by solvolytic disproportionation (Aze et al. 2006, 2007, Bärbel 2009). Plattnerite might be formed if acidic pollutants attack the paint layer (Aze et al. 2007), or when biological colonization is present (Aze et al. 2008). Oxidizing agents such as ozone can causeas well the formation of plattnerite (Bärbel 2009). Red lead is readily affected by organic acids (including the oil binder), which causes the final appearance of lead carbonate, via lead acetate Pb(C2H3O2)2 (De

Laet et al. 2013) and lead hydroxide Pb(OH)2 (Mazzeo et al. 2008), and/or lead soaps (Luxán and Dorrego 1999).

Two types of yellow pigments based on lead and tin are available, type I which is an oxide (Pb2SnO4), and type II which is a stannate silicate of lead (PbSn1-xSixO3). On their nomenclature, chronology, and occurrences, more can be found in literature (Kühn 1968, Roy 1993, Eastaugh et al. 2008), and references therein. As pigments, they show good hiding power and siccative properties when used in oil (Kühn 1968). Other than for use as pigments in paintings, lead and tin-based coloured opacifiers were used in glass making (Kühn 1968, Roy 1993, Eastaugh et al. 2008).

Both pigments are stable to alkali, but sensitive to H2S and sulphides, which cause blackening (PbS) (Kühn 1968). Whereas other lead containing pigment show fire damage, lead tin yellow type I is detected unaltered (Benquerenca et al. 2009), but heating above 900 °C causes decomposition of the material (Kühn 1968, Roy 1993). They are lightfast, opposite to the other Pb based yellow pigment, massicot (PbO), and stable in oil medium (Kühn 1968, Roy 1993). Relative humidity has no negative effect on either pigment (Saunders and Kirby 2004). Oxidizing agents, such as ozone, causes browning/darkeningof lead tin yellow in wall paintings, as plattnerite is formed (Bärbel 2009). On the other hand, acidic components from the paint itself, and of relevance in air quality evaluation in museum environments interact with the pigments, including carboxylic acids from the ageing oil (lead soaps (Salvadó et al. 2009,

50 Mecklenburg et al. 2013, Catalano et al. 2014)), or volatile organic compounds, such as acetic acid (lead acetate Pb(C2H3O2)2 (De Laet et al. 2013)),.

The general understanding of the ongoing potentially harmful processes regarding inorganic mediaeval pigments is almost completed, and benefitted from the development of new analytical techniques. On the other hand, organic pigments and dyes were not considered in this literature review, although it is known that they can be extremely unstable upon exposure to humidity, light or pollutants (Whitmore and Cass 1989, Salmon and Cass 1993, Williams et al. 1993, Saunders and Kirby 1994; and references therein). However, for the purpose of this thesis, the focus is given to inorganic pigments: most of them can be successfully identified by a non-invasive combination of X-ray fluorescence and Raman spectroscopies, as will be demonstrated in the next Chapters 3, 4 and 5. On the other hand, XRF cannot detect low Z elements, and conventional Raman spectroscopy has limited success in the identification of dyes (Casadio et al. 2010). This Sub-chapter, based on chemical and archaeometrical literature, summarizes both evidences of pigment discolouration and degradation in works of art, as well as investigations based on mock-ups and simulations. Also, results from a variety of analytical techniques are here considered, as the complexity of the topic requires. In fact, tracking and understanding the reactions causing discolouration of the studied pigments requires a different approach according to the problem: salt dissolution and formation (Ca-, Cu- and Pb-based pigments); reduction or oxidation reactions (Fe-, Cu-, Hg-, Pb-containing pigments); ions leaching (ultramarine, smalt; Ca-, Cu-, Pb-pigments); polymorphism and structural modifications of the materials (Cu-, As-, Hg- and Pb-pigments). In each case, an adequate selection of analytical and imaging techniques should be made. Moreover, the role of light (illumination, or laser systems) needs to be taken into account, especially for photosensitive pigments (As- and Hg-containing). Finally, humidity, pollutants and contaminants (H2S, SO2/SO3, NOx; Cl-compounds) provide reactive moieties. The collected information regarding the stability of mediaeval inorganic pigments shows how complex the conservation of polychrome objects can be. First, the understanding of pigment stability to specific factors is required. Moreover, the implementation of air quality control systems and climatic monitoring should take into account the different sensitivities of each and every

51 material in museums and exhibitions, in storage or on display, in order to ensure its preservation for the future generations. Even though pigments are not often present without binder in works of art, it is important to know their reactivity. Then, the study of pigment-binder interactions and how they affect the single components, and the paint layer itself needs to be considered. In the next Sub-chapter, the painting technique of Early Netherlandish paintings is described, as it is relevant for the purpose of this thesis.

In this Sub-chapter, the particular case of Early Netherlandish paintings is described. This part combines information from artistic literature and treatises, guild regulations, archival documents, painted and literary representations of artists’ workshops, and is confirmed by the results of analytical investigations of paintings (Billinge et al. 1997, Wadum 1998). Their typical stratigraphic structure is represented in Figure 2.1 (Coremans 1953, p. 73; Thompson 1956, van Asperen de Boer 1975, Billinge et al. 1997). The support is usually oak, radially cut, dried and seasoned (Thompson 1956, Billinge et al. 1997, Wadum 1998, Glatigny et al. 2010). The panel and its frame were prepared at the same time, and on both sides, with a mixture of animal glue and ground chalk (CaCO3) (while in Southern Europe gypsum (CaSO4∙2H2O) was preferred for the purpose) (Thompson 1956, Wadum 1998, Glatigny et al. 2010). The opaque white preparation layer was part of the three-dimensional build-up of the image, contributing to the brightness of the overlaying semi-transparent oil based pigmented layers (Billinge et al. 1997). This porous preparation layer was then smoothed and covered with an impermeabilization layer, on top of which the underdrawing sketch was traced, freehand, or by transferring models, and with varying degrees of detail, shadowing and modelling (Thompson 1956, Billinge et al. 1997). Liquid (ink, black paint) or dry (black chalk, charcoal) drawing are both common in Early Netherlandish paintings, together with incised lines for architectural elements; metal points were also used but limited to paper drawings (Billinge et al. 1997). The paint medium in Early Netherlandish paintings is oil-based. Contrarily to what reported by Vasari (Effmann 2006), it was not Jan Van Eyck who “invented” the technique: in fact, both the treatise of Theophilus, dating to the 12th century,

52 reports indications on how to paint with oil, and Norwegian paintings dating to the 13th and 14th centuries have been recognized as oil-based (Billinge et al. 1997). Linseed and walnut oil are now identified in Netherlandish paintings, and sometimes the latter was preferred for lighter shades as it yellows less (Billinge et al. 1997). Moreover, prepolymerized oil (by heat treatment or sun exposure), or other additives could be used for paints showing poor drying properties (as carbon black, for example), and diluents used to thin the paint mixture (Billinge et al. 1997).

The oil was mixed with the bright pigments lead white (2PbCO3∙Pb(OH)2), lead tin yellows (type I: Pb2SnO4; type II: PbSn1-xSixO3), vermillion (HgS), ultramarine

(Na8[Al6Si6O24]Sn), azurite (2CuCO3∙Cu(OH)2), malachite (CuCO3∙Cu(OH)2) and other copper-based greens, and carbonaceous blacks (C). The yellow, red, brown ochre pigments (Fe2O3, FeOOH, Fe3O4; MnxOy) and the green earths glauconite 3+ 3+ 2+ ((K,Na)(Fe ,Al,Mg)2 (Si,Al)4O10(OH)2) and celadonite (K[(Al,Fe ),(Fe ,Mg)]

(AlSi3,Si4)O10(OH)2)) were also used, as well as a variety of organic dyes and lakes such as indigo (C16H10N2O2), madder (C14H8O4), etc. (Thompson 1956, Eastaugh et al. 2008). The pigments were first ground with a medium using muller and slab (often by an apprentice), and then diluted on a palette. Spatulas and brushes, soft (hermine) and hard (hog), were both used to apply the paint, while for the gilding other tools were required (mussel shell for powdered gold, burnishing tools for foil gilding). In Netherlandish paintings, the sequence of pigmented layers is complex, and the picture is created from light to dark, the darker shades being obtained by applying multiple layers of paint and glaze. Visually, and three-dimensionally speaking, the darker shadows in folds correspond to an increased relief of the surface (Stoner 1985). The layered structure allowed the master and its workshop to optimize the drying times and optical properties by applying a fast-drying, lean tempera underpaint (for cooler tones) (Billinge et al. 1997). Moreover, the use of expensive pigments and materials, such as gold leaf, azurite and ultramarine, could be minimized as they could be applied economically. In fact, the Flemish painters took full advantage of glazing techniques (glacis, glaze in English, is an oil-rich transparent paint where pigment grains are dispersed and/or dyes are dissolved in an oily medium) to adjust the shades and make the most out of expensive pigments (e.g. ultramarine glaze on top of a cheaper opaque azurite-based layer; addition of a little lead white to increase opacity) (Billinge et al. 1997). The stratigraphy is completed by a varnish layer to enhance saturation of the colours and to protect the painting (Thompson 1956). Moreover, the synergy between the properties of oil paint, the empirical knowledge on pigments and paint, and the extraordinary skills of painters as Jan Van Eyck, allowed the creation of incredible masterpieces (Billinge et al. 1997).

53 Varnish

Red glaze

Purplish red layer

Orange-red layer

Impermeabilization layer and underdrawing

Preparation layer

Wooden support

Figure 2.1: Structure of an Early Netherlandish painting. Stratigraphy based on a cross section from the Ghent Altarpiece, panel IV – Deity Enthroned, red mantle (Coremans 1953, p. 73).

The newly created work of art is different, from the material point of view, from what we observe and study today, as with time the materials have interacted among themselves, with the environment, and the object was handled for many reasons by individuals (Brandi 1963, Berrie et al. 2003, Leona and Van Duyne 2009). It appears, from the previous Sub-chapters, that painted objects created centuries ago, while being unique, complex and fragile, convey huge amounts of information on many aspects of culture, including artists’ and artisans’ modus operandi, the material history of an object, and its conservation state. Finally, it is important to be aware of the fact that the materials currently present in works of art can be still chemically unaltered, and give us information on the original composition of the object, or can be different from the originals. The latter is the case of binders and varnishes, which are now polymerized into solid films, and can provide information on the creative process of the polychromy. Also degradation products which might have formed resulting from the interaction of the painting with various factors are included in this category. In this circumstance, it is important to study the ongoing alteration and to take action to mitigate the harmful conditions, to stop the degradation, and to possibly restore the visual integrity of the object by means of digital approaches, in case the appearance is affected (colour changes (Dik et al. 2002), extensive craquelure (Cornelis et al. 2013, Pižurica et al. 2015), etc.). Knowing the materials, their history of use and their sensitivity, is the starting point to better understand the material aspects of objects of our Cultural Heritage. To be able to identify them on real works of art, while minimizing the risk for the

54 object and for the operator yields great added value in the field of Cultural Heritage studies, which is the objective of this work. In the next chapter (Chapter 3) we will deal with the application of non•destructive X-ray based techniques for the study of polychrome objects (Sub•chapter 3.3.1) and pigments (Sub-chapter 3.4.2). The technique of choice is X-ray fluorescence, which is a fast, multi-elemental method to identify the elements present in the object under investigation. Advantages and limitations of such a technique in the identification of pigments (and related materials) for the study of Cultural Heritage materials will be discussed in detail in Sub-chapter 3.5.

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70 In the previous chapters, the reasons to include chemical analysis into an interdisciplinary project of conservation treatment were highlighted (Chapter 1), the most common inorganic mediaeval pigments discussed with respect to their sensitivity to degradation (Sub-chapter 2.1), and the structure of Early Netherlandish paintings illustrated in Sub-chapter 2.2. Starting with Chapter 3, some examples of non-destructive chemical analysis of pigments and paintings in order to answer specific research questions will be given. This Chapter is dedicated to X-ray fluorescence (XRF) techniques for the analysis of polychrome objects in Cultural Heritage. First, the principle of XRF will be described shortly (Sub-chapter 3.1), and then X-ray fluorescence based techniques will be discussed with specific attention to their application to archaeometry (Sub-chapter 3.2). The application of XRF techniques to Cultural Heritage materials is described, first proposing a non-invasive approach to confirm the presence of modern overpaints on the outer panels of the Ghent Altarpiece (Sub-chapter 3.3), and then presenting the preliminary results of a non-destructive (non-sample-consuming) approach to achieve elemental quantification of pigment powder samples (Sub- chapter 3.4). Some final remarks on the advantages (fast, possibility of authentication and chronological consideration, including material history) and disadvantages of XRF techniques (no molecular/structural information, high Z elements, bulk analysis) will be mentioned with specific focus to the analysis of works of art (Sub-chapter 3.5).

71 In this first part of Chapter 3, the principle of X-ray fluorescence analysis will be described. To begin with, X-rays are the part of the electromagnetic spectrum with a wavelength between 0.01 and 10 nm, corresponding approximately to energies of 100 to 0.1 keV (Janssens and Van Grieken 2004, Shackley 2011, Varella 2012). They were discovered by Wilhelm Conrad Röntgen in 1895 (Röntgen 1896). Waves with such properties are often used in material and medical science (Shackley 2011, Varella 2012). On the one hand, X-ray radiographs show differences in X-rays absorption, revealing inhomogeneities and Figure 3.1: Radiograph of the hand of differences in density in the sample Röntgen's wife. In this image the ring and (Figure 3.1) (Leute 1987, Nobuyuki bones appear darker than the flesh. 2005, Uda et al. 2005). On the other hand, those wavelengths match the spacing between atomic layers in crystalline solids, so that diffraction occurs according to Bragg’s law (Leute 1987, Settle 1997, Uda et al. 2005). The obtained information on the layers spacing can be used for identification of the diffracting material. X-ray diffraction (XRD) is a powerful tool for the study of crystalline compounds, allowing the identification and quantification of phases in powders (powder XRD, pXRD), or samples (micro-XRD, μXRD) (Edwards and Vandenabeele 2012). Also because the typical energy range of X-rays is similar to the energy difference between inner electron shells of atoms, this electromagnetic radiation can be used to investigate these atomic energy levels (Settle 1997, Ferretti 2000, Uda et al. 2005, Shackley 2011, Klockenkämper and von Bohlen 2014). This feature is at the base of X-ray fluorescence techniques (XRF), described in the following. When a high energy photon of frequency ν (or an accelerated particle with comparable energy) hits an atom, it can be absorbed by one of the electrons of the inner shells (Settle 1997, Shackley 2011, Klockenkämper and von Bohlen 2014) (Figure 3.2). This phaenomenon happens only if the incoming radiation has

72 sufficient energy to overcome the binding energy φ of the electron, i.e. when hν > φ (photoelectric effect; h = Planck’s constant). The photoelectron is emitted, and a vacancy created (Figure 3.2). The ionized atom is unstable: to reach a more favourable energetic state, the electrons are subjected to rearrangement (Figure 3.2).

Emitted X-rays hν = ΔE = φ’ – φ Photoelectron

E = hν - φK M to K transition: Kβ

L to K transition: Kα

φM φL φK K L M

E = hν Incident photon

Figure 3.2: The photoelectric absorption process and emission of characteristic X-rays. First, the atom is ionized upon irradiation with appropriate radiation (E = hν). The X-ray emission that follows electronic rearrangement is schematized by illustrating two possible options allowed by the selection rules (Kα and Kβ lines). Image after (Kalnicky and Singhvi 2001).

An electron from an outer shell fills the vacancy, and the excess energy is emitted as a photon (secondary fluorescence) whose energy corresponds to the gap between its original electronic level (φ’) and the level where the vacancy was created in the first place (φ) ΔE = φ’ - φ. This energy is released as secondary fluorescence radiation having a frequency ν according to Planck’s law: ΔE = hν, and the photon’s energy is in the range of X-rays (Settle 1997, Shackley 2011, Klockenkämper and von Bohlen 2014). Alternatively, the secondary fluorescence photon can be absorbed by another electron, which is ejected (Auger effect). Fluorescence prevails for heavier atoms, while Auger electrons are favoured for low Z ones. The recording of the induced X-ray photons gives a mean to identify the excited atoms of the material under investigation. As discussed in the previous paragraph, different electronic shells are involved in this process, both as regards

73 the expulsion of a photoelectron (i.e. creation of a vacancy, binding energy φ), and the electron from outer shells filling in the gap (binding energy φ’), according to selection rules. Capital letters K, L, M… indicate the shell where the vacancy was created, Greek letters α, β, γ… the shell from where the vacancy was filled

X-ray source Detector Processing unit

Spectrum

Sample INTENSITY ENERGY

Figure 3.3: Scheme of an ED-XRF instrument. Image after (Kalnicky and Singhvi 2001).

(Settle 1997, Shackley 2011, Klockenkämper and von Bohlen 2014). In practice, the presence of an element in a sample is indicated by the presence of its characteristic lines in the recorded XRF spectrum. A general layout of an energy-dispersive ED-XRF instrument is presented in Figure 3.3. To start with, an X-ray source is used as probe. In this work X-ray tubes based on Mo, W, Rh anode material were exploited to investigate the samples. This radiation is directed to the sample by using crystals, multilayers and polycapillaries (based on diffraction and total reflection) for monochromatization and collimation of the radiation. The detection and separation of the secondary fluorescence emitted upon bombardment with appropriate radiation/highly energetic particles can be performed based on its energy (energy dispersive ED-XRF), or on its wavelength (wavelength dispersive WD-XRF). Energy dispersive detection is currently widely exploited, thanks to the availability of a variety of compact and fast solid state based detectors (Kalnicky and Singhvi 2001, Janssens and Van Grieken 2004, Shackley 2011, Klockenkämper and von Bohlen 2014). As regards the data interpretation, XRF techniques provide fast, non•destructive multi-elemental information on the sample composition, as the fluorescence spectrum (in intensity vs. energy) contains the characteristic emission lines of the elements present, as well as some sort of proportion between the concentration of a specific elements and the number of emitted characteristic X-rays (Janssens and Van Grieken 2004, Shackley 2011).

74 As the number of induced photons is related to the amount of excited atoms, the determination of elemental compositions seems trivial. However, quantification may be challenging, as matrix effects, self-absorption and other phaenomena take place inside a sample when irradiated with X-rays (Shackley 2011). Quantification can be achieved by taking into account the sample composition and structure, instrumental parameters, experimental conditions, and the random aspects of light-matter interactions (Janssens and Van Grieken 2004, Klockenkämper and von Bohlen 2014). So far, the principle and basic instrumentation for XRF analysis were explained, including some general information on the type of results that can be obtained. The following Sub-chapter will contribute to the understanding of the possibilities of application of XRF techniques in archaeology and art history contexts.

Some aspects related to the use of the XRF methodology in archaeometry will be discussed here, in order to point out limitations and advantages of the use of this technique specifically for studying polychrome objects. First of all, it is important to remember that in this thesis the focus is given to pigments and paintings, while the field of archaeometrical research encompasses a wider variety of materials and objects (metals, ceramics, glass, organic materials, etc.) (Edwards and Vandenabeele 2012). A vast variety of polychrome materials of Cultural Heritage can successfully be characterized by means of XRF, for example ceramics, glass, wall paintings, panel paintings, wallpaper, manuscripts (Klockenkämper et al. 1993, Devos et al. 1995, Neelmeijer et al. 2000, Uda et al. 2005, Paternoster et al. 2005, Castro et al. 2007, Van der Snickt et al. 2008, Bonizzoni et al. 2013, van Pevenage et al. 2014, De Langhe 2015). In this sense, in fact, the disadvantages of XRF concern the difficulty in identifying some elements due to overlapping spectral peaks (S-K/Mo-L/Pb-M; Ti•K/Ba-L; As-K/Pb-L). Sulphur is present both in pigments and as a possible effect of degradation (for example HgS, sulphation of frescoes, etc.); lead and arsenic are key-elements for a range of white, yellow, orange and red pigments (Moens et al. 1995, Klockenkämper and von Bohlen 2014, see Sub-chapter 2.1). Titanium and barium are common impurities in many natural materials, but they are also related to synthetic modern pigments, like titanium white, lithopone, barium pigments

75 (Eastaugh et al. 2008; Table 3.1). Also, the overlaps of Ba-L and Ti-K, Pb-L and As-K, and the K-lines (β and α) of adjacent transition metals (21 < Z < 30), as well as escape and sum peaks (e.g. escape of Fe with Ti-Kα, Hunt and Speakman 2015)), can hamper the identification (and quantification) of the elements in the sample. Finally, molybdenum is a common source in XRF instruments, and if not properly monochromatized, the source might interfere with the correct identification of the present elements. Furthermore, the available/chosen X-ray source might introduce some bias on the results. For example: the unfiltered L-lines from the source (Rh, Mo) overlap with the K•lines of elements Z < 18, the L-lines of transition metals of the fifth period (including Ag, Cd, Sn), and the M•lines of lanthanides and transition metals of the sixth period (including Au, Hg, Pb). The availability of multiple X-ray sources can help overcoming this issue (van Hooydonk et al. 1998, Wehling et al. 1999). Moreover, the impossibility of detecting low Z elements (Z > 11, in vacuum, otherwise Z > 12) is particularly relevant, as some widely used pigments cannot be identified by means of this technique, such as organic dyes, carbon based blacks (C), and ultramarine (Na8[Al6Si6O24]Sn). Additionally, extreme care needs to be taken when interpreting the obtained elemental information. In fact, the distinction among different salts of the same metal cannot be successfully performed by this technique (for example: anglesite PbSO4 / red lead Pb3O4 / lead white 2PbCO3·Pb(OH)2 / lead acetate Pb(CH3COO)2; the copper-based green salts (Coccato et al. 2016, Sub•chapter 4.3.2); etc.), which is relevant both in the characterization of the used materials and in the study of degradation processes (Sub-chapter 2.1 and Paragraph 4.3.3). Also, due to the penetrating power of X-rays it is important to remember that the obtained information originates from a certain volume of the sample, and therefore, all stratigraphic information is lost (Varella 2012). Confocal systems can, however, non-invasively provide stratigraphic information, according to the achievable spatial resolution (Janssens and Van Grieken 2004). Notwithstanding these limitations, XRF techniques have the great advantage of being fast, multi-element methods that yield spectra based on the characteristic emission lines of the present elements. Moreover, XRF offers the possibility of non-invasive, and non-destructive analyses, possibly also in situ (Leute 1987, Longoni et al. 1998, Ferretti 2000, Neelmeijer et al. 2000, Uda et al. 2005, Castro et al. 2007, Shackley 2011, Varella 2012, Bonizzoni et al. 2013, Klockenkämper and von Bohlen 2014, Barbi et al. 2014). These aspects explain the widespread use of this technique, often combined with complementary techniques (FORS, PIXE, Raman spectroscopy, SEM-EDX, XANES, XRD), in archaeometrical investigations (Longoni et al. 1998, Neelmeijer et al. 2000, Uda et al. 2005, Paternoster et al. 2005, Ramos and Ruisánchez 2006,

76 Castro et al. 2007, Appolonia et al. 2009, Sawczak et al. 2009, Van der Snickt et al. 2009, Deneckere et al. 2010, Chaplin et al. 2010, Price and Burton 2011, Shackley 2011, Hamdan et al. 2012, Edwards and Vandenabeele 2012, Barbi et al. 2014, Bonizzoni et al. 2015, Schmidt et al. 2016, Rousaki et al. 2016a, Coccato et al. 2017; Sub-chapter 5.1). Finally, in the analysis of works of art, the availability of portable instruments is of great interest. However, for portable XRF analysis, it is important to use an appropriate set-up that guarantees a stable, yet flexible positioning, and that the measurements are carried out by minimizing the risks for both, the operator and the Cultural Heritage object. The non-invasive approach associated with the use of mobile XRF instrumentation for the analysis of a masterpiece of the Flemish Primitives is explained in Sub-chapters 3.3 and 5.1. In the fourth part of this chapter (Sub-chapter 3.4), a non-destructive approach is described: it requires a sample, collected in a virtually non•invasive manner, but the analysis in itself does not consume the sample (therefore “non-destructive”, as defined in Sub-chapter 1.5). In contrast to the traditional (scalpel) sampling, material for total-reflection XRF (TXRF) can be collected by means of a dry cotton swab rolled on exposed paint layers (manuscripts, paintings undergoing varnish removal, etc.) (Devos et al. 1995, Moens et al. 1995, Klockenkämper et al. 2000, von Bohlen 2004). The TXRF technique allows for detection, and quantification of major, minor and trace elements. Sub-chapter 3.5 will conclude this chapter by summarizing the main focal points of XRF analysis in studying pigments and paintings, as emerged from the two selected applications of Sub-chapters 3.3 and 3.4.

This Sub-chapter is introduced by a brief description of the used instrument, as well as of specific measures to safely operate it in a museum environment. The Olympus InnovX Delta handheld XRF (hXRF) spectrometer (Figure 3.4) is a highly-performing instrument for fast in situ non-destructive elemental analysis. It weighs ca. 1.5 kg (without battery), and it has a pistol-like shape. It combines a 4W X•ray tube based on a Rh anode with a silicon drift detector (SDD), allowing for the simultaneous detection of elements from Mg to U (12 < Z < 90).

77 It yields a spotsize of 5×5 mm, but the accurate selection of the point of analysis is slightly hampered by the size and shape of the probehead (ca. 2.5×8 cm). In fact, this instrument is optimized for geological/ industrial applications (bulk analysis), where samples show a greater homogeneity. A more recent version of this same hXRF instrument (in use after the first one was stolen during a mission abroad) includes a collimator, which reduces the spot size to ca. 3 mm2, as well as a built-in camera to have a better control on the measured area. Although it is designed to be held by hand, during this research, for the study of painted surfaces, it was preferred to use the in-house developed holder (Figure 3.5, a and b). It ensures a stable positioning and the greatest radiation safety for the operators, who can control the instrument remotely via a laptop (USB connection). Moreover, cones are used to delimit a safe area with a radius of 3 meters around the X-ray source, to prevent other people in the surroundings to be exposed to dangerous radiation, and operators are provided with a dosimeter, complying with the Standard Operating Procedure of the instrument (Lycke and Monsieurs 2015)).

Figure 3.4: The handheld Olympus InnovX Delta XRF spectrometer.

78 Figure 3.5: a) The InnovX Delta hXRF instrument and its in-house developed holder; b) the holder is attached on cross-mounted sliding elements fixed to a tripod; c) safety measures following the Standard Operating Procedure of the instrument (Lycke and Monsieurs 2015): USB connection to operate the instrument from a distance, and cones to delimit the safe area in MSK, Ghent, Belgium. Photo UGent (Sint•Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW).

79 Figure 3.6: The InnovX Delta hXRF being used to analyse the outer wings (panels and frames) of the Ghent Altarpiece at the MSK, Ghent, Belgium. a) and b) Phases of the positioning; c) and d) measurement of the panel Erytrean Sybil and city view. Photo UGent (Sint•Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW).

This instrument was extensively used during the GOA project (see Sub•chapters 1.4 and 5.1, and Paragraph 3.3.1) to perform fast, non-invasive, multi-elemental analysis on the panels and frames of the Ghent Altarpiece (Figure 3.6) to support the work of the conservators. On the one hand, qualitative analysis using the hXRF enabled the team of UGent to identify overpaints based on key elements present exclusively in modern pigments. On the other, these data were not sufficient to determine the next steps in the conservation project. An interdisciplinary interpretation of the data, and a more complex analytical approach were required, as described in the next Paragraph.

In this part of the chapter dedicated to the evaluation of X-ray fluorescence analysis for the identification and characterization of pigments and paintings, a case study regarding the overpaints on the Flemish masterpiece “The Adoration of the Mystic Lamb” by the brothers van Eyck is presented.

80 The conservators and analysts of the Royal Institute for Cultural Heritage (KIK•IRPA), leading the conservation project of the Ghent Altarpiece (see Sub•chapter 1.4), worked in close cooperation with experts from Ghent University, University of Antwerp, and an international expert panel (Martens 2015). Art History and Chemistry teamed up to support each other to contribute to the conservation campaign, under the guidance of the KIK-IRPA conservators. Visual observations, including high-resolution digital microscopy (Hirox microscope), in situ non-invasive investigations, and the study of samples, were the tools to provide the conservators with valuable information regarding the condition and authenticity of the paint layers. All the analyses performed were subjected to answering specific research questions coming from the conservation team, in this case on the overpaint layers. A Concerted Research Action (GOA) project funded by Ghent University provided support to the interdisciplinary study for the conservation and restoration campaign of the Ghent Altarpiece. Specifically, as it concerns the chemical investigation of the materials carried out by Ghent University, it encompassed the non-invasive in situ study of the panels and frames of the closed polyptych by means of portable instrumentation (discussed here and in Sub-Chapter 5.1), the development of a Raman system for in situ mapping of art objects (Lauwers 2017), and the study of the effectiveness of varnishes to protect the frames’ polychromy (Rousaki et al. 2016b). The current Paragraph focusses on the study of the panel “Cumaean Sibyl and interior” (Figure 3.7). The portable XRF campaign carried out by UGent in 2013 on the green vest of the Sibyl revealed for the first time the occurrence of elements related to the use of modern pigments (see Table 3.1), pointing out the presence of recent overpaints. The findings of the UGent team confirmed on analytical grounds the presence of overpaint, and supported the need for further investigations on this topic. Further observations and analyses allowed the quantification of the overpainted area to a total of ca. 70 % of the surface, i.e. more than 5 m2 of the 8 m2 painted area of the outer wings of the Altarpiece. Some of the results discussed here were presented in the poster “Identification of restorations on the Cumaean Sibyl of the brothers van Eyck’s Ghent Altarpiece (1432)” by F. Rosier, H. Dubois, G. Van der Snickt, J. Sanyova, A. Coudray, C. Glaude, K. Janssens, D. Lauwers, A. Coccato, and P. Vandenabeele, presented at the 4th International Congress Chemistry for Cultural Heritage in Brussels, Belgium (Rosier et al. 2016).

81 Overpaintings are commonly found on works by old masters; they were applied usually by past restorers in an effort to preserve the painting (Martens 2015). They were applied for different reasons, sometimes overpaints were used to disguise damaged areas (including those affected by inappropriate cleaning attempts, like on the green vest of the Cumaean Sybil (“restoration/ REVELATION - The exterior wings of the Ghent Altarpiece” 2016)). In other cases, they were executed simply to refresh the colour palette, according to the taste of the period. In this process, the appearance of the painting could be strongly altered: details were covered (i.e. the bright highlights marking the folds, cobwebs in the corners of the Annunciation scene, etc), or sometimes added (i.e. highlights on the hands and faces of the donors, the fringes on the green tablecloth behind the Virgin Mary, etc.) (“restoration/ REVELATION - The exterior wings of the Ghent Altarpiece” 2016). At least two overpaint campaigns, plus some local interventions, were identified. They covered the damaged areas, imitating the shapes and shades, but often choosing for a simplified appearance. These overpaints resulted in a less accurate rendition of the materials, flattening of the Figure 3.7: Detail of the painting Cumaean Sibyl and three•dimensional illusion, and interior during the conservation treatment. Photo KIK-IRPA (Sint-Baafskathedraal Ghent, copyright discrepancies in the once Lukasweb.be - Art in Flanders VZW). cohesive and coherent

82 construction based on light and shadows. It is important to note that many of these overpaints were already copied by Cocxie in 1557-1558, and are therefore attributed to the intervention of Blondeel and Van Scorel in the 1550s. Later interventions are as well documented in archival sources and, more recently, in photos and reports (Coremans 1953). Traditionally, overpaints cover large areas, overlapping as well with intact zones, as visible at higher magnifications thanks to microscopes, while nowadays the intervention is only carried out on damaged areas, so that the reintegration is kept to a minimum (Martens 2015). Moreover, pigments used in the superimposed layers do not necessarily correspond to the original ones, which makes the application of non-invasive chemical analysis (portable XRF, see Tables 3.1 and Table 3.2) suitable for identifying the presence of different materials used on top of one another. After the layers of ketonic varnish dating to the restoration campaign of 1950•51 (Coremans 1953) were removed, it appeared, under close examination of the painting, that the surface was diffusely overpainted (“restoration/ REVELATION - The exterior wings of the Ghent Altarpiece” 2016). These overpaints showed overlap with the craquelure pattern of the underlying layers, and were stylistically not coherent with the other works by van Eyck. Once it was ascertained that overpaints were present thanks to the results of in situ XRF spectroscopy, additional analyses (macro-XRF scanning, cross sections) and cleaning tests revealed that the underlying layer, although damaged to some extent, was original. It was then decided to proceed in removing the overpaints (Martens 2015), which required more time (2012 till 2016, instead of till 2014) and funding (Gieskes-Strijbis Fund) than what was foreseen at the beginning of the restoration project (“restoration/ REVELATION - The exterior wings of the Ghent Altarpiece” 2016).

Table 3.1: List of some modern pigments encountered in the overpaints on the green vest of the Cumaean Sybil (outer wings of the Ghent Altarpiece), including the key elements and production date (Bell et al. 1997, Burgio and Clark 2001, Eastaugh et al. 2008).

Colour Pigment Key elements Date

Chrome oxide: early 1800s Green Chrome green Cr Viridian: 1838 Chrome yellow Cr 1809 Yellow Cadmium yellow Cd 1845 Zinc yellow Zn 1809

Titanium white Ti Anatase: 1923; Rutile: 1947 White Zinc white Zn 1834 Lithopone Zn, Ba 1874

83 The decision of removing the overpaints was concerted among the international experts and was supported by the relatively good conservation state of the original surface. In fact, it is finally possible to regain the experience of the painting as it was supposed to look originally (Martens 2015).

Table 3.2: List of the most common ancient pigments in use in Europe during the Middle Ages, including the key elements and notes regarding their use (Klockenkämper et al. 1993, Moens et al. 1994, 1995, Bell et al. 1997, Burgio and Clark 2001, Eastaugh et al. 2008). Elements in italic are not detectable by means of XRF.

Colour Pigment Key elements Notes

Carbon black C Bone black C, P, Ca Black Magnetite Fe Manganese oxides Mn Lapis lazuli Na, Al, Si, S Mineral and synthetic (1828) Azurite Cu Mineral Egyptian blue Cu Artificial - 3000 BC Blue Smalt Co Artificial - ca. 1500 Vivianite Fe Indigo C, N

Green earths Fe Mineral, many varieties Malachite Cu Mineral Verdigris Cu Synthetic Cu-resinate Cu Synthetic Green Other Cu-salts (citrate, carbonate, Cu phosphate, sulphate, silicate, chloride, etc.) Mix: yellow + blue Massicot Pb Lead tin yellow I Pb, Sn Yellow Lead tin yellow II Si, Pb, Sn Goethite/yellow ochre Fe Mineral Realgar As Mineral

Haematite/red ochre Fe Mineral Minium Pb Red Vermillion Hg Mineral and synthetic (8th c.) Orange Orpiment As Litharge Pb

Mineral (rare), synthetic White Lead white Pb (before 500 BC) Fluorite F, Ca Purple Caput mortuum Fe Mix: red + blue

84 The overpaint present on the outer wings of the Ghent Altarpiece was carefully documented and analysed before and during removal. Moreover, the information from archival sources as well as from historical photographs from the 20th century was compared, so that it was possible to establish a timeframe for some of the overpaints. Also, many modern pigments have well-known production and discontinuation dates that make them interesting materials for the purposes of dating and authentication (Table 3.1). Considering the research question at hand regarded overpaints on the Sybyl’s vest, their presence, extent and possibility of removal, the results of the portable XRF campaign were preliminary, yet fundamental, as they allowed the define the lines of further research. Anyway, the identification of elements attributable to modern pigments should not be considered conclusive. In fact, some issues related to the ability of detecting and identifying specific elements were mentioned in Sub-chapter 3.2. Additionally, the same elements can be present in different compounds (see Cr and Zn in Table 3.1), so that additional analyses should be performed to obtain information on the present compounds. Finally, portable XRF could not visualize the distribution of the overpaint layer.

Figure 3.8: Comparison of archival photographs of the painting Cumaean Sybil. a) 1902 (Photo RKD, Friedländer archive); b) 1937 (Photo RKD, Van der Veken archive); c) before conservation treatment in 2012. Photo KIK-IRPA (Sint-Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW).

In fact, in the case of the Cumaean Sibyl, it appeared that its green robe, damaged probably during an old cleaning treatment, was overpainted. Black•and•white pictures, taken in 1902 and in 1937, show differences in shape and shading of some folds of the robe (Figure 3.8, a and b). This could be assigned to the restoration campaign performed by Jef Van der Veken (Coremans 1953).

85 However, to evaluate the feasibility of overpaint removal, it was needed to ascertain its extent and the conservation state of the underlying layers. To clarify the composition (type of pigments) and extent of the overpainted areas, XRF analyses were performed. On the one hand, portable XRF was employed for a first screening (point analyses), to evaluate if any suspect element, attributable to the use of synthetic modern pigments, was present in specific areas suggested by the conservators. Then, to have a better overview of the composition and spatial distribution of the recent retouchings, and of the conditions of the underlying layers, X-ray based scanning techniques were exploited. The macro-XRF (MA-XRF) technique, described in detail elsewhere (Alfeld et al. 2011, Alfeld 2013, Alfeld and Broekaert 2013), was used to acquire two-dimensional elemental distribution images. The combination of the qualitative information (ED-XRF instrument, indicating the present elements), with the spatial distribution (scanning system) is a powerful tool to visualise the paint layers and their discontinuity. Moreover, thanks to the penetrating power of X-rays, information coming from underlying layers, not visible to the naked eye, could be obtained (Alfeld 2013, Alfeld et al. 2013). The study was completed by investigating cross sections (in KIK-IRPA), which provided additional details on the stratigraphy, and the conclusive identification of the pigments used.

HXRF point measurements revealed the presence of elements that are related to the use of modern pigments, such as Ti, Ba, Cr and Zn (Figure 3.9, Table 3.1). The signals of Cu and Pb can be assigned to underlying layers, containing traditional pigments such as Cu-based greens and Pb-based pigments (lead white or lead tin yellow), the presence of the latter being suggested by the detected signal of Sn-K lines, see Table 3.2). Calcium is evident in the spectrum recorded on the unpainted wood, it is moreover expected to be present in the preparation layer (calcite CaCO3 + animal glue), and in many other additives and compounds commonly found in paintings. The point analysis performed very close to the edge of the painted area (Figure 3.9, black) seems to reveal less later interventions: in fact, the spectral intensity of elements such as Ti, Ba, Cr and Zn is lower than in the other analysed points, while Cu reaches its highest value among the recorded spectra. In the other areas (Figure 3.9, magenta, grey and blue), those elements are identified as well, showing variable intensities, according to the actual shade. Cr is higher in the yellowish green area (Figure 3.9, blue), Zn is more intense in

86 Figure 3.9: hXRF spectra collected on points indicated by the conservators (different shades of green). In all the points, due to the penetrating power of X-rays, Cu, Sn and Pb were identified, coming from the original underlying layers. The signals of Ti/Ba-L, Cr and Zn are clearly visible in all the measured areas. These spectra allowed the first identification of overpaints based on modern pigments on the outer wings of the Ghent Altarpiece. Photo KIK-IRPA (Sint-Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW). the more yellowish spot (Figure 3.9, magenta), than in the yellowish-green (Figure 3.9, blue), brownish shadow (Figure 3.9, grey) and in the green zone close to the bottom edge of the Sybil’s painting, respectively (Figure 3.9, black). A weak signal of arsenic (11.7 keV (Bearden 1967)) is detected as well in the brownish shadow of the green robe (Figure 3.9, grey). The qualitative characterization of the areas of suspected overpaint by means of hXRF (Figure 3.9) confirms the use of modern pigments to restore the damages in the ancient layer, containing mainly copper, lead and tin, which is consistent with an original green paint, as for example presented in the description of stratigraphic sections from “Saint John the Baptist” and from the vegetation in various panels in (Coremans 1953, pages 71-73), and from the panel Archangel and Prophet Zacharias, in (Coccato et al. 2016), and Paragraph 4.3.1.

87 , copyright , copyright Ghent overpaint removal Photo Photo removal overpaint kathedraal kathedraal L; d) image of Ti; e) image of Cr; Cr; of image e) Ti; of image d) L; - Baafs - Sint of the Cumaean Sybil painting during during painting Sybil Cumaean of the ; b) image of Cu; c) image of Pb of image c) Cu; of image b) ; icture icture P XRF scannings. a) scannings. XRF -

Sint-Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW) Flanders in Art - Lukasweb.be copyright Ghent, Sint-Baafskathedraal : Elemental distribution images as obtained by MA as obtained images distribution : Elemental 10 . 3 KIK-IRPA ( KIK-IRPA f) image of Ba; g) VZW). image of Flanders in Fe; Art h) - image of Lukasweb.be Zn. Images AXES of Group, Research University Antwerp ( Figure Figure

88 It should be pointed out that in both mentioned studies, organic compounds are identified based on microscopy and micro-chemical methods of analysis of cross sections (“colorant organique jaune”, an organic yellow dye with malachite, lead white, and copper resinate (Coremans 1953)), or identified by molecular spectroscopies (indigo with lead tin yellow type I, and copper resinate (Coccato et al. 2016), Paragraph 4.3.1): analysis based on X-rays cannot however clarify the presence and type of such materials. To facilitate the visualization of the overpaint layer extension, the distribution of these elements was determined by means of MA-XRF scanning during the overpaint removal (Figure 3.10, b to h). The MA-XRF measurements highlighted the presence of some damages in the older overpaint layers, as it appears from the images of distribution of Cu and Pb. The robe seems to have been retouched using modern pigments mainly in the areas where the Cu and Pb containing layers were damaged. The images of Ti, Cr, Fe, Zn and Ba perfectly overlap, indicating the use of a complex mixture of modern pigments to replicate the original green shade. In addition to the XRF study, a sample was collected, observed under the microscope and analysed by means of micro-Raman spectroscopy in KIK-IRPA. Two layers of overpaint were identified. Vibrational analyses (Raman spectroscopy) allowed the identification of the exact compounds both in the original and in the overpaint, and revealed the presence of barium sulphate, anatase, zinc dioxide, as well as chromium- and cadmium-based compounds in the overpaint. In situ Raman spectroscopy, on the other hand, was not successful. In fact, the presence of a varnish layer on top of the overpaint did not allow the recording of any signal that could be interpreted. Moreover, the original green pigment is likely to be a copper-based glaze, whose richness in oil causes severe fluorescence, which hampers the in situ Raman spectroscopic identification of such materials.

The presence of elements such as Ti, Cr, Zn, Cd, Ba, etc. in the pigmented layers suggests the use of modern pigments. The identification of overpaints on an Early Netherlandish masterpiece can be successfully performed by means of non•invasive in situ analysis based on XRF. Point analyses (hXRF) were performed by the UGent team in areas indicated by the conservators as a fast and reliable method. In fact, the qualitative characterization of the elements present in the paint layers pointed out for the first time the presence of modern pigments (Shackley 2010). In a later phase, the

89 acquisition of MA-XRF images of elemental distribution confirmed the presence and extent of modern overpaints. Finally, the study of the overpaint was completed by studying a stratigraphic sample (cross section), which confirmed and complemented the X-ray fluorescence data by providing Raman spectroscopic identification of the compounds in the original and overpaint layers. On the basis of the identified materials, the overpaints on the green vest of the Cumaean Sybil could not have been made before 1923 (Bell et al. 1997, Eastaugh et al. 2008). This date is based on the Raman identification of anatase in the overpaint layer, which was not commercially available as a pigment until then, according to the information provided in Table 3.1. This proposed date is consistent with the photographic evidence presented in Figure 3.8, showing differences in the shape of the folds between 1902 and 1937. As shown by this case study, portable instruments can be the most suitable option for large objects (i.e. too big to be analysed in a bench-top instrument), or for materials that cannot leave their position for many reasons (size, fragility, cost of transportation, safety, etc.). The availability of different instrumental setups (hXRF, MA-XRF) makes this non-invasive approach flexible and applicable to a variety of samples, and provides at least two different types of information. On the one hand, simple point analyses can be of interest for the study of selected details, while on the other hand, a more elaborated motorized setup and dedicated data processing (Alfeld et al. 2011, Alfeld 2013, Alfeld and Broekaert 2013) can provide elemental distribution images. The combination of the two approaches proved useful in the identification, study and removal of the overpaints on the outer wings of the Ghent Altarpiece. The next part of this chapter will discuss the applicability of a non-destructive approach to achieve quantification of samples of inorganic pigments. The used technique is total-reflection XRF (TXRF), and a sample, although minuscule, is required for that purpose.

This Sub-chapter begins with a brief description of the differences between conventional XRF and total-reflection XRF (TXRF), and then it moves on to a summary of the peculiar aspects of applying TXRF for the study of pigments (Paragraph 3.4.1).

90 Detector

Secondary fluorescence Totally-reflected X-ray source X-ray beam

Sample carrier

Figure 3.11: Scheme of TXRF measurement geometry. The incident X-ray beam is totally reflected by the optically flat sample carrier. The detector for the secondary fluorescence emitted by the sample is placed above the sample carrier.

In total-reflection XRF, the sample is excited under total-reflection conditions (Figure 3.11). The X-ray source is monochromatized and collimated so that it can be totally reflected by the sample support. The sample happens to be positioned in the region where the incoming and totally reflected radiation interfere, generating a standing wave on top of the support. This doubled excitation of the sample produces an increase of four times in the emitted secondary fluorescence, which is detected by a solid state detector placed on top of the sample (Figure 3.11). This particular geometry (0-90°) is different from the conventional ED•XRF (45-45°), as shown in Figure 3.3, and from the one found in handheld instruments, such as the InnovX Delta used in this thesis (see Sub-chapter 3.3). To obtain an optimal sample excitation, the sample has to be as close as possible to a mono-layer structure (i.e. evaporated diluted liquids, sparse minute particles), allowing for a simplified quantification of major, minor and trace elements, with lower limits of detection (LLD) in the range of ppb (Moens et al. 1994, Klockenkämper 1997, von Bohlen 2004, Klockenkämper and von Bohlen 2014, Coccato et al. 2016b).

91 The detection of key elements to identify pigments is typically used in the field of archaeometry (see Tables 3.1 and 3.2), although the results of elemental analyses need to be interpreted with care (see Sub-chapter 3.2). On the other hand, quantification based on XRF results is often hampered by matrix effects and is affected by the penetration power of X-rays. Contrarily to the non-invasive hXRF approach discussed so far (Sub•chapter 3.3), for the application of total-reflection XRF (TXRF) sampling is required, but this nonetheless permits quantitative studies, as well as a multi•technique study of the sample, which allows the optimization of the obtained information from the unique sample, which is not consumed. Moreover, because only an extremely small amount of material is required to allow for unconventional sampling, this approach is easily applicable to works of art, and well accepted by curators and conservators of museums, libraries and archives (Figure 3.12). Care needs to be taken so that the sample comes from a single paint layer, to facilitate interpretation of the results. Even though the conventional stratigraphic

Figure 3.12: Sampling with a cotton swab performed by a conservator of the Departamento de Conservação e Restauro of Universidade Nova de Lisboa on the Altarpiece of the Cathedral of Funchal, Portugal, during the conservation treatment of the painting (October 2013). Photo UGent (Raman Spectroscopy Research group).

92 Figure 3.13: a): Pigment particles collected between the cotton fibers of a dry cotton swab gently rubbed on an exposed paint layer; b): the cotton swab is tapped gently onto the support for the analysis. Photo UGent (Raman Spectroscopy Research group). sample allows for micro-analytical techniques, such as micro-XRF and/or synchrotron radiation (SR-) XRF analysis, and maintains the layer information, as far as the instrumental spatial resolution allows it, the mentioned non•conventional sampling technique can provide suitable samples for qualitative as well as quantitative characterization of the materials, ranging from major to trace elements (ppb). The detection and quantification of minor and trace elements in paint layers by means of TXRF has already proved effective in discriminating between different manuscript illumination ateliers (Vandenabeele et al. 1999). However, although scientific literature about TXRF applications mentions the possibility of powder analysis (Injuk and Van Grieken 1995, Klockenkämper 1997, Cantaluppi et al. 2013, Klockenkämper and von Bohlen 2014), in practice the studied solid materials are often available in large amounts (ca. 0.5-2 mg), which is not the case when dealing with Cultural Heritage materials. In those cases, dedicated procedures to create a suitable sample for analysis are requested: for example, study of aerosol (Injuk and Van Grieken 1995), of archaeological ceramic samples • digested to obtain their quantitative composition (Cariati et al. 2003, Horcajada et al. 2014), etc.. In practice, the typical sample for TXRF analysis of pigments is collected by rubbing a dry cotton swab on a paint layer (Figure 3.12). The amount of sample is extremely small and trapped between the cotton fibres (Figure 3.13, a). Gentle tapping transfers some of the sample from the cotton swab to the disc (Klockenkämper and von Bohlen 2001, 2014, von Bohlen 2004) (Figure 3.13, b). A recent study titled “Multielemental analysis of powder samples by direct measurement with TXRF” (Cantaluppi et al. 2013) tackled the issue of studying powders, but the specific context of archaeometrical research does not allow for a direct transfer of the methodology. It seems interesting now to point out what are the critical points of TXRF analysis of pigment particles, by commenting the

93 preparation of the sample for analysis, when the sample is environmental (NIST SRM 1648 - Urban Particulate Matter):

“As sample carrier, a double sided adhesive carbon tab of 12 mm diameter and 100 µm thickness (sulfur-free conductive carbon disk C272 for SEM analysis), was placed on the quartz carrier (diameter 30 mm) used for TXRF analysis. The samples in powder form were put for the analysis on the tab using Teflon® tools, distributing homogeneously the sample in a 5 mm spot at the center of the adhesive tab and accurately weighed with analytical grade balance.” (Cantaluppi et al. 2013)

When comparing the published method with the real case of pigment powders, some problems can be highlighted. First, the use of double sided tape should be discouraged, as it might irreversibly contaminate the sample, hampering any further analysis. Also, it complicates the deposition of the sample when it is trapped in a cotton swab (no Teflon instruments can be used in our case). Also, the microscopic size of the few deposited particles does not allow for weighing, which has an impact on the quantification step, as will be clarified later (Paragraph 3.4.2). Next to this, some considerations need to be put forward with respect to the study of paint samples. Paint, as explained in Sub-chapter 2.2 is a complex mixture, prepared in the painter’s workshop by mulling together pigment and binder, to create paint pots, to be diluted, adjusted in shade (Moens et al. 1994), and applied on the prepared support to create illusionistic effects. This implies, on the one hand, that a specific paint pot should have a characteristic trace element pattern, while, on the other, the artistic expression required the adjustment of the shade by addition of other pigments, which alters the trace element pattern. Moreover, the paint layer, once fluid, is now a solid film. This complexity is inevitable, when studying a masterpiece hundreds of years old, and is related as well to the concept of representativity of sampling (Klockenkämper et al. 1993, Moens et al. 1994, Miliani et al. 2010). This study was originated as well in the context of the GOA project “Archaeometrical study of the Ghent Altarpiece” (Sub-chapter 3.3), with the aim of trying to establish a relative chronology both for the original layers, created by Jan and Hubert Van Eyck, and their collaborators in the workshop, and on the different overpaint campaigns. Due to the extent of the overpaints, the study begun with that focus. It appeared that the overpaint was actually a multi-layered system. The sampling (coinciding in this case with the removal of the overpaints by conservators) was then performed with a scalpel, under the microscope, and the removal of paint flakes was precisely documented by means of

94 macrophotographs. This ensured for a detailed knowledge of the samples. However, the simultaneous presence of different pigments in each flake could not be overcome. On the one hand, it was suggested to increase the number of minimally•invasive samples to be collected and studied, in order to be able to perform statistical data treatment on the data (Moens et al. 1994), which could help in a deeper understanding of the used mixtures, and on their trace element patterns. On the other hand, it was proposed to select single-layered flakes under the microscope, to isolate pigment particles from paint flakes, or even from cross sections (in both cases under the microscope), either mechanically by using needles, or chemically by dissolving the polymerized binder, and to perform the analyses on single microscopic grains appropriately selected, transported and correctly placed on a sample carrier for analysis. Unfortunately, neither of the options could be tested further. In the first case, the time required for conservators to sample extensively, yet carefully under the microscope, by using a very sharp blade on a painting by the Van Eyck brothers, while documenting each area of interest, was incredibly long, conflicting with the strict constraints and deadlines of the conservation project. In the latter case, it appeared that the handling steps required in order to obtain the final sample for analysis would have introduced a broad range of unknown parameters having uncontrollable effects on the results of the measurements, especially as TXRF allows trace and ultratrace analyses. Also in this case, the time needed for obtaining the sample would increase noticeably. So far, the study of paint samples coming from oil paintings still poses many challenges, contrarily to the case of manuscripts, where TXRF is straightforwardly applied (van Hooydonk et al. 1998, Vandenabeele et al. 1999, Wehling et al. 1999, Klockenkämper et al. 2000). However, it seemed interesting to study all the steps leading to quantification of the untreated pigment powders, deposited by tapping. Preparing the measurement supports for the analysis, transferring the sample, setting up the quantification procedure are all explored in the next parts of this chapter, as a preliminary study in view of future developments for paint flakes analysis (Paragraph 3.4.2). Particular attention is given to the fact that the sample is in the form of a powder, and that it is potentially unique, so it should not be destroyed (in contrast with the traditional approach for TXRF analyses, which is optimized for liquid samples). On the other hand, when we consider the flow chart proposed in the introduction of this thesis (Figure 1.2), it is suggested that a destructive analysis is performed after the non-destructive ones. In this case, however, the sensitivity of

95 the TXRF technique excludes any unnecessary handling of the sample, as it can introduce impurities that can completely falsify the result of the analysis. The next part of this Sub-chapter (Paragraph 3.4.2) is based on “Pigment particles analysis with a total reflection X-ray fluorescence spectrometer: study of influence of instrumental parameters”, by A. Coccato, B. Vekemans, L. Vincze, L. Moens, and P. Vandenabeele, published in the inArt2016 Topical Collection of Applied Physics A (Coccato et al. 2016b).

Total reflection X-ray fluorescence (TXRF) analysis is an excellent tool to determine major, minor and trace elements in minuscule amounts of samples, making this technique very suitable for pigment analysis. Collecting minuscule amounts of pigment material from precious works of art by means of a cotton swab is a well-accepted sampling method, but posing specific challenges when TXRF is to be used for the characterization of unknown materials. TXRF is based on irradiating material, that is applied on a flat sample carrier, e.g. a quartz disc, with grazing incidence conditions obtained with a 0-90° excitation geometry, causing total reflection of the incident monochromatic X-ray beam (Figure 3.11) (Yoneda and Horiuchi 1971, Aiginger and Wobrauschek 1974). As a result, the spectral background is dramatically reduced so that trace elements down to ppb level of concentration can be detected (Yoneda and Horiuchi 1971, Klockenkämper 1997). Quantification is greatly simplified, as the thin layer approximation can be applied (Klockenkämper et al. 1993, von Bohlen 2004). Major, minor and trace elements concentrations can be obtained in a minute amount of sample (Klockenkämper 1997, Klockenkämper and von Bohlen 2014). Typical fields for application of TXRF are environmental, geological, biological and medical sciences, and the semiconductor industry (Wobrauschek 2007, Ying 2015). Since more than two decades, TXRF is also used for archaeometrical investigations (Klockenkämper et al. 1993, 2000, Moens et al. 1994, von Bohlen and Meyer 1997, van Hooydonk et al. 1998, Wehling et al. 1999, Vandenabeele et al. 1999, von Bohlen 2004, Civici et al. 2005, Fernández•Ruiz and García-Heras 2007, Fernández-Ruiz and Garcia-Heras 2008, Vázquez et al. 2008, Domingo et al. 2012, Bonizzoni et al. 2013). Elemental analysis of inorganic pigments helps in better understanding of the painter’s palette, concerning both pure pigments and complex mixtures; later interventions could be identified thanks to the presence of key elements related to pigments which are incoherent with the chronological attribution of a work of art

96 (some examples are zinc and titanium white (Table 3.1, Vázquez et al. 2008)). As a result of correct pigment identification, more detailed chronological information can be achieved, as well as a deeper insight into potentially harmful degradation processes of sensitive pigments (Moens et al. 1994, Klockenkämper et al. 2000, Vázquez et al. 2008; Sub-chapter 2.1). Various elemental and molecular analytical techniques are now established in the field of archaeometry. Portable instruments for non-destructive analysis are now available for many techniques (Lauwers et al. 2014, Van De Voorde et al. 2014, Brunetti et al. 2016). For TXRF analysis, however, only a minuscule sample is required. Pigment sampling for dry TXRF analysis of particles can be carried out in a virtually non-destructive way, by means of carefully rubbing with a dry cotton swab on a non-varnished surface (Klockenkämper et al. 1993, Vázquez et al. 2008). This sampling is different from the traditional scalpel, or needle, based acquisition. It can only be performed on unvarnished surfaces to ensure that pigment material is actually collected (Figure 3.13), leaving no visible trace on the object (Moens et al. 1994). Collecting only micrograms of material on the cotton swab, even lower amounts (ng) are transferred on the sample carrier in a next step (Klockenkämper et al. 1993, 2000). Von Bohlen (2004) and Klockenkämper et al. (2000) report that a simple and accurate quantification is possible, as long as the deposited weight of the particles does not exceed 50 μg in total and the particle size is only a few μm. Sampling with a cotton swab allows for multiple sample collection, improving the representativity of the analysis (Klockenkämper et al. 1993, Moens et al. 1994). Curators accept such sampling, but still these rare samples are very precious in that respect. Obtaining microscopic pigment particles on a cotton swab poses some specific challenges, when characterizing the used pigments and mixtures using the TXRF methodology (Klockenkämper et al. 1993, Moens et al. 1994, Klockenkämper 1997, van Hooydonk et al. 1998, Klockenkämper and von Bohlen 2014). Considering the very small amounts of material to be characterized with the TXRF method, one needs to take extra care to prepare the necessary “clean” sample carriers. In this respect, the determination of instrumental (spectroscopical) artifacts was necessary. During this period of study, the manufacturer of our G.N.R. TX2000 TXRF spectrometer altered its sample chamber, so that the influence of this instrumental change could also be evaluated. In addition to the qualitative identification of the pigments, (semi)quantitative information can be obtained as well from the spectroscopical data. In order to verify the commercial quantification software SinerX delivered with the G.N.R. TX2000 spectrometer, a quantification scheme was set up for convenience, as reported by (Klockenkämper et al. 1993).

97 The peak and background intensities for these calculations were obtained by means of AXIL software (Vekemans et al. 1994). The transfer of the precious material from the cotton swab right at the correct position on the sample carrier, so that the incoming X-rays can optimally excite it, is a major concern. Because the actual physical deposition of particulate material from the cotton swab to the sample carrier is not fully controllable, the imperfect positioning on the sample carrier was subject to study, in order to better understand the influence it may have on the TXRF results. The final goal of the evaluation of all the steps described above helps to better understand the influence of the different experimental parameters when performing TXRF investigations of pigment particles sampled with cotton swabs, especially when the traditional liquid sample preparation methods conventionally used in TXRF analysis are not applicable (Dargie et al. 1997).

The TXRF measurements were performed with a state-of-the-art G.N.R. TX2000 total reflection X-ray fluorescence spectrometer (Figure 3.14). The selected monochromatic X-ray source is the result of monochromatizing (Si/W multilayer reflector) the Mo-Kα line from a long fine-focus (0.4 × 12 mm) high power (3kW) Mo/W anode. The characteristic radiation induced in the sample is collected using a Peltier cooled silicon drift detector (SDD) with 30 mm2 active area. The energy resolution for Mn-Kα is 140 eV. A sample carousel allows automated batch measurements of 12 samples. All measurements were performed operating the tube at 40kV/30mA, with 1000 seconds measurement time. It should be noted that during the period of this study, G.N.R. replaced the measurement chamber in order to improve the measurement conditions. The software package SinerX, delivered with the instrument, was used for data processing of TXRF spectra. The availability, and use, of “clean” sample carriers seems trivial for a TXRF methodology, but the delivery of batches of effectively clean sample carriers may not be such a trivial task in practice for an instrument that promises ppb levels of limit of detection. In order to reveal the power of detecting trace elements, the NIST SRM 1640a (trace elements in natural water) was measured. The procedure to obtain clean quartz sample carriers used in this work is described by Klockenckämper for the cleaning of digestion vessels (Klockenkämper 1997). The use of a subboiling device (vessel cleaner by ANALAB Sàrl, France) allows the preparation of vast numbers of carriers in an unsupervised way, and with reduced reagent consumption.

98 Figure 3.14: G.N.R. TX2000 total-reflection X-ray fluorescence spectrometer. Photo UGent (Raman Spectroscopy Research group).

The preparation steps are as follows: 1) any residue of previous analysis is removed with paper and Disinfectol®; 2) the sample carriers are inserted in a specifically designed 24-slots Teflon holder (Klockenkämper 1997, Klockenkämper and von Bohlen 2014), which is suited for use in combination with a perfluoroalkoxy alkanes (PFA) subboiling device; 3) the sample carriers are exposed to suprapure nitric acid vapours, obtained by setting the heating unit to 145 °C; 4) the sample carriers are rinsed with MilliQ water; 5) the sample carriers are exposed to MilliQ water vapours, obtained by setting the heating unit to 115 °C; 6) same as step 4; 7) the sample carriers are vacuum dried on their support; 8) finally, they are transferred to Petri dishes for temporary storage. For each batch, one sample carrier was prepared with a 10 μL droplet of a diluted multielement standard, and measured together with the batch. The influence of the cleaning procedure duration was investigated by cleaning batches of sample carriers with varying treatment times for steps 3 and 5 (12, 2 or 1 hour each, for a total cleaning time of, respectively, 24, 4 or 2 hours). After this procedure, each batch of sample carriers was measured five times each for 1000 seconds.

99 When preparing multielement standard solutions (Multi Element Standard Solution IV, Merck, Germany), and the NIST SRM 1640a (trace elements in natural water) samples, a surfactant (Triton) was added to prevent the 10 μL drop from running on the sample carrier, and to ensure it will dry in a circular shape (ca. 8 mm in diameter) in the centre of the sample carrier (Stosnach 2006, Towett et al. 2013, Klockenkämper and von Bohlen 2014, Marguí et al. 2014). A blank solution containing Triton was prepared and studied to check it was not adding unwanted elements to the samples under study. Lower limits of detection (LLD) were calculated from a 1 ppm multielement standard solution and the NIST SRM 1640a measured for 1000s (5 repetitions), according to the following formula (Klockenkämper 1997)

퐶푗 √퐼푏푘푔,푗 퐿퐿퐷푗 = 3 ∗ 퐼푗

where Cj is the concentration of the selected element j, with Ibkg,j and Ij, respectively, the background and the net peak area, as obtained by AXIL (Vekemans et al. 1994). Next to the use of the commercial SinerX software, delivered with the TX2000 instrument, the following quantification calculations were performed. Instrumental relative sensitivities (Rj,IS) needed to be calculated from multielement standard analysis. Rj,IS, were obtained from the known elemental concentrations Cj in the multielement standard and the measured peak intensities Ij for each element j, referred to an internal standard element (IS), for example Fe or Ga, according to the formula (Klockenkämper 1997)

퐼푗 ∗ 퐶퐼푆 푅푗,퐼푆 = 퐼퐼푆 ∗ 퐶푗

From the derived sensitivity curves for K- and L-lines, the relative sensitivities for elements not present in the multielement standard could be obtained by interpolation. Once the Rj,IS are calculated, quantification was performed by applying the following formula, as reported in (Klockenkämper 1997)

퐼푗 ∗ 퐶퐼푆 퐶푗 = 퐼퐼푆 ∗ 푅푗,퐼푆

where CIS can be either the concentration Figure 3.15: Dots of commercial of the added internal standard (e.g. Ga), or eyeliner positioned at 0, 2 and 4 the arbitrary concentration given to a mm (left to right) from the centre of present element, which acts as an internal the support.

100 standard (e.g. Fe = 1 ppm). This latter approach, referred to in this work as Qcalc, was used in this study both for the NIST SRM 1640a and for pigment particles, to obtain semiquantitative information (Klockenkämper 1997, Klockenkämper and von Bohlen 2014). In order to be able to compare the results of the SinerX processing, and of Qcalc with the certified values of NIST SRM 1640a water sample, concentration ratios to Fe were calculated. The correct positioning of microscopic particles on the sample carrier for TXRF analysis is not trivial and it seemed interesting to study. Conventional liquid samples, spiked with a surfactant, or when applied on adequately prepared sample carriers, can be easily positioned in the centre of the sample carrier and dry in a circular shape (Klockenkämper and von Bohlen 2014). On the contrary, particles suitable for TXRF analysis pose some issues, and it seemed relevant to investigate the measured area of the sample carrier. The sampled pigment particles are not visible by naked eye, and they are trapped between the fibres of the cotton swab. Also, because the size of a droplet of surfactant-containing multielement standard as small as 10 μL is extremely hard to control, even after correct pipetting on the centre of the sample carrier, it was decided to use a material, such as a commercial felt-tip eyeliner (Cosnova, Frankfurt, Germany), with which samples of controllable size (ca. 1 mm dots) and position (0, 2 and 4 mm from the centre of the round sample carrier) could be achieved (Figure 3.15). Each prepared sample carrier was also rotated in four steps of 90 ° (n=3 for each position), to take into account instrumental geometry effects. As a final test, samples of Kremer (Germany) pigment particles were prepared by means of dry cotton swabs (Deltalab, Spain), transferred on a clean sample carrier, and immediately analysed. Following the data presentation commonly reported in archaeometrical literature regarding TXRF analysis of pigments, the obtained concentration values as ppm from SinerX and Qcalc were easily converted into percentages to the total of detected elements by normalization to 100%, as no internal standardization can be performed in the case of pigment particles analysis (Klockenkämper et al. 1993, 2000, Klockenkämper 1997, von Bohlen and Meyer 1997, van Hooydonk et al. 1998, von Bohlen 2004, Civici et al. 2005, Klockenkämper and von Bohlen 2014).

After applying the cleaning procedure based on the use of a subboiling device (Klockenkämper 1997, Klockenkämper and von Bohlen 2014), contaminants such as K, Ca, Fe, Ni, Cu, Zn, Pb might still appear. Ni, Cu and Zn are of instrumental origin, while the other elements come from external contamination/handling.

101 Multiple sets of sample carriers cleaned for 24, 4 or 2 hours in total (step 3 + step 5, see above) were measured before chamber replacement, and the obtained intensities are shown in Figure 3.16. The results of the shortest cleaning procedure are comparable with the longest one, on the condition that sample carriers showing potential contamination are removed from further use. Contaminated sample carriers appear randomly, independently to the total cleaning time, in all the cleaned batches, and are most likely related to inaccurate handling of the sample carriers after cleaning and/or accidental deposition of dust particles. A selection procedure was still needed, as no cleaning procedure could ensure a 100 % success rate in assuring clean sample carriers. Figure 3.16 includes as well the unsuccessful cleaning of a sample carrier cleaned for a total of 4 hours (Ca contamination estimated to approximately 2 ppm). During this study, the measurement chamber was replaced, causing a decrease

Sample carriers, 24h cleaning Sample carriers, 4h cleaning Sample carriers, Sample carriers, 2h cleaning 4h unsuccessful cleaning Figure 3.16: Effect of duration of cleaning time. Observed intensity of residual elements after cleaning for different times (step 3 + step 5 = 24, 4 or 2 hours), measured 1000 seconds, as

a function of cleaning time, versus the atomic number. The standard deviation sj is shown for each cleaned batch, measured five times. All the measurements were performed before

chamber replacement. One standard deviation sj is indicated.

102 Sample carriers, Triton Sample carriers, 2h cleaning 2h unsuccessful cleaning Figure 3.17: Study of the effect of surfactant (Triton). Observed intensity of residual elements after cleaning for 2 hours (step 3 + step 5), and of a blank solution of surfactant (Triton). One

standard deviation sj for five repeated measurements of 1000s is given. in Cu interference and the simultaneous slight increase of the signals of Ni and Zn (Table 3.3), related to the detector design. No further tests were performed in the old setup, i.e. prior to chamber replacement. In the present conditions, Ni, Cu and Zn remain stable and can be considered as instrumental background, corresponding to contamination levels of approximately 50-100 ppb of Ni. Ca and Fe are, on the other hand, related to the cleaning procedure or to handling. As regards the cleaned sample carriers, Figure 3.17 shows the intensities and Figure 3.18 the spectra of sample carriers cleaned for a total of 2 hours. The Fe concentration on a sample carrier showing contamination could be estimated in the range of few hundreds ppb. LLD below 10 ppb are observed for elements between Cr and Sr (24

103 Sample carriers, 2h cleaning

Triton Sample carriers, 2h unsuccessful cleaning

Figure 3.18: Study of the effect of surfactant (Triton). Observed spectral signal of residual elements after cleaning for 2 hours (step 3 + step 5), and of a blank solution of surfactant (Triton). A clean sample carrier (full line), a cleaned sample carrier showing Fe contamination (dashed line), and a blank (Triton, dotted line) are shown.

Table 3.3: Effect of chamber replacement on the intensity of some elements on cleaned sample carriers (five repetitions, 1000 seconds), for 4 and 2h total cleaning time.

Ni-Kα Cu-Kα Zn-Kα average average average s s s counts j counts j counts j 4 h, before chamber replacement 78 15 251 64 5 6 4 h, after chamber replacement 409 93 63 8 122 10 2 h, before chamber replacement 42 16 180 28 19 23 2 h, after chamber replacement 266 12 68 24 136 24

As an example of the selection of clean sample carriers, being the LLD for Fe = 7 ppb, the contaminated sample carrier shown in Figure 3.17 and Figure 3.18 has to be discarded from further use, being its estimated Fe content approximately few hundreds ppb. From a 10 ppm multielement standard spectrum, it was also possible to calculate the relative sensitivities values (K- and L-lines) relative to Fe and Ga for the elements of interest (19 < Z < 38; 48 < Z < 95) based on their concentration in the standard and on the recorded net intensities (Figure 3.20) (Klockenkämper

104 1997, Klockenkämper and von Bohlen 2014). Interpolation was performed to obtain information on the elements not present in the multielement standard. Based on the Qcalc method, spectra of the NIST SRM 1640s water sample were processed to obtain the concentration values of the elements of interest, based on a given value of Fe = 1 ppm. Figure 3.21 shows the quantification results for both methods, when no internal standard is added, and Fe, originally present in the sample, is given a value of 1 ppm in order to perform the calculations. The SinerX based quantification overestimates strongly all the elements present, except potassium, while Qcalc yields results closer to the expected values, with some exceptions. Ca is not optimally estimated, as expected from its low relative sensitivity to Fe. V is not considered, as it is present at the LLD level. For Ni, Cu and Zn, it is important to remember that some instrumental background contributes to their signal, making their quantification inaccurate when they are present at the tens of ppb level, as in the present case.

Multi Element Standard solution IV NIST SRM 1640a (1 ppm) Figure 3.19: Calculated LLD (ppb), based on the equation in the text, for 1000s measurement time. The given data correspond to the calculated LLD from a 1 ppm multielement standard solution (full circles), and to the calculated LLD for the studied NIST 1640a water sample (empty circles). For the multielement standard elements not shown (L-lines), the LLD are: Cd = 52 ppb, Ba = 24 ppb, Tl = 5 ppb, Pb = 4 ppb, Bi = 4 ppb. For the NIST SRM 1640a elements not shown (L-lines), the LLD are: Ba = 36 ppb, U = 2 ppb.

105 However, when using the Qcalc method, it should be possible to correct for such a problem, so that the correct concentration is calculated. As regards the quantification of elements whose L-lines are present in the spectrum, Cd is not quantified, as it is present below LLD. Ba quantification is problematic with both methods. Pb, although present in detectable concentration levels, could not be identified in the spectra processed with AXIL. Finally, both methods overestimated the content of U. It appears that the standardless analysis gives better results when Qcalc is used. In general, a different excitation source (for example, W), and the selection of a multielement standard containing different elements might be successfully used to improve the results of quantification for elements such as Cd, Ba, and for other elements Z > 80. Concerning the effect of sample positioning on the quantification results,

Ri,Fe K-lines Ri,Ga K-lines

Ri,Fe L-lines Ri,Ga L-lines Figure 3.20: Calculated relative sensitivities (to Fe and Ga, full and empty symbols, respectively) for elements of interest present in a 10 ppm multielement standard solution (1000s measurement time). A: K-lines, B: L-lines.

106 Qcalc, SinerX, Concentration ratio to Fe Fe = 1 ppm Fe = 1 ppm (NIST SRM 1640a) Figure 3.21: Comparison of elemental concentration ratios to Fe for NIST 1640a water sample, using Fe as an internal standard. The calculated elemental ratios via both methods are

compared to the reference material data. One standard deviation sj is indicated.

Figure 3.22 shows the elemental counts ratio between Fe and Mn, used to evaluate the measured area of the sample carrier. The lowest relative standard deviation of the ratio sFe/Mn (n=3), which gives an indication of the precision of the measurement, is observed for centred samples (4 %), while it increases to 14 % at 2 mm and to 17 % at 4 mm from the centre. On the other hand, the effect of rotations (0, 90, 180, 270 °) is not clear. Elemental ratios are still acceptable for samples placed in an 8 mm ⌀ area in the centre of the sample carrier, even though the relative standard deviation increases with distance from the centre. Thanks to the small particles’ size and the low density of pigment particles on the sample carrier surface, it can be considered that matrix effects are strongly reduced. No internal standardization can be applied to such pigment samples, as the exact weight of the analysed sample cannot be determined (Klockenkämper et al. 1993, 2000, Moens et al. 1994, van Hooydonk et al. 1998). However, a relative quantification is possible. This is an approximation, as matrix effects are absent only for very fine (0.1 μm) grains and for small amounts of material on the sample carrier surface (< 10 μg cm-2). Moreover, the quantification

107 0 mm 2 mm 4 mm

Figure 3.22: Study of the effect of distance from the centre and angle. Fe/Mn intensity ratio of commercial eyeliner when measuring ca. 1 mm diameter dots at angles of 0, 90, 180, 270 ° (from a reference position) as a function of distance from the centre of the sample carrier.

The standard deviation sj was obtained by three repeated measurements of the same sample carrier. excludes the low-Z elements, as they are not detected, and the given results are relative to the total of detected elements (Klockenkämper et al. 1993, 2000, Klockenkämper 1997, von Bohlen and Meyer 1997, van Hooydonk et al. 1998, von Bohlen 2004, Civici et al. 2005, Klockenkämper and von Bohlen 2014). Si is excluded from the calculations, as the quartz sample carrier contributes to the signal, and elements such as Sn and Sb, which are relevant for the study of pigments, are also not considered at this point, based on the results of multielement standard analysis with the Mo source (see Sub-chapter 3.2), as explained in the previous paragraph. In the case of pigment analysis, the quantification can only rely on relative quantification of detected elements based on one selected element as internal standard (i.e. Fe), which are then normalized to 100 %, and finally converted to percentages of detected elements (Klockenkämper et al. 1993, 2000, Klockenkämper 1997, von Bohlen and Meyer 1997, van Hooydonk et al. 1998, von Bohlen 2004, Civici et al. 2005, Klockenkämper and von Bohlen 2014). Also,

108 no certified values from the producer are available regarding the minor and trace elements in the selected pigments (http://kremer-pigmente.com/), so the comparison here regards three different samples for each selected pigment, processed via Qcalc or via the commercial software SinerX. The results (in percentages of detected elements) are summarized in APPENDIX B, Table B.1. The results of quantification on the studied pigment samples are comparable for both the Qcalc and the SinerX calculations, the greatest discrepancy between the results being ca. 5.5 % on the main element (Pb in lead tin yellow I). For the minor and trace elements, differences could be observed, for example Ca in yellow ochre, which is quantified by SinerX as twice the amount resulting from calculations, or of many impurities in magnetite, not identified by the commercial software. The qualitative identification of pigments based on the presence of key elements can successfully be performed, on the condition that conclusive identification is well supported by complementary analysis (see paragraph 3.3.1 and Chapter 5), especially in the case of lead containing yellow pigments, arsenic and copper containing pigments, and natural/artificial ultramarine. As already pointed out in the previous paragraph, as some elements of relevance in pigment analysis are not successfully excited by the Mo tube, the obtained elemental composition on some yellow pigments such as lead tin yellow types I and II (Pb2SnO4, PbSn1-xSixO3)) and Naples yellow (Pb2Sb2O7/Pb3(SbO4)2) is limited to lead, and to other detectable minor elements. In that case, as well as for arsenic based pigments (see Sub-chapter 2.1), a different approach (molecular techniques, such as Raman spectroscopy (Vandenabeele et al. 2000)) could be more suitable and give better results. Among the iron based pigments, yellow ochre and green earth showed a more varied composition, including K and Ca as well, as expected from the mineralogy of these materials.

For the ultramarine and lapis lazuli blue pigments (Na8[Al6Si6O24]Sn), as expected, no specific key elements can be identified by means of XRF based techniques. Misidentification of green pigments based on copper, or of mixtures of Cu- containing blue pigments with (organic) yellow materials, is very likely to happen, when only using elemental analytical techniques (Table 3.2). The TXRF analysis is, however, fast and does not consume material, which allows for further analysis, such as microscopy and micro-Raman spectroscopy (van Hooydonk et al. 1998, Vandenabeele et al. 1999, Wehling et al. 1999, von Bohlen 2004, Civici et al. 2005), preferably done after TXRF measurement, in order to minimize the exposure of the sample to airborne contamination and unnecessary handling.

109 The G.N.R. TX2000 instrument is a highly-performing TXRF spectrometer, showing LLD as low as 2 ppb for Sr (K-lines). The detection power of the instrument was demonstrated by analysing trace elements in natural water (NIST SRM 1640a). For the study of microscopic pigment particles sampled on a dry cotton swab, the effect of positioning is of primary importance, as deviation of the results because of mispositioning is to be expected. Elemental count ratios (Fe/Mn) of a sample deposited in a controlled way, as regards both size and position on the sample carrier, shows that the measurement precision decreases when moving away from the center, as the relative standard deviation increases from 4 % for centred samples to 17 % at 4 mm from the centre. The cleaning procedure using a subboiling device, which allows for the cleaning of vast numbers of sample carriers, requiring limited human intervention and little reagent consumption, still cannot ensure for a cleaning success rate of 100%. This requires checking the sample carriers, to identify possible contaminated ones, to be discarded based on the instrumental LLD. Moreover, from the cleaned sample carrier analysis, it appeared that instrumental interferences of Ni, Cu and Zn occur, causing overestimation of these elements when quantifying them. The final quantification test performed on Kremer pigment powders by means of Qcalc and SinerX showed that both approaches correctly quantify the samples, the greatest uncertainty being related to the correct positioning of the microscopic pigment particles. The present study allowed a better understanding of the effects of some specific instrumental parameters on the obtained TXRF results on pigment particles.

As highlighted in the two previous Sub-chapters 3.3 and 3.4, the advantages of XRF analysis for the study of Cultural Heritage objects are clear (Janssens and Van Grieken 2004, Shackley 2011, Varella 2012, Klockenkämper and von Bohlen 2014). First of all, the identification of key elements is a fundamental starting point for further analysis, as it can support the identification of pigments present in the

110 stratigraphy. Moreover, thanks to the fact that inorganic modern pigments often contain specific elements (Table 3.1) which do not overlap with the key elements of traditional mediaeval pigments (Table 3.2), the presence of overpaint can be easily ascertained. Moreover, thanks to the known production dates of the pigments listed in Table 3.1, chronological considerations can be put forward. Secondly, the availability of portable/transportable instruments makes it possible to study objects without the need of sampling. Care needs to be taken when interpreting the results, due to instrumental features, spectral interferences, and to the penetrating power of X-rays (Shackley 2010). However, differences should be highlighted between point measurements as collected by the handheld instrument (hXRF) and the visual outcome of MA•XRF scans. The ease of interpretation of the results is enhanced in the second case, although the time consumption is much higher, and dedicated softwares for data treatment are needed. In both cases, however, instrumental artifacts or interaction of the characteristic radiation with the multilayer structure of a Flemish painting (as explained in Sub-chapter 2.2) might complicate the situation. As it concerns the possibility of quantification of major, minor and trace elements in a minute sample, it can help in obtaining information on the used pigments and mixtures. Moreover, trace element analysis on paint layers is, in theory, of primary importance in order to distinguish between different paint pots, or for material provenance studies (Vandenabeele et al. 1999). However, the situation is much more complex as pigments were rarely used purely (light and shadow rendering, hues), so that the trace element pattern is complicated by the fact that paint layers are complex mixtures, which means that trace elements from each pigment contribute to the overall pattern. Moreover, the use of “dirty” tools and recipients by the artist himself could even release spurious elements in the paint. The binders and varnishes were commonly mixed with siccatives and additives. Moreover, migration of ions is now recognized between paint layers, and it can never be assured that, during the sampling, a single paint layer is sampled, and obtaining appropriate material for the analysis requires substantial handling of the sample, leading to increased risk of contamination. All these aspects make the analysis of oil paint layers at the trace element level extremely challenging, as highlighted by the observations at the end of paragraph 3.4.1. To conclude, XRF techniques are powerful tools to perform the identification of pigments on the basis of the presence of key elements. However, care needs to be taken when interpreting XRF results, especially if no other data are available. In fact, for in situ analyses, the information comes from a multitude of superimposed paint layers (bulk analysis), and in general no information on the low Z elements can be collected. Moreover, the overlap of emission lines from the

111

elements in the sample, instrumental artefacts and limitations require additional care in the interpretation of the results. Therefore, neither the identification of carbon-based black pigments, organic dyes, ultramarine, nor the characterization of some classes of pigments, like the copper containing greens is achievable. These materials actually play an important role in the study of works of art, as they can inform us on the skills and preferences of the painter. In the next Chapter 4, special attention is given to black and green compounds. The availability of recipes for producing various carbon blacks suitable for the use as pigments contrasts with the limited range of traditionally mentioned green materials. In the first case, the carbonaceous black pigments identified on works of art is simply referred to as “carbon black” (with the only exception of bone black, thanks to the presence of phosphates), while in the second case, chemical analyses on polychrome objects revealed the use of a broad range of pigments, which does not correspond to the written sources (possibly as a matter of terminology). Carbon-based blacks and green pigments require a deeper molecular/ structural comprehension, for which the use of different, still non-destructive and potentially portable, analytical techniques is necessary. On the one hand, Raman spectroscopy is chosen, and on the other, an interdisciplinary literature review is attempted, to shed light on nomenclature issues, as will be shown in the next Chapter (Paragraphs 4.3.1 and 4.3.2). Moreover, Raman spectroscopy is used for monitoring pigment degradation in simulated museum environments (Paragraph 4.3.3), and for the in situ study of Flemish panel paintings (Paragraph 4.4.1).

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115 Paternoster, G., R. Rinzivillo, F. Nunziata, E. M. Castellucci, C. Lofrumento, A. Zoppi, A. C. Felici, G. Fronterotta, C. Nicolais, M. Piacentini, S. Sciuti, and M. Vendittelli. 2005. Study on the technique of the Roman age mural paintings by micro-XRF with Polycapillary Conic Collimator and micro-Raman analyses. Journal of Cultural Heritage 6:21-28. van Pevenage, J., D. Lauwers, D. Herremans, E. Verhaeven, B. Vekemans, W. De Clercq, L. Vincze, L. Moens, and P. Vandenabeele. 2014. A combined spectroscopic study on Chinese porcelain containing ruan-cai colours. Analytical methods 6:387-394. Price, T. D., and J. H. Burton, editors. 2011. An introduction to archaeological chemistry. Springer, Amsterdam, The Netherlands. Ramos, P. P. M., and I. Ruisánchez. 2006. Data fusion and dual-domain classification analysis of pigments studied in works of art. Analytica Chimica Acta 558:274-282. restoration/REVELATION - The exterior wings of the Ghent Altarpiece. 2016. . Röntgen, W. 1896. On a new kind of rays. Science 3:227-231. Rosier, F., H. Dubois, G. Van der Snickt, J. Sanyova, A. Coudray, C. Glaude, K. Janssens, D. Lauwers, A. Coccato, and P. Vandenabeele. 2016. Identification of restorations on the Cumaean Sibyl of the brothers van Eyck’s Ghent Altarpiece (1432). Page 103. 4th International Congress Chemistry for Cultural Heritage. Rousaki, A., A. Coccato, C. Verhaeghe, B.-O. Clist, K. Bostoen, P. Vandenabeele, and L. Moens. 2016a. Combined Spectroscopic Analysis of Beads from the Tombs of Kindoki, Lower Congo Province (Democratic Republic of the Congo). Applied Spectroscopy 70:76–93. Rousaki, A., P. Vandenabeele, J. Sanyova, H. Dubois, and L. Moens. 2016b. A protocol to evaluate how good varnishes protect silver gilding against atmospheric pollutants. Page 81. 4th International Congress Chemistry for Cultural Heritage, Brussels, Belgium. Sawczak, M., A. Kamińska, G. Rabczuk, M. Ferretti, R. Jendrzejewski, and G. Śliwiński. 2009. Complementary use of the Raman and XRF techniques for non-destructive analysis of historical paint layers. Applied Surface Science 255:5542-5545. Schmidt, B. A., M. A. Ziemann, S. Pentzien, T. Gabsch, W. Koch, and J. Krüger. 2016. Technical analysis of a Central Asian wall painting detached from a Buddhist cave temple on the northern Silk Road. Studies in Conservation 61:113-122. Settle, F. A., editor. 1997. Handbook of instrumental techniques for analytical chemistry. Prentice Hall PTR, Upper Saddle River , New Jersey, USA. Shackley, M. 2010. Is there reliability and validity in portable X-ray fluorescence spectrometry (PXRF). The SAA Archaeological Record:17-20. Shackley, M. S., editor. 2011. X-Ray Fluorescence Spectrometry (XRF) in Geoarchaeology. Springer New York, USA. Van der Snickt, G., W. De Nolf, B. Vekemans, and K. Janssens. 2008. μ-XRF/μ-RS vs. SR μ-XRD for pigment identification in illuminated manuscripts. Applied Physics A 92:59•68. Van der Snickt, G., J. Dik, M. Cotte, K. Janssens, J. Jaroszewicz, W. De Nolf, J. Groenewegen, and L. Van der Loeff. 2009. Characterization of a degraded cadmium yellow (CdS) pigment in an oil painting by means of synchrotron radiation based X-ray techniques. Analytical chemistry 81:2600-2610. Stosnach, H. 2006. On-site analysis of heavy metal contaminated areas by means of total reflection X-ray fluorescence analysis (TXRF). Spectrochimica Acta Part B: Atomic Spectroscopy 61:1141-1145. Towett, E., K. Shepherd, and G. Cadisch. 2013. Quantification of total element concentrations in soils using total X-ray fluorescence spectroscopy (TXRF). Science of the Total Environment 463-464:374-388.

116 Uda, M., G. Demortier, and I. Nakai, editors. 2005. X-Rays For Archaeology. Springer, Amsterdam, The Netherlands. Vandenabeele, P., B. Wehling, L. Moens, B. Dekeyzer, B. Cardon, A. von Bohlen, and R. Klockenkämper. 1999. Pigment investigation of a late-medieval manuscript with total reflection X-ray fluorescence and micro-Raman spectroscopy. Analyst 124:169-172. Vandenabeele, P., A. von Bohlen, L. Moens, R. Klockenkämper, F. Joukes, and G. Dewispelaere. 2000. Spectroscopic examination of two Egyptian masks: a combined method approach. Analytical Letters 33:3315-3332. Varella, E., editor. 2012. Conservation Science for the Cultural Heritage: Applications of Instrumental Analysis. Springer Science & Business Media, New York, USA. Vázquez, C., A. Albornoz, A. Hajduk, D. Elkin, G. Custo, and A. Obrustky. 2008. Total reflection X-ray fluorescence and archaeometry: Application in the Argentinean cultural heritage. Spectrochimica Acta Part B: Atomic Spectroscopy 63:1415-1419. Vekemans, B., K. Janssens, L. Vincze, F. Adams, and P. Van Espen. 1994. Analysis of X•ray spectra by iterative least squares (AXIL): New developments. X-Ray Spectrometry 23:278-285. Van De Voorde, L., J. Van Pevenage, K. De Langhe, R. De Wolf, B. Vekemans, L. Vincze, P. Vandenabeele, and M. P. J. Martens. 2014. Non-destructive in situ study of “Mad Meg” by Pieter Bruegel the Elder using mobile X-ray fluorescence, X-ray diffraction and Raman spectrometers. Spectrochimica Acta - Part B Atomic Spectroscopy 97:1-6. Wehling, B., P. Vandenabeele, L. Moens, R. Klockenkämper, A. von Bohlen, G. Van Hooydonk, and M. De Reu. 1999. Investigation of pigments in medieval manuscripts by micro Raman spectroscopy and total reflection X-ray fluorescence spectrometry. Microchimica Acta 130:253-260. Wobrauschek, P. 2007. Total reflection x-ray fluorescence analysis-a review. X-Ray Spectrometry 36:289-300. Ying, L. 2015. Low Power Total Reflection X-Ray Fluorescence Spectrometry: A Review. Advances in X-ray Chemical analysis 46:13-26. Yoneda, Y., and T. Horiuchi. 1971. Optical Flats for Use in X-Ray Spectrochemical Microanalysis. Review of Scientific Instruments 42:169-170.

117

The previous Chapter 3 described X-ray fluorescence techniques, which are very useful in archaeometry. In the proposed case studies, they allowed the identification of modern overpaint on a Flemish Primitives masterpiece, which contributed to a deeper understanding of the material history of the polyptych (including past conservation treatments), and allowing for chronological considerations (Paragraph 3.3.1). Moreover, XRF techniques can support a quantitative characterization of the samples under investigation (Sub-chapter 3.4). However, in some cases, these element-specific techniques are not able to solve the challenge on hand, and therefore the use of molecular spectroscopic techniques, such as Raman spectroscopy, can be helpful. The current chapter is dedicated to Raman spectroscopic techniques for the study of pigments and paintings. First, the Raman effect will be described shortly (Sub-chapter 4.1), followed by some consideration on the archaeometrical applications of Raman spectroscopy (Sub-chapter 4.2). The use of Raman spectroscopy in the scientific examination of materials of Cultural Heritage will be discussed next. In fact, the correct identification of materials in works of art can support authentication and chronology issues, clarify the choice of materials by the painter, and highlight specific conservation problems. Two lines are proposed. First, a laboratory one for characterizing pigments and their degradation (Sub•chapter 4.3), secondly the implementation of portable instrumentation for in situ measurements of panel paintings is proposed (Sub•chapter 4.4). Some final remarks on the advantages and disadvantages of the technique will be mentioned with specific focus to the analysis of works of art (Sub-chapter 4.5).

119 This first part of Chapter 4 begins with a brief explanation of the Raman effect. It was first discovered and described by sir C.V. Raman in 1928 (Raman 1928, Raman and Krishnan 1928a, 1928b), who was awarded the Nobel prize in Physics in 1930 (H. Pleijel 1931). During his studies on the scattering of light, Raman realized that the scattering of light is not only explainable by Tyndall, and Rayleigh effects. His first experiments were performed by using sunlight, coloured filters, and the eye to detect any change Figure 4.1: C.V. Raman and the spectrograph in colour between the incident used to collect the first Raman spectrum, and scattered radiation (Raman photographed at the Indian Association for the and Krishnan 1928a). Cultivation of Science, where he carried out his By using a mercury lamp, experiments on the scattering of light. Image whose emission was from Indian Association for the Cultivation of appropriately filtered to ensure it Science. had only one specific wavelength (436 nm), a spectrograph (Figure 4.1), and photographic plates, he confirmed that the light scattered by a medium did not correspond exactly to the initial ray. Through the spectrograph he was actually able to observe other wavelengths, not present in the original beam of light (Raman 1928, Raman and Krishnan 1928a, 1928b). Sir C.V. Raman and his colleague K.S. Krishnan proposed that the observed spectral lines originated by the interaction of an incident quantum of radiation, as a whole or in part, with molecules of a fluid (Raman and Krishnan 1928b). He then observed as well that the frequency difference between the incident and inelastically scattered light is comparable to the frequency of infrared absorption, and that different molecules showed different patterns (Raman and Krishnan 1928b, H. Pleijel 1931). He nonetheless observed that this effect is extremely weak, especially for some substances (Raman 1928). Contrarily to Rayleigh scattering, where the photons maintain their frequency, the Raman effect explains the inelastical scattering of light by molecules. For the Raman effect to happen, there needs to be an interaction between a molecule and an oscillating dipole (i.e. light), so that energy can be transferred, and the scattered photon’s energy is different from the incoming one’s (Vandenabeele 2013).

120 The incident photon carries an electromagnetic field 퐸⃗ oscillating at the frequency ν0. At the same time, molecular bonds oscillate following the spring model at a frequency vm. A molecule interacts with an electromagnetic field 퐸⃗ 0 oscillating at the frequency of light ν0, and an oscillating dipole moment 푝 = α퐸⃗ 0 is generated (Vandenabeele 2013). An indication of a molecule’s sensitivity to external electromagnetic fields is given by their polarizability α, which indicates how easily the electronic cloud is distorted. If the polarizability α of a molecule is constant (and different from 0), there is only elastic Rayleigh scattering, that means no modulation of the electric field, which keeps its frequency ν0. On the contrary, if α is different from 0 and oscillates at the molecular vibrational frequency vm, the induced dipole moment is modulated according to trigonometry (prostapheresis formulas) by the incident radiation ν0 and by molecular frequencies νm (Vandenabeele 2013):

푝 = 푝 0 (ν0) + 푝 AS(ν0 + vm) + 푝 S(ν0 - vm)

The first term of this equation describes the Rayleigh scattering, the other two the Raman effect (Figure 4.2). If the frequency of the scattered light is higher than that of the incoming photons, we talk about Stokes lines (푝 S). On the contrary, anti-Stokes lines show a lower frequency (푝 AS). The two sets of lines are symmetric around the elastically scattered radiation. In fact, in a Raman [anti•]Stokes transition, the decay from the virtual energy level happens while switching on [switching off] a molecular vibration. This implies a diminution [increase] in energy of the incoming photon. The frequency difference between the incident and scattered photon equals the vibrational frequency of the molecule vm. For convenience, the Stokes lines are usually preferred, as they are more intense, as a consequence of the Boltzmann distribution (Vandenabeele 2013). It is important to note that the incoming radiation does not excite the molecule to a “real” energetic state (electronic excited state), but to a virtual energy level, which is not stable (Vandenabeele 2013). If not, this would lead to absorption and fluorescence, or in some cases to the resonance Raman effect (Clark 2005, Bersani et al. 2016), The energy level diagram of Figure 4.2 provides a qualitative schematization of the processes described so far. In practice, a laser is used to excite the sample and the scattered radiation is then collected and filtered in order to detect only the Raman scattering. In a dispersive Raman spectrometer, the polychromatic light produced by the inelastic scattering is separated by means of grating systems, and detected in the form of a Raman spectrum.

121 While excitating the sample with a laser, other phaenomena occur that negatively affect the detection of the Raman signal. Fluorescence is one of them, and occurs when the laser stimulates the molecule to an excited electronic state (Ferraro et al. 2003, Clark 2005, Vandenabeele 2013, Bersani et al. 2016). The decay from that state occurs first via non-radiative transitions, and then by emitting fluorescence radiation upon transition to the ground state (Vandenabeele 2013). Resonance effects can however be favourable, as they can enhance the Raman scattering (Withnall et al. 2003, Clark 2005, Vandenabeele 2013, Bersani et al. 2016). Other relevant side-effects of performing Raman analyses are related to the use of a laser source. Thermal effects might temporarily/irreversibly alter the sample, light-induced alterations, and even laser ablation might happen as well (Clark 2005, Vandenabeele 2013, Bersani et al. 2016). More details on laser•sample interactions with specific regards to Cultural Heritage materials will be described in Sub-chapter 4.2. Finally, concerning the identification of compounds in Raman spectroscopic studies, it is important to note that the identification of molecules can be either performed by simulating the spectrum based on the molecular properties, or by comparison with a database (Lafuente et al. 2015). The latter case is widely exploited in pigment identification: on the one hand, the number of traditional pigments is limited (Sub-chapter 2.1, Table 3.2, (Burgio and Clark 2001, Bouchard and Smith 2003, Smith and Clark 2004, Castro et al. 2005, Edwards and Chalmers 2005, Gatto Rotondo et al. 2010)) and allows for manual comparison of reference spectra with the unknown one, also based on its optical properties (i.e. the colour) (Edwards and Chalmers 2005). On the other hand, the availability of databases and of searching algorithms is fundamental in the study of the more complex organic molecules, including

Electronic excited state

Rayleigh Anti-Stokes Stokes Virtual energy level

ν0

ν0 ± vm

Vibrational excited state, vm

Electronic ground state, vibrational ground state v0

Figure 4.2: Energy level diagram explaining the Rayleigh effect, and the Raman effect. Image after (Baker et al. 2011).

122 modern synthetic organic pigments (Bruni et al. 2011), and of corrosion products (Hayez et al. 2004), among others. To conclude this first part on the Raman effect, it seems worth to go back to the Presentation Speech by Professor H. Pleijel, Chairman of the Nobel Committee for Physics of the Royal Swedish Academy of Sciences, on December 10, 1930. He declares:

“Thus the Raman effect has already yielded important results concerning the chemical constitution of substances; and it is to foresee that the extremely valuable tool that the Raman effect has placed in our hands will in the immediate future bring with it a deepening of our knowledge of the structure of matter.” (H. Pleijel 1931)

Almost 90 years later, we can successfully state that Raman spectroscopy has now become a widely used technique for the study of materials in many fields of research, from medicine to geology, from biology to archaeometry (Lewis and Edwards 2001, Ferraro et al. 2003, Edwards and Chalmers 2005). This growth took full advantage of the development of electronics (infrared and charge•coupled devices (CCD) detectors), lasers, starting from the 60s (Maiman 1960), and to the miniaturization of components leading to the design of portable instruments. The next Sub-chapter summarizes some interesting aspects of the application of Raman spectroscopy in Cultural Heritage studies.

In this Sub-chapter, the specific application of Raman spectroscopy to study materials of relevance for archaeology and art history is tackled in detail. First, a brief review of specific needs of archaeometrical research will be pointed out, and then the features of Raman spectroscopy are discussed in relation to them. A wide range of phaenomena, namely absorption, resonance, fluorescence, photosensitivity, weak Raman cross-section apply to Cultural Heritage materials as well. In fact, low Raman scattering properties, dilution of the materials (often in a fluorescent organic medium), or small particle size can limit the quality of the recorded Raman spectrum (Clark 2005, Vandenabeele et al. 2007b, Colomban 2012, Bersani et al. 2016, Centeno 2016, Casadio et al. 2016). Moreover, a precise calibration of the instrument is required to obtain reproducible and accurate Raman band positions (Vandenabeele et al. 2007a, Vandenabeele 2013).

123 For the purpose of archaeometrical research, it is important to remember that it is an interdisciplinary field of investigations (Yoshimura 2005, Madariaga 2015, Carò et al. 2016). It is devoted to the characterization of materials in works of art, which have been subjected to natural and accidental changes, due to interactions among them, with the environment, and as an effect of human intervention (Coremans 1960, Vandenabeele et al. 2014, Centeno 2016). The application of analytical techniques can in fact inform us on

“the “chaine operatoire” of art and archaeological artifacts: from manufacturing through use to degradation, burial (where applicable), collection, conservation and fruition” (Casadio et al. 2016).

This can be achieved by identifying the pigments and alteration products, which inform us on the mentioned aspects (Clark 2005, Vandenabeele et al. 2007b, 2014, Bersani et al. 2016, Edwards and Vandenabeele 2016, Centeno 2016, Casadio et al. 2016). First, the possibility of obtaining a specific molecular signature from organic and inorganic materials without direct contact or any sample preparation, and the non-destructive character of the technique make it very appealing for such a field of research (Clark 2005, Vandenabeele et al. 2007b, 2014, Colomban 2012, Vandenabeele 2013, Madariaga 2015, Bersani et al. 2016, Edwards and Vandenabeele 2016, Vandenabeele and Donais 2016, Centeno 2016, Casadio et al. 2016, Łydżba-Kopczyńska and Madariaga 2016, Gázquez et al. 2017). According to Gázquez et al. (2017), Raman spectroscopy is “compatible with preservation”, for the reasons mentioned above. A Raman spectrometer can be coupled to a confocal microscope system with a spatial resolution down to 1 μm (Clark 2005, Vandenabeele et al. 2007b, 2014, Bersani et al. 2016, Centeno 2016, Casadio et al. 2016), allowing the study of single grains of pigments. The availability of motorized xy stages can even allow the recording of micro-Raman mappings of flat samples (such as small paintings, or polished cross sections) (Vandenabeele et al. 2014, Bersani et al. 2016, Centeno 2016, Casadio et al. 2016, Lauwers 2017). Thanks to a series of technological improvements in the last decades, Raman spectroscopy gained an even greater range of positive aspects to support its extensive application to the study of polychrome objects and archaeological materials. On the one hand, the development of lasers since the 1960s, and the production of low-power solid state lasers, available in different wavelengths ever since, and on the other hand, the progress in designing efficient detectors and filters to limit the interference of the Rayleigh scattering created the basic ground for developing mobile instrumentation (Clark 2005, Vandenabeele et al. 2007b,

124 2007a, 2014, Colomban 2012, Bersani et al. 2016, Edwards and Vandenabeele 2016, Centeno 2016, Vandenabeele and Donais 2016, Casadio et al. 2016). Issues that are associated with the use of mobile instrumentation are related to the accessibility of the site and transportation of more or less heavy devices, as the instrument itself (Colomban 2012, Vandenabeele et al. 2014, Bersani et al. 2016, Vandenabeele and Donais 2016), the set-up (as illustrated in Table 1.2 and Sub-chapter 4.4), and the generator in case the instrument is not designed to be battery operated (Vandenabeele et al. 2014, Bersani et al. 2016). This might imply flight connections, car travels and/or man-powered transportation: limitations in volume, weight, size, number of pieces apply. The miniaturization conjuncted to the design of mobile instruments requires a compromise between robustness and performance, so that the achievable spatial and spectral resolutions, the focusing process and the time needed to record a spectrum are generally worse than in a bench-top instrument (Colomban 2012, Vandenabeele and Moens 2012, Lauwers et al. 2014c, Vandenabeele et al. 2014). Moreover, it is important for a mobile instrument to be used in the examination of works of art to have the possibility of adjusting parameters such as:  the choice of the most suitable laser wavelength, in order to optimize the measurement (Vandenabeele et al. 2007b, Lauwers et al. 2014c, Lauwers 2017)  the laser power, which could cause damage to the materials, making the technique “destructive” (see Sub-chapter 1.5; Clark 2005, Vandenabeele et al. 2007b, 2014, Lauwers et al. 2014c, Casadio et al. 2016, Lauwers 2017)),  the duration of signal acquisition, as the access to the object under study is exceptional, planned in advance, and limited in time (Clark 2005, Vandenabeele et al. 2014, Centeno 2016, Casadio et al. 2016, Lauwers 2017). For mobile instruments, positioning is also crucial. It needs to be stable, to guarantee accurate selection of the measurement point, yet flexible, as many different approaches might be required (Vandenabeele et al. 2014, Madariaga 2015, Bersani et al. 2016, Casadio et al. 2016, Lauwers 2017). As it concerns the optics, it is as well possible to connect the Raman system to a fiber optic probe, equipped with an appropriate lens, so that a more flexible application can be exploited (both in the laboratory and in situ) (Lauwers et al. 2014c, Vandenabeele et al. 2014, Bersani et al. 2016, Casadio et al. 2016). Such a probe can be mounted on motorized devices for Raman mappings of large/unmoveable objects (Lauwers et al. 2016, Lauwers 2017). The fact that the Raman effect is very weak means that it is easily overwhelmed, by fluorescence, or by environmental light. In the first case,

125 Figure 4.3: Environmental light shielding in practice. In the case of in situ analyses, a variety of possibilities are available: a) shielding the windows with blinds or with black paper (right side of the picture), b) using black cloth and some supports to enclose the sample, in this case a manuscript, and the probehead with its holder; c) positioning under a black tent-like structure in the MSK, Ghent, Belgium; d) use of a sliding black plastic tube with a foam rim to prevent any damage to the painted surface (used for panel paintings); e) use of a copper tube of appropriate length, lined with tape to reduce damage (for wall paintings). The used laboratory instrument (f), equipped with sliding elements to surround the objective lenses and sample stage. Photos UGent (Raman Spectroscopy Research Group). fluorescence can be limited by selecting a longer wavelength laser, or it can be suppressed by comparing spectra recorded using different sources (Conti et al. 2016a). The balance between fluorescence and Raman signal for each material, in fact, depends on the used laser (Vandenabeele 2013). The problem of environmental light interference is solved by performing the analysis in the dark (Colomban 2012, Vandenabeele 2013, Vandenabeele et al. 2014, Casadio et al. 2016) (Figure 4.3). This could mean to measure at night, in a dark room (Figure 4.3, a), or by using a shielding around the sample being analysed (Figure 4.3, b to f). The availability of reference spectra for many common materials in works of art, including degradation products, are published as “databases” or collections of reference spectra specifically related to archaeometry (Bell et al. 1997, Vandenabeele et al. 2000, 2007b, 2014, Burgio and Clark 2001, Bouchard and Smith 2003, Castro et al. 2005, Edwards and Chalmers 2005, Scherrer et al.

126 2009, Fremout and Saverwyns 2012, Colomban 2012, Madariaga 2015, Pozzi and Leona 2016, Centeno 2016, Casadio et al. 2016, Łydżba-Kopczyńska and Madariaga 2016), but still not all the materials are systematically studied, and not always accessible as spectral data for comparison. On the other hand, there are databases available designed for related disciplines, such as mineralogy (RRUFF database (Downs 2006, Lafuente et al. 2015); (Frezzotti et al. 2012)). The lack of reference spectra can strongly hamper the identification of materials when using Raman spectroscopy (Edwards and Chalmers 2005, Vandenabeele et al. 2007b, 2014, Colomban 2012, Madariaga 2015, Centeno 2016, Pozzi and Leona 2016, Casadio et al. 2016, Łydżba- Kopczyńska and Madariaga 2016). Moreover, it is important to know that different instrumentation (laser power and wavelength, filters and optics, gratings, detector, measurement conditions, calibration, etc.) can yield slightly different spectra in terms of spectral resolution, band positions, band relative intensity. Care needs to be taken in comparing the recorded spectra with those from databases (Vandenabeele et al. 2014, Bersani et al. 2016, Casadio et al. 2016, Coccato et al. 2016; Paragraph 4.3.1). Other Raman approaches are now in use in the field of cultural heritage analysis, such as surface-enhanced Raman spectroscopy (SERS, for dyes and organic molecules) (Colomban 2012, Madariaga 2015, Bersani et al. 2016, Pozzi and Leona 2016, Casadio et al. 2016), and spatially-offset Raman spectroscopy (SORS, which can provide depth information without sampling, on certain conditions of layers thickness and turbidity (Conti et al. 2014a, 2015b, 2015a, 2016b). Nevertheless, a tailored approach is required, clearly depending on the wide range of materials to be studied, on the environmental conditions and on the research question (Madariaga 2015, Bersani et al. 2016). A multi-technique methodology also needs to be included into this tailored approach (Clark 2005, Vandenabeele et al. 2007b, 2014, Madariaga 2015, Bersani et al. 2016, Vandenabeele and Donais 2016; Chapter 5). Complementary techniques to Raman spectroscopy (which is a molecular technique), especially for in situ analyses, usually focus on the elemental composition of the materials. The combination of Raman and XRF spectroscopies is widely applied in this field (Paternoster et al. 2005, Ramos and Ruisánchez 2006, Aceto et al. 2006, Castro et al. 2007a, Sawczak et al. 2009, Deneckere et al. 2010, Deneckere 2011, Edwards and Vandenabeele 2012, Hamdan et al. 2012; Chapter 5). Moreover, other molecular and structural techniques can be considered complementary to Raman spectroscopy, such as XRD (Artioli 2013). In fact, the latter allows for qualitative and quantitative studies of ordered phases, while it is not suitable for detecting amorphous and glassy materials, contrarily to Raman spectroscopy. In both cases, absorption of the used radiation can hamper the

127 result of the analyses, and the availability of reference spectra is fundamental for successful identification of the compounds. XRD, as Raman spectroscopy, can be performed on a sample at a micrometric resolution (μXRD, micro-Raman spectroscopy) or in situ. In the case of portable XRD, however, the positioning and alignment of source and detector appears to be more challenging (Duran et al. 2009, Nakai and Abe 2012, Artioli 2013, Van de Voorde et al. 2015). To conclude, Raman spectroscopy shows interesting features that make it very suitable for applications in archaeometry, and explain its wide use (Clark 1995, Smith and Clark 2004, Vandenabeele 2004, Bellot-Gurlet and Coupry 2006, Madariaga 2010, Bersani and Lottici 2016). It is in fact versatile (organic/inorganic materials; solid/liquid/gas samples; polymorph distinction), non-destructive (see Sub•chapter 1.5), fast, and it requires no sample preparation (Clark 1995, Settle 1997, Ferraro et al. 2003, Vandenabeele 2013). It can be coupled to microscope systems to achieve micrometric spatial resolution (micro-Raman spectroscopy, Sub•chapter 4.3), but it can as well be used in situ, as portable instrumentation is available (Vandenabeele et al. 2007a, Edwards and Vandenabeele 2012, Vandenabeele and Donais 2016; Sub-chapter 4.4). Raman spectroscopy has been described, in relation to the analysis of art and archaeological objects, as “quasi-indispensable” (Gázquez et al. 2017), “the technique of choice” (Edwards and Vandenabeele 2016), “the most effective technique [for pigment identification]” (Clark 1995) and even “the best single technique” (Clark 2005). It is important to remember the limitations highlighted so far, which can be either related to the studied materials, or to the analytical technique itself, or to the chosen instrument. In the following Sub-chapters, the Raman spectrometers used in this thesis, two bench-top instruments (4.3) and a mobile one (4.4), will be described. Their application to the study of pigments and painted artworks will be tackled in Paragraphs 4.3.1, 4.3.2, 4.3.3 and 4.4.1. In the case of mobile Raman spectroscopic analysis, a multitechnique approach is also considered, and described in the next Chapter 5.

128 A general scheme of a Raman spectrometer is given in Figure 4.4. The light of a laser is directed to the sample by means of appropriate optics. The presence of a microscope system allows the focusing of the laser beam on the desired point. Moreover, neutral density filters allow the reduction of the laser power, to avoid thermal damage to the sample. The scattered light is collected, filtered to remove the Rayleigh scattering, and directed to the dispersion system (in this thesis based on diffracting gratings). The dispersed radiation is detected by a multi-channel detector (charge•coupled device, CCD) (Vandenabeele 2013). The laboratory Raman spectrometer used was a Bruker Senterra dispersive instrument. The choice of the instrumental parameters is done via the Bruker OPUS software. The spectrometer is equipped with two lasers (532 and 785 nm) whose power can be adjusted by means of neutral density filters (respectively 14.3 to 0.03 mW, 30.8 to 0.28 mW at the sample). The laser can be focused on the sample by using a microscope equipped with a 5, 20, 50 and 100× objectives (achievable spotsize of 50, 8, 4 and 2 μm, respectively). The spectral resolution can be high (3-5 cm−1) or low (9-18 cm−1), depending on the used grating. The covered range is 80-3500 cm-1 for the red laser and 60•3700 cm-1 for the green one. Lower quality, lower resolution spectra are acquired faster, as the entire range is collected at once. Confocality is obtained by using a pinhole. The calibration of the Raman spectrometer was checked before every session, by recording the spectrum of cyclohexane, whose main band is at

Figure 4.4: General scheme of a Raman spectrometer. 1: Optics to focus the laser beam on the sample, neutral density filters, microscope system; 2: optics to collect the scattered light, filters. Image after (Vandenabeele 2013).

129 802 ± 2 cm•1. Peltier cooling ensures that the CCD detector is maintained at •65°C. Moreover, the motorized xy stage allows the recording of micro-Raman mappings on flat surfaces. Both lasers of this instrument were used to record the Raman spectra of green materials (Paragraph 4.3.1) and of carbon-based black pigments (Paragraph 4.3.2). In the case of the measurements performed in the framework of the MEMORI project (Paragraph 4.3.3, (De Laet et al. 2013)), the green laser of this instrument was used for obtaining the spectrum of the green pigment malachite, which strongly absorbs the red laser, so that hardly any Raman band can be observed. On the other hand, for the other pigments, a Kaiser System Hololab 500R modular Raman microspectrometer was used. This instrument is equipped with optical fibers to guide the laser light to the sample, a Leica DMLP optical microscope (100× objective). A 785 nm diode laser whose intensity at the sample was set to maximum 40 mW, was used. A HoloSpec VPT System spectrograph and a back-illuminated deep depletion CCD were used for the detection of the scattered photons. Laboratory instruments were used during this thesis to study pigments and coloured materials of relevance for Cultural Heritage (Paragraphs 4.3.1 and 4.3.2), and their sensitivity to airborne pollutants in museum-like environments (Paragraph 4.3.3).

The first application of micro-Raman spectroscopy to be found in this Chapter 4 is based on the paper “Raman spectroscopy of green minerals and reaction products with an application in Cultural Heritage research”, by A. Coccato, D. Bersani, A. Coudray, J. Sanyova, L. Moens, and P. Vandenabeele, published in the Journal of Raman Spectroscopy (Coccato et al. 2016). It is related to the specific problem of database availability, and tries to fill an existing gap by providing reference spectra of green-coloured compounds of relevance to the investigation of Cultural Heritage. It is common knowledge for the (amateur) artist that green paint can be obtained by mixing the primary colours yellow and blue (subtractive synthesis). This was known to artists and artisans since millennia, as can be observed in many examples from antiquity to modern times (Gilbert et al. 2003, Jurado-López et al. 2004, Guineau 2005, Pastoureau 2007, Edwards et al. 2007, Aliatis et al. 2009, Clark et al. 2010, Moretto et al. 2011, Lo Monaco et al. 2013, Buzgar et al. 2014). Moreover, a huge variety of green compounds is suitable for the use as

130 pigments and is now identified in Cultural Heritage objects, in contrast to the supposedly restricted variety of green pigments existing according to traditional sources (mainly malachite and verdigris) (Gilbert et al. 2003, Valadas et al. 2015). The range of studied green materials is wide and encompasses Fe-based (green earths and synthetic organic pigments), Cu-containing compounds (natural and synthetic reaction compounds, more or less known as pigments or degradation products, including polymorphs of the same formula), modern Cr- and Co-green pigments. Glauconite, celadonite and other green earth minerals have been used as pigments (Feller 1987, Ospitali et al. 2008). Natural Fe-containing green pigments (green earths) are found in many local deposits, which makes these pigment less likely to be traded over long distances. Copper-based green pigments such as malachite, verdigris and copper resinate are well known as painting materials (Gilbert et al. 2003, Valadas et al. 2015), while sulphates, chlorides and other Cu-salts were mainly investigated in relation to corrosion processes (Martens et al. 2003, Hayez et al. 2005, Chiavari et al. 2007, Bertolotti et al. 2012, Ropret and Kosec 2012). However, more detailed investigation of works of art revealed that a huge variety of Cu-salts and reaction compounds were also used as pigments: sulphates, phosphates, chlorides (van Asperen de Boer 1977, Kühn 1970, Derbyshire and Withnall 1999, Scott 2000, Gilbert et al. 2003, Eremin et al. 2006, Sanyova and Saverwyns 2006, Klipa et al. 2010, Valadas et al. 2015); citrate (Eremin et al. 2008). This fact implies that the identification of Cu in a green area cannot be straightforwardly interpreted as related to the presence of the traditional pigments malachite or verdigris, but a deeper investigation is required (Gilbert et al. 2003). For example, the simultaneous detection of Cu and Cl could be related, at the same time, to the natural mineral atacamite or to the synthetic copper phtalocyanine green, which is a modern synthetic pigment (Irazola et al. 2012). Moreover, this elemental association can be related to the degradation of the blue pigment azurite in presence of chloride ions or to verdigris, in case it was prepared with additives such as salt, honey or urine (Dominguez-Vidal et al. 2014). In fact, the specific case of copper-based green materials in works of art is given attention, as it appears to be more complex than expected, and care needs to be taken in drawing conclusions if only elemental information (e.g. XRF) is available (see Paragraph 3.3.1). Raman spectroscopy is a powerful technique for the characterization of materials and is of valuable use in archaeometrical research in general. Green compounds of natural or synthetic origin are found in many research areas, ranging from mineralogy, to pigment identification, to corrosion studies. However,

131 a detailed and comprehensive database of spectra and references is still missing in the literature. Raman spectroscopy can succesfully discriminate among different molecular structures, and be of help in identifying green pigments. However, some limitations have to be taken into account, like for example the weak Raman scattering properties of some of the investigated materials in relationship to a specific laser wavelength, or the overlap of the main band of anatase -1 (TiO2, 143 cm ) to the ν(O-Al-O) stretching vibration of kaolinite •1 (Al2Si2O5(OH)4, 143 cm ), which is of importance in investigating clay pigments (Košařová et al. 2013). Therefore, using a combination of sensitive analytical techniques, including other molecular techniques such as XRD, seems to be of extreme importance, in combination with having access to an extended, accessible Raman database. This paragraph provides to the researcher dealing with green materials in Cultural Heritage both, a literature review and downloadable Raman spectra of reference products, which are available for download at http://www.analchem.ugent.be/RAMAN/. Nomenclature problems have to be faced, as the correspondence between a traditional name and a specific chemical composition is not always certain, especially when referring to ancient sources (for example the term “chrysocolla” had a different meaning to Pliny the Elder in comparison to a 20th and 21st century geologist) (Pliny the Elder: Naturalis Historia, Feller 1987, Aliatis et al. 2009). The collected spectra are discussed in relation to the preliminary/commercial identification of the material itself and to the published data. Practical aspects regarding the laser wavelength selection are also discussed with regards to the comparison to published reference spectra. This approach is illustrated by analysing a cross section of a green zone of the Ghent Altarpiece by the Van Eyck brothers. Additional information on the studied samples, and on the Raman spectroscopic features can be found in APPENDIX C.

Raman spectra of the green materials were recorded with both lasers of a Bruker Optics ‘Senterra’ dispersive Raman spectrometer, i.e. with laser wavelengths of 532 and 785 nm. Spectra were recorded in the high resolution mode (3-5 cm-1) of the instrument. Full range spectra (60-3700 (532 nm laser) and 80•3500 (785 nm laser) cm-1) were recorded using a 20× objective, giving a spotsize of ca. 10 µm.

132 The laser power was adjusted so that no degradation was caused on the materials, and it was always 4 mW and 1.4 mW at the sample for the 785 nm and the 532 nm laser respectively. If a lower laser power was used, it is mentioned. The acquisition time and accumulations were defined for each case, in order to obtain a good signal-to-noise ratio. At least three spectra per sample were acquired, on different positions. The system uses a thermo-electrically cooled CCD detector, operating at -65 °C. All the measurement parameters and instrumental conditions are controlled by the OPUS software.

Table 4.1: Overview of green materials used as pigments and glazes, possibly related to degradation processes of interest for archaeometry.

Cu-containing green pigments

Cu2(CO3)(OH)2 Malachite

(Cu,Al)2H2Si2O5(OH)4·zH2O Chrysocolla Cornetite, libethenite pseudomalachite, Cu (PO ) (OH) x 4 y z reichenbachite, ludjibaite

CuSO4·yCu(OH)2·zH2O Antlerite, brochantite, posnjakite, langite

Cu2Cl(OH)3 Atacamite, botallackite, clinoatacamite Fe-containing green pigments

3+ ((K,Na)(Fe ,Al,Mg)2(Si,Al)4O10(OH)2) Glauconite 3+ 2+ (K[(Al,Fe ),(Fe ,Mg)](AlSi3,Si4)O10(OH)2) Celadonite Synthetic green pigments

xCu(CH3COO2)·yCu(OH)2·zH2O Verdigris Copper resinate

2+ (Ca,Cu )3(SiO3)3 Egyptian green

C32H3Cl13CuN8 to C32HCl15CuN8 PG 7

C32Br6Cl10CuN8 PG 36

C30H18FeN3O6Na PG 8

Cr2O3 Chrome oxide green

Cr2O3·2H2O Viridian

For the analysis of copper resinate and of a paint sample containing this pigment also a 514 nm green laser was used. The Raman spectra were acquired with a Renishaw InVia multiple laser Raman spectrometer with a Peltier cooled (•70 °C) NIR enhanced deep depletion CCD detector. The laser power was kept below 300 μW. Embedded stratigraphic samples can be analysed at

133 magnifications of 5× to 100× in the direct-coupled Leica DMLM microscope with enclosure. To minimize the high reflection, a set of polarizers is added to the microscope when viewing the cross sections under white light. Data processing was performed by using GRAMS 8.0 software (Thermo). All the spectra presented in the figures, as well as the downloadable files are without baseline correction.

Green reference materials of natural and synthetic origin were either purchased by Kremer Pigmente (Aichstetten, Germany), were available in the laboratories (copper resinates) or on the mineral market, from collectors. Most of the Cu•containing minerals of relevance for Cultural Heritage belongs to the third group. Table 4.1 provides an overview of the materials of interest for Cultural Heritage investigations, while Table C.1 reports a list of the studied samples, including details about their origin and preliminary identification. The experimental conditions and the Raman identification of the materials (including some modifications to the preliminary labelling) are also given in Table C.1. In addition to these reference materials, a sample coming from the Ghent Altarpiece panel painting was investigated with micro-Raman spectroscopy in order to establish the potentialities of this technique in the detection of copper resinate in cross sections.

The results are presented separately for natural copper containing minerals of interest for Cultural Heritage, natural iron based green pigments, which are the green earths glauconite and celadonite, and finally synthetic pigments (containing Cr, Fe, Cu, Co). The elements Cr, Fe, Cu, Co can be detected easily by elemental techniques such as XRF, but molecular techniques (e.g. Raman spectroscopy, XRD) are needed to fully clarify the molecular structure of the various compounds. As regards the copper containing materials, the discussion will be split according to the anionic species (carbonates, silicates, phosphates, sulphates, chlorides). An overview of the detected Raman bands, including their relative intensity (Vandenabeele and Moens 2012b) with both 785 and 532 nm laser excitation and of the identified materials in all the studied samples can be found in Table C.2.

134 Malachite (Cu2(CO3)(OH)2), together with the artificial bluish-green reaction product verdigris (xCu(CH3COO2)·yCu(OH)2·zH2O), is known as a painting material since antiquity (Pliny the Elder: Naturalis Historia, Gilbert et al. 2003).

The blue copper•containing pigment azurite (Cu3(CO3)2(OH)2) is also known to degrade into green compounds (Dei et al. 1998; Paragraph 2.1.5). Historical sources are found in Cyprus, Macedonia, Spain, and Armenia (Aliatis et al. 2009). Moreover, green Cu-containing salts now recognized as pigments are found as degradation products of copper or bronze artifacts (Frost et al. 2002a, Shen et al. 2006, Bertolotti et al. 2012, Kosec et al. 2012, Ropret and Kosec 2012), or of Cu•containing pigments (Castro et al. 2005, 2008, Irazola et al. 2012; Paragraph 2.1.5). The formation of chlorides, sulphates, phosphates and mixed salts is related to a variety of environmental and intrinsic factors, so that the presence of specific secondary minerals is related to local conditions leading to degradation and to the stability of the compounds (McCann et al. 1999, Ropret and Kosec 2012, Yu et al. 2013; Paragraph 2.1.5).

Figure 4.5: Raman spectra of selected Cu-containing carbonates and silicates (532 nm). a: Malachite (Kremer 10300); b: chrysocolla (Kremer 1035); c: subtraction of the spectrum of malachite from that of chrysocolla. The bands of the latter are more clearly visible.

135 These observations are crucial to a correct approach to the interpretation of analytical results of green pigments in works of art, as the detection of Cu in a green area has no straightforward implications on the identification of the actual pigment. In all these circumstances, it is necessary to couple the elemental technique with a molecular one (see for example the combination of PIXE with Raman spectroscopy for the study of green pigments in manuscripts (Gilbert et al. 2003); XRD (Duran et al. 2009, Nakai and Abe 2012, Artioli 2013, Van de Voorde et al. 2015)).

The green basic carbonate malachite is described in treatises already in the Roman times (Pliny the Elder: Naturalis Historia), and the term “chrysocolla” seems to have been used as a synonym for that (Gettens and Fitzhugh 1974) (see the next paragraph for more details). It is also widely found in works of art from various cultures, from the Egyptians (Eastaugh et al. 2008) to 19th century artists (Correia et al. 2007). A list of findings (Gettens and Fitzhugh 1974), as well as of malachite Raman spectroscopic identification (Appolonia et al. 2009, Burgio et al. 2010, Buzgar et al. 2014, Lauwers et al. 2014a, van Pevenage et al. 2014) is reported in literature. Malachite is typically present in Cu ore deposits (Hungary, East Central Europe, France (Gettens and Fitzhugh 1974); Cyprus, Macedonia, Spain, Armenia (Aliatis et al. 2009)), and it is often associated with azurite, chrysocolla and cuprite (Gettens and Fitzhugh 1974). Malachite is also found as a corrosion product on bronze (McCann et al. 1999, Yu et al. 2013, Stranges et al. 2014). Differences in the Raman spectra of malachites are attributable to different formation processes (mineral vs. corrosion product) (Yu et al. 2013). However, the main bands still allow for the identification of the monoclinic carbonate (Guineau 1984, Frost and Martens 2002, Correia et al. 2007, Aliatis et al. 2009, Yu et al. 2013, Buzgar et al. 2014) (Figure 4.5, a).

The most intense and diagnostic bands are the CO3 stretching modes at 1493, -1 1462 and 1368 cm (asymmetric ν3 (Frost and Martens 2002, Buzgar and Apopei -1 2009)), the doublet at 1097-1059 cm (doubly degenerated ν1) and the ν4 and ν2 at -1 -1 ca. 720 and 750 cm and 820 cm respectively (Buzgar and Apopei 2009). Below 600 cm-1, the lattice mode at 435 cm-1 is very intense, but also other bands are observable (Table C.2) (Buzgar and Apopei 2009). The ν(OH) stretching and bending vibrations are found at 3380, 3382 and 1638 cm-1, in good agreement with published data (Frost and Martens 2002, Buzgar and Apopei 2009). All these bands are observed with the 532 nm laser only, as absorption of red light hampers the recording of a Raman spectrum with the 785 nm laser (De Laet et al. 2013).

136 The mineralogical term chrysocolla ((Cu,Al)2H2Si2O5(OH)4·zH2O) now indicates a series of hydrated copper silicates, while in the past it has been used as a synonym for malachite (Aliatis et al. 2009). It is currently believed that chrysocolla is a hydrogel containing Si and Cu, associated with copper oxides and carbonates (Frost and Xi 2013). Three samples described as chrysocolla were available (Table C.1): the commercial pigment 1035 Kremer, and two samples from Northern Italy (Libiola and Reppia mines). The Kremer sample 1035 (532 nm, Figure 4.5, b) showed mainly the bands of malachite, but after careful observation of the spectrum and subtraction of a malachite reference spectrum (after normalization to the most intense malachite band at 180 cm-1), additional bands attributable to chrysocolla were identified (Figure 4.5, c; Table C.2). The bands above 3600 cm-1 and at 1460 cm-1 are attributed respectively to the symmetric ν(OH) stretching and to the δ(OH) bending modes; while silicate related -1 -1 bands are those at ca. 680 cm (ν4 SiO3) and between 500 and 300 cm (ν2 SiO3) (Frost and Xi 2013). These bands are in good agreement with published values (Downs 2006, Frost and Xi 2013). Also, the bands at 1073, 988 and 973 cm-1 could possibly be related to antlerite (CuSO4·yCu(OH)2·zH2O) (Gilbert et al. 2003). The sample labelled as chrysocolla from Libiola mine, a classical locality for Italian chrysocolla, gave very weak Raman spectra with both lasers, and was finally attributed to brochantite (CuSO4·yCu(OH)2·zH2O) as the main observed band is that at 973 cm-1 with both lasers (Schmidt and Lutz 1993, Hayez et al. 2004, Makreski et al. 2005, Chiavari et al. 2007, Burgio et al. 2010). Finally, the sample from the Reppia mine, labelled as chrysocolla-allophane, was studied on many different points, but it always gave the same spectrum with the 532 nm laser, with bands at 3420 (asymmetric and very broad), 2942, 1638, 1357, 1103, 982, 858, 720, 502, 396 (shoulder) and 364 cm-1. These bands do not correspond to any of the published spectra of chrysocolla (Downs 2006, Frost and Xi 2013), while they show better correspondence to allophane

(Al2O3(SiO2)1.3•2.0·2.5-3.0H2O). This is not surprising because Cu-rich blue allophane and chrysocolla are both present in the Reppia mine and they are usually found mixed together; it is impossible to discriminate between them by simple observation, even by microscope. One of the most prominent bands, at 858 cm-1, seems related to the in plane δ(Si-OH) bending (Creton et al. 2008). Another Raman band, related to silicatic species but not expected in the spectrum of allophane, is here observed at 1103 cm-1. This can be interpreted as the stretching of disilicatic units (Phillips 1985). As regards the presence of water,

137 the δ(OH) bending is observed at 1638 cm-1 (Frost and Martens 2002, Buzgar and Apopei 2009), while the stretching modes seem to correspond to those of bonded OH (Sun 2009). The Raman spectrum recorded with the red laser, although of lower quality, showed a band at 859 cm-1, which corresponds to what observed with the 532 nm excitation (Si-OH bending (Creton et al. 2008)).

The copper phosphates of interest for archaeology and art history are cornetite

(Cu3(PO4)(OH)3), libethenite (Cu2(PO4)(OH)), and the polymorphs ludijbaite, pseudomalachite, and reichenbachite (Cu5(PO4)2(OH)4) (Naumova et al. 1990, Frost et al. 2002b, Kharbish et al. 2014). These bright green copper containing minerals have been identified in Cultural Heritage objects (Derbyshire and Withnall 1999, Eastaugh et al. 2008).

Other Cu-containing phosphates are sampleite (NaCaCu5(PO4)4Cl·5H2O)

(Fabrizi et al. 1989) and veszelyite ((Cu,Zn)2ZnPO4(OH)3·2(H2O)) (Garcia Moreno et al. 2008). Raman spectra of libethenite acquired with different laser wavelengths are published in literature (780 nm (Downs 2006), 633 nm (Frost et al. 2002b, Kharbish et al. 2014), 532 nm (Downs 2006), 514 nm (Downs 2006, Belik et al. 2007)), and show good agreement with our sample (Table C.1) measured with 3- the 532 nm laser (Figure 4.6, a). The PO4 vibrations are depending on orientational effects and on which crystal face is exposed to the laser (Frost et al.

2010b). The most diagnostic bands are the phosphate ν1 symmetric stretching at -1 -1 975 cm , the ν3 antisymmetric stretching at 1069 cm . -1 The ν4 bending modes are found at 650, 628, 585 and 558 cm , while the ν2 is at 450 cm-1 (Frost et al. 2002b, 2010b). Lattice modes are dominated by a feature at ca. 300 cm-1, which is always clear, independently on the orientation of the sample (Frost et al. 2002b, Belik et al. 2007, Kharbish et al. 2014). Other bands are observed below 300 cm-1 (Table C.2) (Kharbish et al. 2014). The ν(OH) stretching band is observed at ca. 3470 cm-1, while the deformation bands are at 815 and 862 cm-1 (Frost et al. 2002b, Kharbish et al. 2014). It seems that the band at ca. 865 cm-1 is only present in natural samples of libethenite, and might be orientation-dependent (Downs 2006, Belik et al. 2007, Frost et al. 2010b). The Raman spectrum of libethenite acquired with the 785 nm laser shows only weak bands at 975 and 300 cm-1.

138 Figure 4.6: Raman spectra of selected Cu-containing phosphates and sulphates. a: Libethenite (Libethen, Slovakia; 532 nm); b: brochantite (Baccu Locci, Sardinia, Italy; 532 nm); c: langite (Voltaggio, NW Italy; 785 nm); d: langite (Voltaggio, NW Italy; 532 nm); e: antlerite (Morocco; 532 nm). Spectra c and d were recorded on a sample described as langite.

Copper sulphates and mixed salts are of significance for pigment identification (Castro et al. 2007b, Burgio et al. 2010), pigment degradation studies (Pérez•Alonso et al. 2006, Castro et al. 2008) and for corrosion studies of copper and bronze artefacts (Martens et al. 2003, Hayez et al. 2004, Chiavari et al. 2007), as well as for geological and mineralogical applications (Martens et al. 2003). Green copper sulphates were also identified in Netherlandish paintings from the period 1520-1530 (van Asperen de Boer 1977, Spring 2000, Sanyova and Saverwyns 2006, Klipa et al. 2010) and on 16th century Portuguese-Flemish paintings (Valadas et al. 2015). It seems that brochantite has been intentionally used as a pigment.

139 Antlerite (Cu3(SO4)(OH)4), brochantite (Cu4(SO4)(OH)6), posnjakite

(Cu4(SO4)(OH)6·H2O), langite (Cu4(SO4)(OH)6·2H2O) are some of the most interesting basic copper sulphates from those points of view.

Others are chalcantite (blue, CuSO4·5H2O) (Hayez et al. 2004), serpierite

(Ca(Cu,Zn)4(OH)6(SO4)2·3H2O) (Frost et al. 2004, Zaharia 2012), connellite

(Cu36(SO4)(OH)62Cl8·6H2O) (Frost et al. 2002c, Stranges et al. 2014) and chalcophyllite (Cu18Al2(AsO4)4(SO4)3(OH)24·36H2O) (Frost et al. 2010a). Brochantite and antlerite seem to be more stable than the other sulphates (Hayez et al. 2004). The sulphate symmetric stretching vibration is centred between 970 and 990 cm-1 (Martens et al. 2003, Hayez et al. 2004). Brochantite Raman spectrum (532 nm excitation, Figure 4.6, b) shows multiple bands in the ν(OH) stretching region, two at ca. 3588 and 3567 cm-1, and some broader ones at 3403, 3371 and 3262 cm-1. In one of the measured samples, these bands were sharper and those at ca. 3400-3370 cm-1 were resolvable into four components (3411, 3402, 3383 and 3372 cm-1) (Schmidt and Lutz 1993, Hayez et al. 2004, Makreski et al. 2005,

Chiavari et al. 2007, Burgio et al. 2010). The ν(SO4) stretching vibrations are -1 -1 observed at 972-973 cm (ν1), 1129, 1105, 1097 and 1076 cm (ν3) and 620, 609, -1 595 cm (ν4) (Schmidt and Lutz 1993, Hayez et al. 2004, Makreski et al. 2005, Chiavari et al. 2007, Burgio et al. 2010). The OH librations are relatively weak and correspond to published values: 910, 871, 776 and 728 cm-1 (Schmidt and Lutz 1993, Hayez et al. 2004, Makreski et al. 2005, Chiavari et al. 2007, Burgio et al. 2010). The Cu-O vibrations and lattice modes are numerous and in good agreement with literature (see Table C.2) (Schmidt and Lutz 1993, Hayez et al. 2004, Makreski et al. 2005, Chiavari et al. 2007, Burgio et al. 2010). The most intense of these bands are those at ca. 480, 390, 140 cm-1. The Raman spectrum of brochantite recorded with the red laser (785 nm, Figure 4.6, d) shows most of the bands detected with the green one (see Table C.2). The reference sample from Voltaggio (456, NW Italy) showed both green brochantite and bluish crystals of langite. The Raman spectrum of langite (532 nm) shows good agreement with published values (Gilbert et al. 2003, Hayez et al. 2004, Downs 2006). The ν(OH) stretching vibrations are observed at 3587, 3578, 3399 and 3370 cm-1 while the bending modes are weak at 770 cm-1; -1 the very strong ν(SO4) symmetric stretch is found at 970 cm . Other bands are those at 1156, 1127, 1093, 611, 510, 487, 450 (shoulder), 432 (strong) and 242 cm-1. The Raman spectrum collected with the red laser shows bands at 970, 428 and 239 cm-1. (Figure 4.6, c and d; Table C.2). Among the available reference samples, more sulphates were detected and identified. Antlerite was present together with brochantite (the first as light green aggregates, the second as dark green elongated crystals): with the 532 nm

140 excitation, the OH bands are sharper and in number of two in the stretching region (3581 and 3489 cm-1) (Gilbert et al. 2003, Martens et al. 2003, Hayez et al. 2004) and two in the deformation modes (784 and 748 cm-1) (Martens et al. 2003). The sulphate symmetric stretching band is found at 988 cm-1 and the antisymmetric ones at 1171, 1122 and 1076 cm-1. The sulphate bending region (510 to 410 cm-1) is complex (Table C.2) (Martens et al. 2003, Hayez et al. 2004). Lattice modes are weaker and reported between 341 and 70 cm-1 (Martens et al. 2003, Hayez et al. 2004) (Figure 4.6, e; Table C.2). To the best of our knowledge, we are the first to report the Raman bands of antlerite below 125 cm•1. Serpierite (Figure 4.7, a) was also identified together with brochantite in a sample from Corchia (Northern Italy). The recorded band positions (Table C.2) are in good agreement with the spectrum presented in RRUFF (Downs 2006), and with some published data (Frost et al. 2004, Zaharia 2012).

Pyromorphite (Pb5(PO4)3Cl) was also identified in some light yellowish green crystals occurring together with brochantite in one sample. The recorded spectrum corresponds to published data (Bartholomäi and Klee 1978, Downs

Figure 4.7: Raman spectra of selected Cu-containing mixed sulphates. a: Serpierite (Corchia, NW Italy; 532 nm); b: connellite (Monte Fucinaia, central Western Italy; 532 nm); c: chalcophyllite (Cap Garonne, France; 532 nm); d: spangolite (Monte Fucinaia, central Western Italy; 532 nm).

141 2006) as regards the region 1120-150 cm-1 (Table C.2). Some features have been recorded outside this range: very weak ones at 1150, 1501, 1529 cm-1, broad bands at 3252, 2662, 2534, 2422 and 2041 cm-1 and two strong features at 106 and 89 cm-1, which are for the first time here reported.

Connellite (Cu36(SO4)(OH)62Cl8·6H2O) is recognizable by the bands at 984, 585, 446, 404, 350, 262, 236, 192, 184 (shoulder) and 132 cm-1 (Frost et al. 2002c, Downs 2006, Stranges et al. 2014) (Table C.2). As the mineral contains OH units, stretching bands are expected. In all the spectra of connellite that we recorded, a broad band at ca. 3550 cm-1 is observed (Frost et al. 2002c) (Figure 4.7, b). Its formation on copper containing artefacts can be described as bronze disease (Frost et al. 2002c, Stranges et al. 2014).

Chalchophyllite (Cu18Al2(AsO4)4(SO4)3(OH)24·36H2O) is another emerald green copper mineral, containing sulphate, arsenate and hydroxyl anions. The stretching bands related to these species are found at 979, 843 (broad and asymmetric) and 3400-3600 cm-1 respectively (Frost et al. 2010a). The bending vibrations and lattice modes are in good agreement with published data (Downs 2006, Frost et al. 2010a), however, symmetry lowering produces multiple features in the region 620-110 cm-1 (Figure 4.7, c; Table C.2). When the sample was measured with the red laser, the signal of quartz (464, 208 and 128 cm-1 (Burgio and Clark 2001)) was recorded, in addition to (broad, asymmetric) bands at 981 and 854 cm-1, corresponding respectively to sulphate and arsenate ions vibrations (Frost et al. 2010a).

Spangolite (Cu6Al(SO4)(OH)12Cl·3(H2O)) Raman bands are observed at 968, 615, 520, 410 and 168 cm-1 for both lasers (Figure 4.7, d; Table C2). They correspond to published spectra of the RRUFF database (Downs 2006), which are the only available spectra recorded so far.

Atacamite (Cu2(OH)3Cl) is the orthorhombic polymorph of hydroxychlorides, while clinoatacamite and botallackite are monoclinic, and anatacamite is triclinic (Frost et al. 2002a, Hayez et al. 2005, Bertolotti et al. 2012, Ropret and Kosec 2012,

Engelbrekt et al. 2014); paratacamite is trigonal (Cu,Zn,Ni)2(OH)3Cl. Botallackite is the most unstable polymorph (Bertolotti et al. 2012). Many issues still need to be faced, as the term paratacamite has sometimes been used as a synonym for clinoatacamite (Fleet 1975, Jambor 1996, Frost et al. 2002a, Bertolotti et al. 2012) even in some X-ray diffraction databases leading to bad naming of mineralogical samples. It seems that the ν(OH) stretching region is effective for discriminating among the different polymorphs. The expected bands of atacamite are related to the ν(OH) stretching region (3200-3600 cm-1), the δ(OH) or δ(Cu-OH) bending region (800-1000 cm-1), and the low wavenumber region (below 600 cm-1) containing the

142 O-Cu-O and Cl-Cu-Cl modes (Frost et al. 2002a, Bertolotti et al. 2012). The most significant band for these compounds is the one around 510 cm-1, which is attributed to ν(CuO) stretching (Frost et al. 2002a). However, strong orientational effects are observed (Bertolotti et al. 2012) (Figure 4.8, a, b and c). The Kremer pigment atacamite (103900) was identified as malachite on the basis of the Raman spectrum recorded with the green laser. The spectrum recorded on our atacamite sample from Monte Fucinaia (Italy, Table C.1) shows bands at 3436, 3346, 3330 cm-1 (this last one only detected in a sample from Atacama (Frost et al. 2002a)); the other bands, as reported in Table C.2, are in agreement with literature (Frost et al. 2002a, Hayez et al. 2005, Bertolotti et al. 2012). The sample labelled as paratacamite, on the other hand, did not correspond to the Zn, Ni-rich copper hydroxychloride, but to clinoatacamite on the basis of the bands reported in Table C.2 (Frost et al. 2002a, Bertolotti et al. 2012, Ropret and Kosec 2012) (Figure 4.8, d). Such labelling mistakes are not unusual, due to the confusion in the past on the classification of copper hydroxychlorides.

Figure 4.8: Raman spectra of selected Cu-chlorides. a, b, c: Atacamite (Monte Fucinaia, central Western Italy; 532 nm); d: clinoatacamite (Piombino, central Western Italy; 532 nm). The spectrum d was collected on a sample labeled as paratacamite. For more details about the nomenclature issue, refer to the text.

143 Moreover, this spectrum showed broad features at ca. 2960-2850 and 1450, 1300 cm-1. These features are attributed to organic compounds, such as those characteristic of human skin. It seems logic that handling of the sample is responsible for the detection of phospholipidic materials: symmetric and -1 antisymmetric ν(CH2) at 2850 and 2930 cm respectively; symmetric and -1 antisymmetric ν(CH3) at 2870 and 2960 cm respectively; δ(CH2) scissoring and twisting at 1450 and 1300 cm-1 respectively (Lawson et al. 1998).

Green earths are widely used in space and time (Feller 1987, Ospitali et al. 2008). Local deposits of clay materials of appropriate green shade are easily found, and it is likely that local sources were exploited (Verona, NE Italy; Cyprus; Spain (Hradil et al. 2003, Aliatis et al. 2009)). The two minerals composing the so called 3+ 2+ green earths are celadonite (K[(Al,Fe ),(Fe ,Mg)] (AlSi3,Si4)O10(OH)2) and 3+ glauconite ((K,Na)(Fe ,Al,Mg)2 (Si,Al)4O10(OH)2). The chemistry of these micas is similar, however they are formed, respectively, in metamorphic/volcanic and sedimentary rocks (Hradil et al. 2003, Correia et al. 2007, Aliatis et al. 2009, Cristini et al. 2010). Depending on the origin, they have different molecular structures, and hence different Raman spectra (Bell et al. 1997). This also means that the two minerals do not occur together (Eastaugh et al. 2008). However, the associated minerals are common (quartz, clays, calcite, iron oxides, feldspars, anatase) and are therefore not useful for discrimination (Feller 1987, Aliatis et al. 2009). Additional green clay-like materials can be referred to as green earths (Feller 1987, Ospitali et al. 2008). The green colour of green earths is explained by the simultaneous presence of divalent and trivalent iron (Hradil et al. 2003, Moretto et al. 2011, Košařová et al. 2013). Powder X-ray diffraction (pXRD) is not always successfully discriminating between the two silicates (Feller 1987), while Raman and IR spectroscopies can yield good results (Feller 1987, Ospitali et al. 2008, Aliatis et al. 2009, Moretto et al. 2011), on the condition that the sample is not degraded (Cristini et al. 2010). It is reported in literature that green earths are not very Raman active when excited with a 1064 nm laser (Burgio and Clark 2001, Košařová et al. 2013), although both red and green lasers are successfully used to characterize and identify these silicates, even if they are relatively poor Raman scatterers (Tlili et al. 1989, Bell et al. 1997, Correia et al. 2007, Ospitali et al. 2008, Moretto et al. 2011, Košařová et al. 2013, Wang et al. 2015). It is important to note that Raman studies of phyllosilicates are affected by the natural complexity of the chemical structure (variation in ions and ionic groups), the small size of the mineral grains (<2 μm), the low degree of crystallinity and

144 possibly from the contamination from organic molecules. Also, some resonance effects may occur in clay materials for specific laser wavelengths (Cristini et al. 2010, Košařová et al. 2013, Wang et al. 2015). For all these reasons, and for the simultaneous presence of other minerals together with glauconite and celadonite, Table C.2 reports only band positions, without indication of relative intensity. The Raman bands of glauconite and celadonite are reported in literature, however, it is sometimes hard to differentiate between the two green earths (Bell et al. 1997, Moretto et al. 2011), due to similarity in the molecular structure (Aliatis et al. 2009). It seems that the most useful bands for discrimination are the ones in the -1 lower wavenumber region (100-300 cm , MoO6 octahedra vibrations): celadonite presents a doublet at 174-202 cm-1 and a band at ca. 280 cm-1, while glauconite has a single band at 170 cm-1 and presents a downshift in the band at ca. 270 cm-1 (Ospitali et al. 2008, Aliatis et al. 2009), as a consequence of increasing Al3+ content (Tlili et al. 1989, Ospitali et al. 2008, Aliatis et al. 2009). Also, the ν(OH) stretching region seems diagnostic, as glauconite shows a weak broad feature, while celadonite has resolved bands, both in the Raman (Ospitali et al. 2008) and FT-IR spectra (Ospitali et al. 2008, Aliatis et al. 2009, Moretto et al. 2011).

Figure 4.9: Raman spectra of green earth pigments. a: Green earth from Bavaria, identified as glauconite (Kremer 11100, 532 nm); b: celadonite from Cyprus (Kremer 17410, 532 nm); c: green earth from France (Kremer 40830, 532 nm); d: PG 7 (532 nm).

145 Among the studied reference samples it was possible to identify both glauconite and celadonite (Table C.2). However, in some cases the distinction was not feasible.

Glauconite samples are the green earths from Bavaria, Bohemia and Russia (11100, 40810 and 11110 Kremer respectively). The bands which allowed the assignment to the Al-rich green earth glauconite are at ca. 700, 544, 450, 380, 270 and 180 cm-1. The latter band is actually broad and reported between 188 and 200 cm-1 (Ospitali et al. 2008). Resonance effects are described in (Ospitali et al. 2008), so that it is not surprising that spectra recorded with the green laser are dominated by the features at 700, 450 and ca. 265 cm-1 (Figure 4.9, a), while with the red laser the most prominent features are the 590 cm-1 and 384-389 cm-1 ones (Ospitali et al. 2008). Moreover, a variety of spectra can be ascribed to glauconite (Ospitali et al. 2008, Aliatis et al. 2009, Košařová et al. 2013), and it is not surprising that not all the recorded spectra are identical. Moreover, the absence of sharp bands of ν(OH) supports the glauconite identification, both in Raman and FT-IR spectra (Aliatis et al. 2009).

The Raman spectrum of celadonite is expected to show narrower bands than the glauconite spectrum, as a result of greater order, as the substitution of Al3+ for Si4+ is limited (Ospitali et al. 2008). Celadonite containing samples are the two green earths originating from Cyprus (blue green and green earth, Kremer pigments 17410 and 17400 respectively. Table C.1. Figure 4.9, b)). These spectra show a doublet at 170-200 cm-1, and bands at 270, 394-400 cm-1. The higher wavenumber band (550 cm-1) is affected by resonance effects (Ospitali et al. 2008). Cyprus green earth (celadonite is geologically associated to basaltic rocks) was used in works of art since antiquity (Eastaugh et al. 2008). In these cases, the bands at 272-276 cm-1 and the sharp feature at 3560 support the identification (Ospitali et al. 2008, Aliatis et al. 2009, Košařová et al. 2013). Moreover, in the case of the Cyprian green earth, a feature at 970 cm-1 is also visible, which is only present in celadonite (Ospitali et al. 2008). A good agreement is observed with spectra of celadonite from Cyprus previously published (Ospitali et al. 2008, Košařová et al. 2013). Other pigments, described as green earths by the supplier, did not necessarily contain green earths only. It seems relevant to highlight that names provided by suppliers usually refer to the hue or the colour, and not automatically to the mineral

146 composition of the material. In some cases such as green earth from France (40830 Kremer, Figure 4.9, c; Table C.2), and green earth from Verona (40821 Kremer, Table C.2), synthetic organic pigments were detected such as PG 7 (Figure 4.9, d). In some other cases, the signature of anatase overwhelmed the signal from the other components. Indeed, anatase is a very good Raman scatterer, while in general the Raman intensity of silicates is weaker. Anatase was detected together with goethite, calcite and clay minerals (halloysite, dickite) in powdered green earth from Verona (11000 Kremer, Table C.2). The bulk Verona green earth sample yielded a complex Raman signature including anatase, calcite, haematite and silicates. The green earth mineral does possibly contain celadonite, as indicated by the presence of OH stretching vibrations at 3606 and 3566 cm-1.

Some copper containing green pigments are also artificially made reaction compounds, prepared according to various recipes (Kühn 1970, Guineau 2005, Chaplin et al. 2006, Eastaugh et al. 2008). All the treatises require the exposition of copper plates to different acidic media (urine, acetic acid, etc.) with a variety of additives (salt, honey, etc.) to produce blue-green reaction compounds that are now named verdigris (Kühn 1970, Eastaugh et al. 2008). This material could be dissolved in resin to produce the so called copper resinate, appreciated for the use as a glaze (Kühn 1970). Nomenclature issues have been found: in fact a variety of names have been used to identify the reaction product of metallic copper and acetic acid, such as «viridis cupris», «ærugo» or «viride salsum» (De la Roja et al. 2007, Eastaugh et al. 2008). Raman spectroscopy proved successful in identifying these compounds, with some significant differences related to the excitation wavelength used (Conti et al. 2014b). The best overall results seem to be obtained with a 785 nm laser (detection of both verdigris and copper resinate, even when mixed with linseed oil) (Conti et al. 2014b).

Seen the variety of reagents that have been listed in recipes from the past as necessary to produce verdigris, it is clear that a variety of (similar) chemical compounds has to be expected in the final pigment. Different blue-green copper acetates have actually been synthesized following the traditional recipes, and characterized (Chaplin et al. 2006, Eastaugh et al. 2008, San Andrés et al. 2010, Conti et al. 2014b). Raman spectroscopy has shown good results in this sense (Chaplin et al. 2006, De la Roja et al. 2007, San Andrés et al. 2010, Conti et al. 2014b), with the

147 successful identification of different copper acetates, Cux(CH3COO)y(OH)z·nH2O (San Andrés et al. 2010) and other compounds (cuprite, atacamite, etc. (Chaplin et al. 2006, De la Roja et al. 2007)). These latter materials seem to disappear after recrystallization/purification of the pigment, as they are converted into acetates (De la Roja et al. 2007). Reference values for different acetates can be found in literature (Bell et al. 1997, Chaplin et al. 2006, Pereira et al. 2006, De Laet et al. 2013): it seems that verdigris is always a mixture of compounds, not necessarily related to the pigment synthesis (Chaplin et al. 2006, De la Roja et al. 2007). The most characteristic bands which are used for successful identification of this copper pigment are related to methyl groups (stretch: 2930-3020 cm-1; bend: 1350-1450 cm-1; rock: 1050-1060 cm-1) and to carboxylates (antisymmetric and symmetric stretch: 1540-1650 and 1260-1450 cm-1); the ν(C-C) stretching vibration is observed at 930-940 cm-1; the δ(O-C-O) deformations at 600-680 cm-1 (San Andrés et al. 2010). In case OH groups are present, bands appear at 3050•3650 cm-1 (San Andrés et al. 2010). Slightly different band positions and assignments can be found in

Figure 4.10: Raman spectra of Cu-containing artificial green pigments. a: Verdigris (Kremer 44450, 532 nm); b: Cu resinate Kremer 1220 (532 nm) 3x60s; c: Egyptian green (Kremer

11100, 532 nm); d: Tridymite SiO2 R090042; e: Co green (Kremer 4410, 532 nm).

148 (Bell et al. 1997, Conti et al. 2014b). Also, the results from the study of the pigments themselves are not straightforwardly applicable to real samples, as the bands of binding media overlap with the organic part of these greens (Conti et al. 2014b), and binding media alteration due to ageing can also have an effect (Nastova et al. 2012). The best results seems to be obtained both with the 532 and 785 nm lasers, for the identification of the pigment, even mixed with linseed oil (Conti et al. 2014b). In the present study, Raman spectra of verdigris (44450 Kremer) were recorded both with the 785 and 532 nm lasers (Table C.2). In both cases the identification was successful, even if the spectrum recorded with the red laser is noisier and shows only a few diagnostic bands (948, 320, 180 cm-1 (Conti et al. 2014b)). On the other hand, with the green laser (Figure 4.10, a) it is possible to better understand the chemistry of copper acetates present in the sample. When comparing our spectrum with published values, the greatest similarity is observed with sample B2 [Cu(CH3COO)2·Cu(OH)2·5H2O] and sample C

[Cu(CH3COO)2·2(Cu(OH)2)] published in (Chaplin et al. 2006), with specific bands at 1057 and 2695 cm-1 pointing to each of the two compounds respectively.

Copper resinate is a transparent green glazing material obtained by mixing resinous materials with verdigris (Conti et al. 2014b). Copper salts of abietic acid are considered diagnostic for a positive identification of this material (Colombini et al. 2001, Abdel-Ghani et al. 2009). Such a pigment has been identified in artworks from different cultures and time periods (Kühn 1970, Eastaugh et al. 2008). However, it has to be noted that copper resinate can also be a product of interaction of copper pigments, such as verdigris, with organic binding media (Conti et al. 2014b). Therefore, it is difficult to conclude if the identified copper compound was prepared as copper resinate before its application, or if it results from the alteration of a Cu containing painting layer. The 1220 Kremer sample was brittle and naturally aged in our laboratories (Figure 4.10, b). Two more naturally aged reference materials prepared in KIK•IRPA laboratories were also studied. The sample KIK-CR1 was prepared in 1981 using the resin from Abies pectinata, while KIK-CR2 (1973) is a mixture of copper resinate and oil. Also these samples were brittle, but the second one was sticky, probably as a result of the oil component. Due to the glaze-nature of this pigment, its identification is difficult. As regards Raman spectroscopy, the main issue is fluorescence (Conti et al. 2014b). In comparison to verdigris, the Raman spectrum of copper resinate is showing bands of ν(C-C) stretch (1600-1700 cm-1) (Conti et al. 2014b). The increase in the

149 ratio 1612/1648 cm-1 can suggest a higher aromatic unsaturation or slower oxidation of abietic acid (Conti et al. 2014b). The best laser wavelengths for studying copper resinate, according to a recently published systematic study (Conti et al. 2014b), are 785 and 830 nm. A Raman spectrum attributable to a copper containing organo-complex from an illuminated manuscript has been recorded with 633 nm source (Nastova et al. 2012). No results could be obtained with the 785 nm laser, while green lasers (532 nm and 514 nm) provided better results. However, the spectra recorded on sample CR-KIK2 were overwhelmed by fluorescence (probably related to the oily component) and no bands could be observed. The band positions are in good agreement with published values (Conti et al. 2014b) for both 532 and 830 nm lasers (Figure 4.10, b). Differences in acquisition time and accumulations have to be pointed out and could explain the absence of some Raman bands (Conti et al. 2014b) and the change in the Raman profile of some bands. Moreover, different instruments, having different setups, will give slightly different results in term of Raman spectra. A very strong, broad, asymmetric band with maximum at 2933 cm-1 is attributed to the antisymmetric stretch of ν(CH2) and ν(CH3) units (with the symmetric stretching visible at ca. 2870 cm-1); the ν(COO) stretching vibration is also intense, broad and asymmetric, with a maximum at 1614 cm-1, while its in plane -1 bending is at 709 cm . Bands related to CH3 vibrations are observed at 1445 and 1074 cm-1. The ν(CC) stretching and δ(CH) wagging modes are observed respectively at ca. 970 and ca. 770 cm-1. Some features attributable to copper acetates are those at 1410 and 120 cm-1 (Conti et al. 2014b). Lattice vibrations involving Cu might explain the band at 437 cm-1, which is found as well in malachite (Bell et al. 1997, Frost and Martens 2002, Yu et al. 2013). The band at 202 cm-1 (Nastova et al. 2012, Conti et al. 2014b) is however missing from our spectra, unless it is corresponding to the one at 220 cm-1, therefore displaying a significant Raman shift which could be explained by compositional differences or by ageing.

The synthetic pigment Egyptian green was obtained from different proportions of the same ingredients used for producing Egyptian blue (Pagès-Camagna et al. 1999, Bianchetti et al. 2000, Pagès-Camagna and Colinart 2003, Eastaugh et al.

2008). This latter is a calcium copper silicate of formula CaCuSi4O10, while the green material has been defined as copper containing wollastonite (Ca,Cu)3Si3O9 (Pagès-Camagna et al. 1999, Bianchetti et al. 2000, Pagès-Camagna and Colinart 2003, Eastaugh et al. 2008). The insertion of ca. 2 % Cu in the formula, however, does not allow for distinction between the Raman spectra of wollastonite and

150 cupro-wollastonite (Pagès-Camagna et al. 1999). There are no ancient recipes available for this green pigment production (Pagès-Camagna et al. 1999). It is known that differences in the production process of this pigment (firing temperature, atmosphere, cooling rate (Pagès-Camagna and Colinart 2003)) influence the resulting material, however a silicatic enriched compound is expected as a final product. The Raman spectra recorded on the commercial sample of Egyptian green (10064, Kremer, Figure 4.10, c), however, show different compounds, such as monoclinic tridymite (bands at 210, 302, 355, 433, 789 and 1075 cm-1 (Kihara et al. 2005), Figure 4.10, d; Table C.2) and Cu-containing materials. Tridymite is reported to be the dominant crystalline phase in recently produced pigment produced at temperatures higher than 950 °C (Bianchetti et al. 2000). The weak bands at 406, 391, 253, 142 and 110 cm-1 may be assigned respectively to ν(Cu-Cl) stretching (Frost et al. 2002a), symmetric stretching ν(O•Cu-O) (Liu et al. 2011), bending δ(O-Cu-O) (Liu et al. 2011), bending δ(O-Cu) (Frost et al. 2002a, Liu et al. 2011). Raman spectra of wollastonite are published mainly from a mineralogical point of view (Huang et al. 2000), and correspondences are observed with archaeological samples from Egypt (Pagès-Camagna et al. 1999).

Cobalt containing greens are described as zincates or titanates, commercially sold as pigment green 50 (PG 50) (Eastaugh et al. 2008, Casadio et al. 2012). The studied sample (44100 Kremer, Figure 4.10, e) is described as cobalt titanate green spinel, however, this name may apply to a variety of compositions and structures (Casadio et al. 2012). Cobalt titanate green pigments show shifts in the main Raman band position, which is the one at ca. 700 cm-1. This band shows as well broadening. This depends on the presence of other metallic ions in the lattice (Casadio et al. 2012).

This band is attributed to the symmetric stretching of CoO6 octahedra in CoTiO3, which suggests that it might be assigned to the same octahedral vibration in

Co2TiO4 green pigments (Casadio et al. 2012). Other weaker bands are reported in Table C.2, which are in reasonable agreement with published values of reference cobalt greens (Casadio et al. 2012), considering the already mentioned band shifts. The observed band positions at 1135, 982, 620 and 460 cm-1 suggest the presence of an extender of the family of sulphates (possibly epsomite

MgSO4·7H2O (Makreski et al. 2005)). It is known that such additives have been used for the production of modern pigments, but this finding is in contrast with what previously identified in cobalt green (44100 Kremer), which was barite

151 (BaSO4) (Casadio et al. 2012). A cobalt titanate green pigment has been recently identified in a 20th century painting (Casadio et al. 2012). Chromium oxide greens were produced starting from the 19th century and following various recipes (FitzHugh 1997, Eastaugh et al. 2008). The end product is however chromium (III) oxide, anhydrous or dihydrated (viridian Cr2O3·2H2O) (FitzHugh 1997, Eastaugh et al. 2008). “Chrome green” was also used as a name for the mixture of Prussian blue and chrome yellow (FitzHugh 1997, Desnica et al. 2003, Rosi et al. 2004, Abdel-Ghani et al. 2008), while viridian was often mixed with barium sulphate, which negatively affects the saturation of the pigment, but makes it easier to disperse it in binding media (FitzHugh 1997). Notable occurrences of chrome oxide greens can be found listed in literature (FitzHugh 1997), however, Raman spectroscopic identification is more limited (Ropret et al. 2008).

The structure of Cr2O3 corresponds to the mineral eskolaite and Raman spectra of chromium oxide are published both in relation to geology and archaeometry (Hardcastle and Wachs 1988, FitzHugh 1997, Bell et al. 1997,

Figure 4.11: Cr-containing green pigments. a: Chrome oxide green Cr2O3 (Kremer 44204);

b, c, d: viridian Cr2O3·nH2O (Kremer 4425). It seems that the pigment Viridian is a

mixture of anhydrous and hydrated Cr2O3. The term “viridian” refers to the hydrated form (spectrum d).

152 Maslar et al. 2001, Bouchard and Smith 2003, Bouchard 2007, Ropret et al. 2008, von Aderkas et al. 2010).

The most intense Raman band of Cr2O3 (Figure 4.11, a) is the A1g mode at ca. 550 cm-1 (Sammelselg et al. 2010), and weaker bands are observed at 597, 391, 340 and 304 cm-1 with the 532 nm laser for both chrome oxide greens studied (44200 and 44204 Kremer; Table C.2). These band positions are in good agreement with published data (Castro et al. 2005, von Aderkas et al. 2010). With the 785 nm laser, bands are observed at 613, 555, 351 and 299 cm-1 (Castro et al. 2005, Ropret et al. 2008). It seems important to highlight that the position of the main Raman band of chromium oxide shifts according to the used laser (532 nm: 542 cm-1; 785 nm: 555 cm-1).

The term viridian refers to the hydrated form Cr2O3·2H2O (Castro et al. 2005, Correia et al. 2007, Eastaugh et al. 2008). It seems that the pigment viridian (4425 Kremer) is a mixture of anhydrous and hydrated chromium oxides, as the recorded Raman bands (532 nm, Figure 4.11, b, c and d) at 583, 552, 487 and 266 cm-1 are attributed to the hydrated oxide, and those at 543, 345, 307 and 266 cm-1 to the anhydrous one (Maslar et al. 2001).

Green synthetic organic pigments are based on a variety of molecular structures, such as phtalocyanines, azo and monoazo pigments. The pigments studied during this research are the polycyclic phtalocyanine greens PG 7 and PG 36 (Cu•containing, chlorinated and brominated, respectively) and PG 8 (Fe•containing, azo metal complex) (Scherrer et al. 2009). The detection of such pigments in archaeological objects allows the identification of recent restoration treatments or forgeries (Burgio and Clark 2000, Aliatis et al. 2009), as these pigments were synthesized starting from the beginning of the 20th century (Eastaugh et al. 2008). The Raman spectra of the phtalocyanine greens PG 7 and PG 36 were successfully recorded both with the red and the green lasers, showing different relative intensities for some of the bands (see Table C.2). The PG 7 spectrum (Figure 4.9, d) is in good agreement with published values (Schulte et al. 2008, Scherrer et al. 2009), even if the spectra are recorded with a different source (488 and 514 nm respectively). In fact, some bands present a downshift of ca. 5 cm-1 when compared with those references (Schulte et al. 2008, Scherrer et al. 2009), and some bands reported for the 488 nm laser excitation are also visible in our 532 nm excited spectrum (642, 506, 368, 346, 332, 163 and 146 cm•1 (Schulte et al. 2008)) and some in the 785 nm spectrum (740, 345, 331, 288, 165, and 147 cm-1 (Schulte et al. 2008, Fremout and Saverwyns 2012)). The band at 740 cm-1 is the most intense in the Raman spectrum acquired with the 785 nm laser in

153 Figure 4.12: Raman spectra of synthetic organic green pigments. a: PG 36 (532 nm); b: PG 36 (785 nm); c: reference spectrum of PG 36 (785 nm (Fremout and Saverwyns 2012)); d: PG 8 (785 nm). this study, while another published spectrum displays a very strong 1560 cm-1 band (Fremout and Saverwyns 2012). PG 36 is a copper phtalocyanine containing chlorine and bromine as well. Our spectra show generally a good agreement with published values (Scherrer et al. 2009, Fremout and Saverwyns 2012). Again, different lasers give slightly different spectra (Figure 4.12, a and b). The band positions and intensities recorded with both lasers are reported in Table C.2 and are in good agreement with published values (Scherrer et al. 2009, Fremout and Saverwyns 2012). A downshift in some bands is observed when comparing different green lasers, as the published values (514 nm) are 1539 and 1498 cm-1, and the observed ones (532 nm) are 1527 and 1490 cm-1. PB 15 is also detected by the bands at 1528, 950 and 748 cm-1 (Scherrer et al. 2009). PG 8 is based on Fe and azo groups. Our sample is in good agreement with the only reference spectrum published so far (Figure 4.12, d), with a 785 nm laser (Fremout and Saverwyns 2012), while some differences are observed with the

154 532 nm excited spectrum. To the best of our knowledge, we are the first ones to report the Raman spectrum of PG 8 with a 532 nm excitation (Table C.2).

It has been reported in literature that the behaviour of the reaction compound copper resinate as a pure pigment and in paint samples, together with oleo- resinous binders and other components, is different (Conti et al. 2014b). During this research we had the opportunity to investigate a sample from the Ghent Altarpiece. This masterpiece of Early Netherlandish art (Jan and Hubert van Eyck, second quarter of 15th century (Coremans 1953)) is currently being restored and investigated for conservation purposes. A sample, taken from a green area of panel Archangel and Prophet Zacharias (Figure 4.13, a) during the conservation campaign of years 1951-1952, was re-investigated. The stratigraphy of the sample is complex (Figure 4.13, b), and shows the superposition of green layers on top of a greyish imprimatura. The lowest layer (layer 4) shows a bluish green hue, the upper one (layer 5) is light green. Finally, a semitransparent green layer (layer 6) is applied before a yellow glacis. Copper resinate has been identified in this sample, as well as in six others taken from the panels IX, X, XII and XX of the Ghent Altarpiece. The identification was performed in 1979 by Kockaert in the KIK-IRPA laboratories, using micro-chemistry. Copper resinate was identified also in other 27 samples from various Netherlandish paintings (Kockaert 1979). In this study, micro-Raman analysis were carried out in order to confirm the presence of copper resinate in the cross section and to evaluate if any difference could be observed between spectra recorded with different green lasers as

Figure 4.13: a) detail of the painting Archangel and Prophet Zacharias. The sample was collected during the campaign of 1951-1952 on the green book cover; b) microphotograph of the studied cross section. Photos KIK-IRPA (Sint-Baafscathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW).

155 excitation sources (514 and 532 nm, see Figure 4.14). For this last purpose, care was taken in order to acquire the spectra in the same spots with the different instruments. The results obtained with the two instruments proved comparable, even if the quality of the recorded spectra is not the same. Differences in acquisition time and accumulations have to be pointed out and could explain the observation of additional Raman bands and some changes in the band profile, see for example: 3x60 seconds (Conti et al. 2014b) (Figure 4.10, b) versus 60x100 seconds (Figure 4.14, d). Also, different Raman spectrometers will affect the results, as a consequence of the different setups (Figure 4.14: spectra a and b are from the sample, spectra c and d from the reference material). It is therefore of extreme importance to be careful when comparing spectra with published data. We are the first ones to report the Raman spectrum of copper resinate with a 514 nm laser excitation (Figure 4.14, c). In layer 4, indigo (1706, 1584-1570, 1364, 1310, 1252, 1148, 598 cm-1) and -1 lead tin yellow type I (Pb2SnO4) (129 and 196 cm ) were identified. Once again, the mixture of yellow and blue to obtain green is noted. The Raman spectrum of

Figure 4.14: Raman spectra of a Ghent Altarpiece cross section. a) C10.020, layer 5 (532 nm) 10x150s; b) C10.020, layer 5 (514 nm) 10x40s; c) Cu resinate (Kremer 1220, 514 nm); d) Cu resinate Kremer 1220 (532 nm) 3*60x100s. The bands marked with dashed lines correspond to lead tin yellow type I (195 and 128 cm-1)

156 indigo seems better excited with the 514 nm laser. Layers 5 and 6 contain both copper resinate (identified by the features at ca. •1 -1 2900 cm and that at ca. 1610 cm ). It is important to mention that the ν(CH2) -1 and ν(CH3) broad stretching band at 2900 cm can be related to the oily binding medium, and that the most diagnostic feature of copper resinate is that of ν(CC) stretching vibrations at ca. 1610 cm-1 (Conti et al. 2014b). In some spectra, -1 additional bands at 1437, 1305 and 700 cm , also referable to Cu resinate δ(CH2) bending and ν(CC) stretching modes, were detected with both lasers (Figure 4.14, a and b). In addition to copper resinate, in layer 5 also lead tin yellow type I was identified by the bands at 195 and 128 cm-1 (Bell et al. 1997) (Figure 4.14, a and b). In layers 4 and 5 an intense broad band centred at 549 cm-1 was recorded with both instruments. The assignment of this band to lazurite was considered at first, however, various aspects contradict this hypothesis. First, the lack of the medium intensity band at 1096 cm-1 does not allow for a reasonable attribution to this blue pigment (Bell et al. 1997). Also, considering the high price and symbolic value of this blue pigment, especially during the Middle Ages, its use in the green area of a character of secondary importance (compared to the Virgin Mary, whose vest was traditionally painted with ultramarine), seems suspicious (Eastaugh et al. 2008). This band could not be assigned. Moreover, it is important to note that the detection of copper resinate mixed with drying oils was never published so far with green lasers. Both the 514 and 532 nm lasers were considered not suitable for the identification of pure copper resinate or mixed with oleoresinous binding media (Conti et al. 2014b).

The present study highlighted some remarkable aspects regarding the study of green materials of interest for the investigation of Cultural Heritage. An extensive literature survey has been performed regarding the potentialities of Raman spectroscopy in identifying different green pigments in a variety of objects, as well as to find published spectra for comparison purposes. By doing this, it appeared clear that not all of the necessary reference spectra were available, and that the low wavenumber (below ca. 200 cm-1) and ν(OH) stretching (above 3400 cm-1) regions are not always considered, although they provide many interesting features useful for spectral identification. Therefore, the extended range spectra have been discussed in this paragraph and all the spectra collected on reference materials during this research can be requested online at http://www.analchem.ugent.be/RAMAN/.

157 By means of Raman spectroscopy, several commercial green pigments were fully characterized. It was possible to define which “green earth” was present and to identify impurities and additives, thanks to the combined use of different excitation wavelengths. In the case of Cu-containing materials, most of the suggested identifications were confirmed, but in some cases a new interpretation was suggested on the basis of non-destructive analyses. The study of modern pigments also allowed the collection of reference spectra with different wavelengths of some less usual materials, which increases the possibilities of successful identification of pigments in forthcoming studies. The traditional pigment copper resinate has been investigated both as reference material and in a cross section, with different green lasers, which both proved effective in detecting it, even if the measurement conditions and instrumental parameters may affect the quality of the recorded spectra. The successful identification of copper resinate in cross sections from the Ghent Altarpiece was achieved thanks to the comparison of the recorded spectra with those of reference materials. However, this comparison needs to be careful, as differences in the acquisition of the Raman spectra affect the result of the measurements. In this case, green lasers of different wavelength (532 and 514 nm) proved effective in detecting the synthetic pigment copper resinate in a cross section from an aged oil painting. The Raman characterization of the cross section additionally allowed the identification of lead tin yellow type I, and of indigo, which is not feasible by elemental techniques such as XRF or SEM-EDS. Also, the identification of copper resinate, which is used as a glaze, and no pigment particles can be identified in the layer, poses some specific issues. In fact, it cannot be identified by means of XRD (which can, on the contrary identify and discriminate among the different verdigris compounds (Kuhn 1970, Altavilla and Ciliberto 2006, Švarcová et al. 2014)), Other techniques that proved successful in the identification ond study of copper resinate are XAS (Cartechini et al. 2008) and XPS (Altavilla and Ciliberto 2006). Moreover, thin layers of low-diffracting materials embedded in resin can be a challenge for μXRD measurements (Švarcová et al. 2014). In addition to the discrimination among different green materials (including pigments and corrosion products), Raman spectroscopy offers the advantage of detecting compounds such as carbon, which was extensively used as a black pigment. Moreover, additional considerations on the nature of the materials under investigation become possible thanks to the sensitivity of the technique to disorder. The next paragraph focusses on carbonaceous materials of relevance for the study of Cultural Heritage objects.

158 Similar to what has been observed in relation to the use of XRF techniques for obtaining a complete characterization of green materials (especially the Cu•based ones), it appears that carbon blacks can be successfully studied by means of Raman spectroscopy. In fact, carbon cannot be detected by means of XRF techniques, and its presence is sometimes inferred by the lack of other black pigments based on iron or manganese (Edwards and Vandenabeele 2012). Moreover, the possibility of obtaining structural information from Raman spectroscopy, thanks to the link between Raman spectral features and the specific raw material used for making the black pigment, might have interesting archaeological/art historical implications. The next paragraph tackles the topic of carbon-based black pigments, on the same note of exploring the advantages of vibrational characterization of materials in works of art. It is based on the paper “Raman Spectroscopy for the investigation of Carbon-based black pigments” by A. Coccato, J. Jehlicka, L. Moens, and P. Vandenabeele, published in 2015 in the Journal of Raman Spectroscopy (Coccato et al. 2015). Additional information on the Raman bands of carbonaceous materials and on the observed Raman signatures can be found in APPENDIX D. The use of Raman spectroscopy to characterize carbonaceous materials of geological interest is widespread, and partially overlapping with archaeometrical research. However, different terminology is used in these fields, making the interpretation of the Raman spectra of carbon complicated. Raman spectroscopic studies of carbonaceous materials are, until now, mainly devoted to geological and industrial materials. On the other hand, it is known from artistic literature that many varieties of carbon-based black pigments were produced and used in different places and times, and according to the artist’s preferences. The ability of Raman spectroscopy to analyse particles down to 1 µm and its non-destructiveness make it an ideal tool for pigment investigation. Anyway, the discrimination among different types of carbon-based black pigments is affected by various aspects, one of which is the lack of reference spectra as well as of specific nomenclature. Carbonaceous matter can be found in nature as geological deposits of graphite and related materials, or can be produced by firing organic materials. The composition of carbonaceous matter from rocks can vary from pure carbon (graphite, diamond, lonsdaleite, chaoite, fullerene), through polymers (e.g. fossil resins, kerogen) to more or less liquid complex mixtures of hydrocarbons (e.g. petroleum, bitumens). These materials are widely distributed in sedimentary series throughout the world, originating from biota from different environments and of different ages of the Phanerozoic era. The use of carbonaceous matter is

159 recorded in a variety of fields. Coal and petroleum have served as sources of energy and of derived chemicals for a long time. Natural geological carbonaceous compounds are also used as gemstones: jet is a fashionable carbonaceous matter derived from carbonification of wood-rich sediments from few sites of Jurassic age (UK, Asturias, Whitby); diamond is a rare transparent and hard phase used in jewellery. As a pigment or as a drawing material, in the field of Cultural Heritage, carbon•based materials play an extremely important role. Carbon materials, of mineral, vegetable or animal origin, composed ideally of pure carbon, were largely employed during prehistory (Gettens and Stout 1966, Chalmin et al. 2003, Ospitali et al. 2005, Berrie 2007, Rampazzi et al. 2007, D’Errico 2008, Tomasini et al. 2012b, Olivares et al. 2013) and were never abandoned by artists and artisans (Castro et al. 2007a). Carbon-based materials are suitable for both dry and liquid drawing (in the form of graphite, charcoal sticks, black chalk, pastels and inks, respectively (Berrie 2007)), and have been used as pigments in paintings (Menu and Walter 1992, Zoppi et al. 2002, Chalmin et al. 2003, Edwards et al. 2003, Languri 2004, van der Weerd et al. 2004, Ospitali et al. 2006, Striova et al. 2006, Rampazzi et al. 2007, Prinsloo et al. 2008, Giachi et al. 2009), in polychrome objects (Edwards et al. 2004, Castro et al. 2007a), and for pottery (Ospitali et al. 2005, Caggiani and Colomban 2011). The continuity of use through time and the worldwide distribution of this kind of materials requires the establishment of a well-defined terminology that could be easily used for archaeometrical applications, but that keeps into account the major contributions of geological and industrial research to the study of carbon•based materials, along with the information from artistic literature (treatises (Cennini: Il libro dell’arte, Berrie 2007, Eastaugh et al. 2008). The lack of archaeometrical literature on the specific topic of distinguishing among different carbon-based black pigments (Tomasini et al. 2012b, 2012a) made necessary a survey of geological and industrial literature, which indeed suggested some useful ideas for data processing and interpretation, while many aspects were not considered. This latter fact is mainly due to an important consideration, which is the non-correspondence between geological and artistic nomenclature (Winter 1983). The scope of this work is of offering a general terminology for archaeometrical purposes and of providing some indications for a better comprehension of carbon•based black pigments. Besides carbon-based blacks, other black pigments are, for example, iron and/or manganese containing oxides/hydroxides with spinel structure (Spring et al. 2003, van der Weerd et al. 2004, Ospitali et al. 2005, Mironova-Ulmane et al.

160 2009, Caggiani and Colomban 2011, Buzgar et al. 2013), pure metals as powdered bismuth (Buzzegoli et al. 1996, 2000, Spring et al. 2003, Trentelman 2009), black minerals as tenorite (Eastaugh et al. 2008), enstatite (Eastaugh et al. 2008), stibnite (Spring et al. 2003), galena (Spring et al. 2003) and tourmaline, and other compounds of synthetic origin (copper nitrate derivative, hydrous molybdenum oxide (Eastaugh et al. 2008)). All these materials, except tourmaline, have been identified in archaeological and art historical manufacts.

Table 4.2: Forms of carbon according to (Winter 1983, van Krevelen 1984, Marshall and Fairbridge 1999) and JJ.

Crystalline hexagonal form of carbon, of natural or artificial origin. d002 = 0.3356 GRAPHITE nm, P63/mmc, different uses since prehistoric times up to today.

FLAME Carbon produced in the gas phase, from incomplete combustion of hydrocarbons. CARBONS/SOOTS Turbostratic structure. Product of wood pyrolysis. Non-graphitizable carbons: the solid phase structure is CHARS maintained during carbonization. Man-made material and fuel produced by carbonization of bituminous coals or COKES bitumen. Graphitizable carbons: the precursor is liquid/plastic immediately before carbonization. Cokes made from coal are highly porous. Organic rocks (lithified plant remains) composed of macerals (i.e. the equivalent of minerals for the inorganic rocks) that were formed during the Carboniferous Period. Initially unaltered sedimentary material containing plant remains change COALS progressively (these changes include physical, biochemical and chemical processes) during diagenesis and catagenesis. Allochtonous materials are also present, as quartz and carbonates.

Concerning the archaeometrical study of carbon blacks, it is necessary to point out a fundamental issue, which is the simultaneous presence of different carbon types, according to geological definitions, in a specific carbon-based black pigment (Winter 1983). Elementary carbon exists in one of its allotropic crystalline forms as graphite. More or less disordered materials occur, and comprehend flame carbons, cokes, chars and coals (Winter 1983) (see Table 4.2). Historical carbon-based pigments cannot be described properly using the previous categories, because they can contain some different substances (from a compositional or structural point of view). For studying carbon-based black pigments a different classification is proposed (Berrie 2007, Winter 1983), which is based on the production processes: carbon•blacks of mineral origin, pigments from firing vegetable materials or from animal ones, pigments obtained in the form of soots and smokes. Still, some carbon-containing pigments cannot be ascribed to any of these categories, as

161 iron•gall inks (salts of tannic acid) and the black ink produced by Cephalopoda (Sepia black). Both these organic compounds are enriched in Fe. It is important to stress that the term “carbon black” is often used to encompass pigments, while “black carbon” is used for particulate atmospheric residual carbonaceous matter sometimes deposited from atmosphere to soils.

Common geological phases of carbon (e.g. graphite and diamond, as well as rare minerals chaoite and lonsdaleite) occur in nature as crystalline. Other carbonaceous compounds, generally not pure carbon, from rocks, are frequently more or less amorphous (Ulyanova et al. 2014). Bitumens, solid bitumens, resins and fossil resins as well as associated organic compounds in soils and rocks show always a lower structural organization than the ordered form of hexagonal carbon (i.e. graphite). Hence, X-ray diffraction (XRD) or electron diffraction (ED) techniques are not suitable for analysis, unlike their primary role in analytical mineralogical studies. XRD, being sensitive to crystalline arrangements, shows clearer results for graphite and for graphitizable carbons (Winter 1983, Bernard et al. 2010). Non-crystalline carbons show diffuse bands, but some interesting features can be observed in the small-angle scattering region (Winter 1983); other compounds can be detected as well, like quartz and iron oxides in black earth, and hydroxyapatite in bone black (Winter 1983, Tomasini et al. 2012). SEM backscattered electrons images are also very informative on the raw materials used to prepare the carbon black (Winter 1983, Tomasini et al. 2012). Transmission electron microscopy (TEM) represents a useful tool to recognize and visualize the heterogeneity of the samples and describe the so-called microtexture i.e. mutual arrangement of structural units (Rouzaud and Oberlin 1989). Raman spectroscopy has played and continues to play an important role in the structural characterization of graphitic/carbonaceous materials of different origin (Oberlin 1984, Dresselhaus et al. 2005), as the spectrum is affected by the degree of crystallinity. This technique has been hugely used to characterize graphitic materials (highly oriented pyrolytic graphite, carbon fibers, glass-like carbons (Wang et al. 1990) and carbonaceous nanomaterials (fullerenes, and carbon nanotubes (Dresselhaus et al. 2005, Jehlicka et al. 2005)). Raman spectroscopy has been found to be very sensitive to structural changes that modify translational symmetry in carbonaceous materials, thus being an ideal method for characterizing the carbonaceous matter structure. Careful Raman investigation of the carbonaceous matter of geological materials (metamorphic rocks, inclusions in magmatic rocks, dispersed carbonaceous matter) permits us to learn more about the structural order in carbonaceous matter and to obtain

162 salient information on provenance. Because of the amorphous character of many sedimentary and even metamorphic carbons, as well as of carbons used frequently in fine arts artifacts, other appropriate techniques of characterisation can be evoked, typically TEM. This method permits to visualise basic structural units (BSU) or molecular units and their orientation in space, but sampling is needed, which is rarely allowed for cultural heritage objects. It could be indeed very usefully combined with Raman spectroscopy. Ordered graphite consists of arrangements of two-dimensional graphene sheets parallel-oriented with carbon atoms being arranged in hexagonal rings 2 through localized in-plane 2s, 2px and 2py (sp ) orbitals. The individual sheets are weakly bonded by delocalized out-of-plane 2pz orbitals, which overlap to give a delocalized electron system (Dresselhaus et al. 2005). All other carbons (disordered graphite, soot, “carbon black”, “lampblack”, and the different forms of amorphous carbon) display less periodical structure (generally with only biperiodic arrangement of BSU) (Oberlin 1984), that modifies the structural order, and thus can be monitored in the Raman spectrum. A proper understanding of the disorder-related features in the Raman spectrum of carbon will certainly improve the chances of reconstructing the formation process of a certain carbon-based black material. Some ideas have been collected from geological literature and carefully transferred to the archaeometrical field of interest, always keeping into account that the experimental conditions of measurement (first of all, the laser wavelength) and the spectral processing vary. The spectral region between 1000 and 1800 cm-1, the carbon first-order Raman spectrum, contains most of the structural information for carbonaceous materials. In hexagonal graphite a single band at 1582 cm-1 occurs and is assigned to the G band in-plane aromatic ring CC stretching vibrational mode with E2g2 symmetry (Tuinstra and Koenig 1970). However, when disordered sp2 carbons are present, significant spectral modifications can be observed (the G band broadens and new bands arise) as a consequence of decreased order (crystallite size, presence of impurities, etc.) (Figure 4.15). The so-called D bands have been correlated with different causes of disorder. The 1355 cm-1 band (D1) has been first interpreted as particle size effect, as the

A1g mode becomes active due to finite crystal size (Tuinstra and Koenig 1970). This band was also assigned to edge effect, plane defects and heteroatoms as oxygen, or double bonds. Other impurities are considered responsible for this broad band. Vidano et al. (1981) observed the absence of this band in compression-annealed carbons, which was later suggested as an indicator of increasing degree of metamorphism (Pasteris and Wopenka 1991). This band is also absent in highly ordered graphites (Beyssac et al. 2002, Lünsdorf et al. 2014).

163 The D1 band position is found to cover the whole range 1240 to 1400 cm-1 (Yoshikawa et al. 1988, Fonti et al. 1990, Nikiel and Jagodzinski 1993, Dippel and Heintzenberg 1999, Marshall and Marshall 2013) and it shifts with the used laser wavelength (Yoshikawa et al. 1988, Sadezky et al. 2005, Lünsdorf et al. 2014). Its intensity depends on the carbon type (Wang et al. 1990), on the amount of disorder (Tuinstra and Koenig 1970), on the orientation of the crystal with respect to the laser (Jehlička et al. 2003) and on the measurement conditions (Table D.1). The D2 band at ca. 1600 cm-1 is assigned to the splitting of the degenerated

E2g vibration (Wang et al. 1990, Jehlicka et al. 2005). This band is expected whenever the D1 band is present (Sheng 2007). The G and D2 bands positions overlap, creating a more or less symmetric feature in the Raman spectrum of disordered carbons. This band appears (and its intensity increases) in disordered

Figure 4.15: a) Ordered graphite; b) graphite from metamorphic rocks; c) black earth, 785 nm laser, 4mW, 30x10s; d) black earth 532 nm laser, 1.45mW, 50x10s. The selected spectra show some examples of carbonaceous materials Raman spectra. Different measurement conditions were required to obtain those signatures, which correspond to ordered structures (a and b, graphites), and to less ordered materials (c and d, black earth). Also, the effect of the laser wavelength on the band positions and relative intensities is visible in black earth Raman spectra (c and d).

164 materials, but it is found to be resolvable from the G band in ordered ones (Jehlička et al. 2003). The D3 band position shifts from 1440 to 1550 cm-1. Its assignment is still debated but it suggests the presence of disorder both as structural defects (Cuesta et al. 1994, Urban et al. 2003, Beyssac et al. 2003) and impurities (Ferrari and Robertson 2001, Sadezky et al. 2005, Ulyanova et al. 2014). The fourth disorder band of carbon, D4, has the lower wavenumber. It is found below 1290 cm-1 so that it appears mainly as a shoulder on the D1 band (Dippel et al. 1999). It is observed in extremely disordered materials (Beyssac et al. 2002), as bitumens (Jehlička et al. 2003), soots (Sadezky et al. 2005, Ulyanova et al. 2014) and wood charcoal (Yamauchi and Kurimoto 2003, Ishimaru et al. 2006, 2007). Care has to be taken to avoid spectral artefacts as a consequence of inappropriate band fitting (Angoni 1993, Ferrari and Robertson 2001, Ulyanova et al. 2014). To complete the overview of carbon bands, we also mention the one of diamond (1332 cm-1, (Ferrari and Robertson 2001)), those of fullerenes (1459•1469 and 930-970 cm-1, (Jehlicka et al. 2005)) and rhombohedral graphite (1400 cm-1 (Yoshikawa et al. 1988)), as summarized by Table D.1.

Amorphous carbon as a pigment was identified in prehistoric sites worldwide (e.g. Ekain cave, Basque country, 16500-12500 BP (Chalmin et al. 2003), ancestral Puebloan artefacts, 1000-1200 AD (van der Weerd et al. 2004)), in dynastic Egyptian manufacts (Edwards et al. 2004), in Greek-Roman remains throughout Europe (Berrie 2007, Giachi et al. 2009). Some raw materials (crayons from prehistoric sites and paint pots from Pompei) have also been investigated (Giachi et al. 2009). Pliny the Elder (Naturalis Historia) is probably the oldest source mentioning the use of carbon-based black pigments; while we have plenty of medieval artistic literature describing in detail the preparation of various pigments and polychromic artefacts (paintings, statues, ceramics…). The written recipes, anyway, are difficult to correlate with specific materials (Guineau 2005). This is related to the variety of used materials and to the problematic identification of materials on the basis of traditional names. Moreover, the empirical needs of painters and artisans, as regards obtaining specific colour shades or paint texture, certainly had an influence on the traditional recipe, which was adjusted accordingly. Furthermore, different treatises use different terms as synonyms, while sometimes it is also possible that the same material has completely different names, or that spelling issues may lead to misidentification of materials (Berrie 2007, Eastaugh et al. 2008). Artists’ material suppliers, who produce pigments according to ancient recipes, are also to be considered in this field of study, since they play an important role in restoration

165 processes. Pigments currently produced, even following old treatises, may have different characteristics with respect to the old ones due to technical aspects of the process (temperature control, anaerobicity, etc.) or to the used raw materials. It is also important to mention the existing (and also potential) confusion in terminology in recipes coming from artistic literature itself and from correlating it with scientific papers.

The previous paragraphs highlight a quantity of issues:

 inhomogeneity and general inconsistence in the naming of carbonaceous matter in literature: terminological discrepancies exist among archaeologists/ art historians, chemists/ physicists/ spectroscopists, and geologists; pigment names may have not been constant through time and space; commercial pigments may not necessarily correspond to what expected (Bouchard and Smith 2003);

 no general agreement on the origin of the so-called disorder D bands;

 Raman band shift due to intrinsic characteristics of the materials and measurement setup (orientational effects, wavelength dependent band shift and intensity, etc.), band overlap.

Reference black pigments were selected among Kremer Pigmente (Aichstetten, Germany) productions. Care was taken in purchasing pigments that have a historical use (i.e. have been already identified in works of art) or that have been described in artistic literature (Berrie 2007) (Eastaugh et al. 2008). In addition to these pigments, ordered graphite and graphite from metamorphic rocks were analyzed for comparison. A list of the studied pigments, their composition as declared by the producer, as well as some additional information as regards different ways of classifying carbonaceous black pigments are given in Table D.2. Also, the laser used for obtaining the spectra is there noted. The Raman spectrometer used is a Bruker Optics ‘Senterra’ dispel rsive Raman spectrometer with an Olympus BX51 microscope. The instrument is equipped with 532 (Nd:YAG) and 785 nm (diode) laser sources. High resolution (3-5 cm-1) spectra are recorded in the range 60-2750 cm-1 and 80-2640 cm-1 for the 532 and 785 nm lasers, respectively. The system uses a thermo-electrically

166 cooled CCD detector, operating at -65 °C. The power of each laser is controlled via the software in discrete steps. For the red laser, powers of ca. 4 and 15 mW were used; the green laser was operated mainly at 1.4 mW. The 20× objective was used for all the samples, with a spot size of approximately 10 µm. The microscope contains a joystick-controlled motorised stage and the analysed area is displayed on screen with the aid of an attached video-camera. The instrument is controlled via the OPUS software. At least three spectra per sample were acquired. Both lasers were used in order to achieve good quality Raman spectra: the red laser was used first to test all the pigments, starting with the lowest achievable laser power. If no spectra could be recorded (flat spectrum or just fluorescence), first the laser power was increased and then the number of accumulations adjusted. Different laser powers were used in order to obtain good quality spectra, while keeping the integration time constant (10 seconds). The number of accumulations varied from 10 to 300 to improve the signal to noise ratio. The laser power on the sample was selected case by case, to maximize the scattering while minimizing potential damage to the pigment itself and acquisition time. The reason for preferring the 785 nm excitation line is in its suitability for the study of many historically used pigments and for some geological applications (Marshall and Marshall 2013). It is also known that the shorter the laser wavelength, the higher is the fluorescence background. However, for geological carbon studies the most used wavelengths are in the green region of the visible spectrum. The green (532 nm) laser sometimes made the acquisition possible for those samples that showed no bands with the 785 nm excitation, while on the other hand it sometimes increased the fluorescence signal so that the carbon bands were then not recognizable at all. The variety and complexity of the studied materials made necessary this inhomogeneous approach. The excitation wavelength will be always specified in the results presentation and discussion section. The subsequent data processing was homogeneous for all the spectra. First, a linear baseline was subtracted in the range ca. 800 to ca. 1850 cm-1, then the spectra were deconvoluted in GRAMS 8.0 (Thermo Scientific) using Laurentzian shaped curves for the carbon bands, and Gaussians (for example for fitting the valley between the D and G bands) (Angoni 1993, Ferrari and Robertson 2001). It seems that a linear baseline correction is the safest option, as it is easily applicable with little bias. At the same time, it is important to be aware of the possibility of artefacts rising both from the baseline subtraction, and from the fitting procedure (Angoni 1992, Ferrari and Robertson 2001). The values obtained through this deconvolution procedure (position, width, intensity and area of the bands) were compared with the literature, but the identification of the expected G and D bands was not straightforward, especially as regards D1.

167 This band (see Table 4.2) proved to be the most difficult to identify, due to the presence of multiple bands in the range where D1 can be found. This made the calculation of ratios, such as R1 and R2 (Beyssac et al. 2002), which are related to the structural order of carbonaceous matter, unfeasible. As these ratios could not be calculated with sufficient reliability, they were not considered further. The spectrum as a whole was also considered, for identifying mineral impurities and additional features. Moreover, laser-induced heating can affect the appearance of carbonaceous materials Raman spectra (Vidano et al. 1981, Wang et al. 1990, Everall et al. 1991, Angoni 1992, Kagi et al. 1994, Beyssac et al. 2002). Total-reflection XRF analyses (TX2000 by G.N.R., Italy, described in Sub•chapter 3.4) were carried out only for qualitative purposes on selected pigments samples, to obtain more information on the chemical composition of carbon-based black pigments (characteristic impurities). The spectra were acquired for 1000 seconds (live time), using a Mo anode (40 kV, 30 mA).

The studied carbonaceous materials and carbon-based black pigments cover different grades of crystallinity, graphite corresponding to the highest order in the group, but at the same time represent the different forms of carbon listed in (Winter 1983). In addition to Winter’s classification, it is also possible to consider the origin of the pigments, as both natural and artificial materials have been used as black pigments. Among natural materials are listed minerals and organic matter that are used as they are as black pigments, as for example graphite, humic earths and sepia. For artificial black pigments, i.e. products of carbonization, it is important to consider the starting materials, so we can have carbon from vegetable, animal and mineral raw materials (Table D.2). According to the characteristics of the starting materials (graphitizable vs. non-graphitizable carbons (Winter 1983)), the resulting product will show a more or less organized crystalline structure. Berrie, in (Berrie 2007), uses a slightly different classification for the “practical pigments” (graphitic, soots and smokes, vegetable origin pigments, animal origin pigments, coals and related materials, pp. 1-38 (Berrie 2007)). A separate chapter in the book is dedicated to asphalt (pp. 111-150 (Berrie 2007)). As expected, all the pigment samples showed the two, more or less intense, broad features typical of carbon, as shown in Figure 4.15 (a spectrum of ordered graphite is given for comparison). It is known that disorder, in addition to a

168 broadening of the Raman features, causes an increase in the number of Raman•active bands (Nakamizo et al. 1974). In many cases, the red excitation allowed the recording of a good quality spectrum (graphite, Figure 4.16), but for most samples, both lasers were tested, since no clear results were given with the 785 nm diode laser excitation: sometimes the quality of the spectrum was much improved when using the green excitation (black chalk, Figure 4.15), while in some other cases, as for bituminous coals, only the green excitation allowed the recording of Raman bands instead of just fluorescence. Also, in some cases, the attempt to obtain a higher signal to background ratio by switching from the red to the green laser (as the green is preferred in geological studies) did not help in improving the signal, so that the considered spectra are those obtained with the 785 nm laser. Table D.2 summarizes the excitation conditions. The G band, which is always clearly identifiable, is univocally assigned to ordered carbons’ E2g modes and is considered a marker for crystalline graphite. It broadens with increasing disorder (Zerda et al. 1981). The D2 band arises in disordered carbons, at ca. 1600-1635 cm-1 (therefore overlapping with the G band, Table 4.3) and is associated also with the D1 feature, which means that all the studied pigments samples have a D2 band component, since they all show disorder bands between 1300 and 1400 cm-1. In the following, for disordered carbons we will name the E2g mode as band G*, if the D2 component is not resolvable, while band G will identify the ordered lattice vibration, without D2 disorder contribution.

The producer’s description of graphite pigment suggests an artificial origin for the material, according to the presence of ashes (Table D.2). Both lasers were tested for excitation (Figure 4.16). The spectra show two broad bands after baseline correction: the G* band is at 1593 cm-1 and the D1 at 1308 cm-1 with 785 nm excitation and at 1596 cm-1 (G*) and 1346 (D1) cm-1 for the 532 nm laser. The presence of the D1 band suggests that this pigment is a disordered graphite. No additional disorder bands were needed for the deconvolution of the green excited spectrum, but as explained above, and as indicated by the use of the G* symbolism, the D2 band has to be considered present. The red excited spectrum required the D3 band to fit the valley between the G and D1 bands (Figure 4.16). The weak band at ca. 1165 cm-1 could be identified as the D4 band. Effects of mechanical sample alteration, differences between the orientation of graphitic crystallites or platelettes can be the origin of slightly different ratios of G and D bands. Especially the wavenumbers can be used for practical comparative purposes.

169 From Figure 4.16, it is also clear the effect of excitation wavelength on the band position, the D1 being ca. 40 cm-1, and the G ca. 10 cm-1 downshifted for the diode laser. Different theories were developed to explain the bandshift observed with different laser wavelengths. Vidano et al. (1981) correlates the D1 shift to different depth of sampling of different lasers, therefore recording the signature of different structures under the surface. Wang et al. (1990) proposes also resonance effects to justify the shift of the D1 band: different lasers can selectively excite subpopulation of crystallites. More recent studies (Everall et al. 1991, Kagi et al. 1994, Beyssac et al. 2002), however, agree in attributing the bandshift of the G band on laser•induced non-permanent heating effects, which alter the vibrational spectrum (expansion of the bond lengths resulting in lower wavenumbers). These effects are both dependent on the used laser, laser power, and on the sample characteristics, like particle size, presence of heat dissipating/absorbing surrounding media, etc. This latter aspect is relevant in the case of archaeometrical

Figure 4.16: Raman spectra of graphite pigment, obtained with different laser wavelengths. A) graphite pigment 785 nm laser, 7.4mW, 40x10s; b) graphite pigment (baseline corrected, with deconvolution) 785 nm laser, 7.4mW, 40x10s; c) graphite pigment 532 nm laser, 1.4mW, 40x10s; d) graphite pigment (baseline corrected, with deconvolution) 532 nm laser, 1.4mW, 40x10s.

170 applications, as pigments are likely to be mixed with a binder to create paint layers, and if the analysis is performed on a cross section, the presence of the embedding resin might affect the thermal properties of the sample. A much lower fluorescence background for the lower frequency laser is also noticeable. Moreover, both excitations produced a broad (ca. 50 cm-1) low wavenumber feature at ca. 1040 cm-1. The assignment of this feature is still unknown, but a few hypothesis can be proposed, based on literature. Vibrations of the methyl groups -1 (C-CH3) are reported between 1065 and 1205 cm (Ulyanova et al. 2014), and C•N stretching between 1040 and 1150 cm-1 (Sun et al. 1999). It seems that sp3 carbons excited with UV lasers produce a strong feature in this region (Nakamizo et al. 1974, Gilkes et al. 1998, Ferrari and Robertson 2001), but in this study visible wavelengths are used, so this assignment seems inappropriate. Contrarily to this, weak Raman features are observed with all used wavelengths in the intermediate frequency region (700, 850 and 1050 cm-1) by (Fantini et al. 2004, Fantini et al. 2005, Skákalová et al. 2007). The pigment black earth was also tested with both lasers (Figure 4.17). The spectra obtained with the green excitation were clearer than the ones collected with the red laser (cfr. Figure 4.15): the higher signal to background ratio allowed the identification of other phases in addition to carbon. The two carbon bands maxima after baseline correction are 1592 and 1325 cm-1 and 1605 and 1352 cm•1, for the red and green laser respectively. The detected mineral impurities are calcite (narrow band at 1088 cm-1 (Castro et al. 2005, Downs 2006)), quartz (463 cm-1 (Castro et al. 2005, Downs 2006)) and possibly anatase (145 cm-1 (Burgio and Clark 2001, Downs 2006)). A weak broad band was observed at ca. 1043 cm-1 for the 785 nm excited spectrum, and at ca. 1082 cm-1 for the 532 nm one. Shungite (Figure 4.18) proved to be very similar to graphite as regards to band positions (G*, D1) and eventually showed a lower fluorescence background when single particles were measured, instead of the fine powder. However, the structure of shungite is far from that of graphite. Calcite was detected (bands at 154 and 281 cm-1, (Burgio and Clark 2001, Downs 2006)), as well as quartz (460 cm-1, (Burgio and Clark 2001, Downs 2006)). The spectra recorded on finely powdered shungite show a very intense background, and broader G band with respect to the spectra recorded on a single particle, which resemble very closely the spectra of the pigment graphite. Black chalk is usually considered as a synonym of black earth. The pigment black chalk was measured with the red laser. The two broad bands maxima are at 1600 and 1316 cm-1, with hardly resolvable G and D2 bands, respectively at 1592 and 1610 cm-1. The fluorescence background was extremely low, similarly to the pigment black earth.

171 Figure 4.17: Raman spectrum of black earth pigment. a) Black earth 532 nm laser, 1.4mW, 50x10s; b) black earth (baseline corrected) 532 nm laser, 1.4mW, 50x10s. Raman spectrum of black earth pigment: c) black earth 532 nm laser, 1.4mW, 50x10s, (zoom in); d) Calcite R050009 (RRUFF database); e) Quartz R050125 (RRUFF database); f) Anatase R070582 (RRUFF database).

172 Figure 4.18: Ordered carbons Raman spectra collected with the red laser: a) graphite 785 nm laser, 7.4mW, 40x10s; b) shungite 785 nm laser, 15mW, 40x10s; c) black chalk 785 nm laser, 4mW, 30x10s.

No additional bands were needed towards the lower wavenumber region of the linearly baseline corrected spectrum (Figure 4.18). Ordered carbons can be easily excited with both the 785 nm (Figure 4.18) and the 532 nm lasers (Figure 4.16) and show a relatively narrow G band (51±3 cm-1). The D1 band is weak, and, if present, it points out the presence of the D2 band, more or less overlapping with the E2g mode. Care has to be taken as the G band position, width and area can be strongly affected by this unresolvable feature.

As regards graphitizable carbons (cokes), two materials were available in three different forms: ivory black (powder and pieces) and bone black powder (Figure 4.19). The distinction between bone/ivory black and other carbon-based black pigments is normally feasible by means of Raman spectroscopy thanks to the presence of a band at ca. 960 cm-1 (phosphate stretching (Castro et al. 2007a, Tomasini et al. 2012a)). This is expected on the basis of the composition

173 of bone material (collagen and hydroxyapatite Ca5(OH)(PO4)3). This band intensity varies in comparison to the carbon bands, but has been successfully identified (Bell et al. 1997, Edwards et al. 2000, Smith and Clark 2004, Pérez-Alonso et al. 2006, Castro et al. 2007a). Among the studied materials, different animal blacks were available, as powder or as pieces (Figure 4.19). First, ivory black powder and pieces require completely different measurement conditions in order to achieve good quality Raman spectra, respectively 785 nm excitation (Figure 4.19, a and b) and 532 nm (Figure 4.19, c). Moreover, the phosphate band was clearly identifiable only in powdered ivory black measured with the green laser (band at 961 cm-1, Figure 4.19, c). Samples measured with the red laser did not show this band. The pigment bone black was measured with the red laser and no phosphate band was detectable either. It is known that it is sometimes difficult to detect the phosphate signal (Castro et al. 2005, 2007a, Tomasini et al. 2012a), possibly because of the overlap of the carbon signal with the broad and weak band of nanocrystalline apatite, which is therefore hard to detect. For this reason, it seems that the negative criterion normally used for the identification of vegetable origin carbon

Figure 4.19: Raman spectra of coke pigments: a) ivory black pieces 785 nm laser, 4mW, 30x10 s; b) ivory black 785 nm laser, 4mW, 30x10 s; c) ivory black 532 nm laser, 1.4mW, 80x10 s.

174 black might be questionable, at least when using the 785 nm laser (Smith and Clark 2004, Ambers 2004, Edwards et al. 2005, Tomasini et al. 2012a). A shoulder on the broad, asymmetric D band is visible around 1040 cm-1. The results of TXRF analysis on powdered ivory black showed the presence of calcium and phosphorus, which confirms the Raman identification (with 532 nm excitation) of a black pigment of animal origin. For cokes, either laser could be used. In the case of carbon-based black pigments, cokes are identified by the presence of the phosphate band, as bone material give rise to graphitizable carbons. Anyway, the band at ca. 960 cm-1 is not always detected, so additional analyses are suggested for confirmation (Tomasini et al. 2012a).

The only available sample of char-type carbon is pyrolized beech (Fagus sylvatica) charcoal. Good quality spectra were obtained with the red laser. A G* band (Figure 4.20) and the other carbon disorder bands (D1 to D4) are all

Figure 4.20: Raman spectra of charcoal. a) Wood charcoal 785 nm laser, 15mW, 40x10s; b) wood charcoal (baseline correction) 785 nm laser, 15mW, 40x10s; c) wood charcoal (baseline correction, with deconvolution) 785 nm laser, 15mW, 40x10s.

175 present and in good agreement with literature (D1: 1345 cm•1, D3: 1510 cm-1, D4: 1280 cm-1), but two more bands were required to obtain a good fitting. One is a band at ca. 1310 cm-1, and the other is at ca. 1031 cm-1. The need for many bands to completely fit the spectrum suggests a high degree of disorder, which is the case for non-graphitizable carbons, as chars.

A variety of soot materials were available for analysis, as various fruitstone soots (genuine peach, grape and cherry stones soot) and furnace black. Also, the pigment bistre is washed and treated beechwood (Fagus sp.) soot. Good quality spectra of the three fruitstone soots (Figure 4.21) were recorded with the 785 nm laser, and all looked homogeneous: 1576-1587 cm-1 for the G* band (average bandwidth of 85 cm-1) and for the disorder band it has to be mentioned that most of the time one extra band at ca. 1340 cm-1 was required for optimizing the fitting, in addition to the one at ca. 1311 cm-1. The need for

Figure 4.21: flame carbons: fruitstone soots Raman spectra, collected with the red laser. a) Peach black 785 nm laser, 15.4 mW, 35x10s; b) cherry black 785 nm laser, 15.4 mW, 40x10s; c) grape black 785 nm laser, 15.4 mW, 20x20s; d) grape black (baseline corrected, with deconvolution) 785 nm laser, 15.4 mW, 20x20s.

176 additional bands for deconvolution suggests a certain degree of disorder. Furnace black and bistre (Figure 4.22) were better excited with the 532 nm laser, the first one having a flat background, the latter a very intense fluorescence signal. The G* band positions are 1593 and 1600 cm-1, respectively. The main disorder band of bistre is strongly shifting (1345 to 1397 cm-1, sometimes an extra band at ca. 1300 cm-1 was also needed), while furnace black required up to seven bands for the deconvolution of the range 800-1800 cm-1. Three of these bands fall in the region where the D1 band is expected (1326, 1377 and 1378 cm•1). Additional bands were needed at both edges of the deconvoluted region (< 1000 cm-1 and > 1700 cm-1). The need of more bands than expected to deconvolute the disorder region is becoming more and more important when dealing with soots with respect to graphite and ordered carbons. As already mentioned, the more disorder, the more Raman active bands can appear (Nakamizo et al. 1974). Moreover, these two latter smoke carbons behave differently from the fruitstone ones, and also differently one from the other,

Figure 4.22: Raman spectra of flame carbons: bistre and furnace black, both green laser excited. a) Bistre 532 nm laser, 1.4mW, 100x10s; b) bistre (baseline corrected, with deconvolution) 532 nm laser, 1.4mW, 100x10s; c) furnace black 532 nm laser, 1.4mW, 300x10s; d) furnace black (baseline corrected, with deconvolution) 532 nm laser, 1.4mW, 300x10s.

177 probably in relation with the properties of the starting materials (wood for bistre, oil or bitumen for furnace black) or of the fabrication process (bistre is traditionally collected close to the flame which will give more tarry residues).

Both pigments Van Dyck brown and Cassel earth gave no results with the red excitation, and the 532 nm laser produced a high fluorescence background

Figure 4.23: Raman spectra of humic earths/bituminous materials all collected with the green laser excitation. a) Van Dyck brown 532 nm laser, 1.4mW, 30x10s; b) Cassel earth 532 nm laser, 0.17mW, 30x10s; c) asphaltum 532 nm laser, 1.4mW, 60x10s; d) atramentum 532 nm laser, 1.4mW, 30x10s; e) Van Dyck brown 532 nm laser, 1.4mW, 30x10s (zoom in).

178 (Figure 4.23). Weak, asymmetric bands are common to both pigments. No clear difference between them could be obtained from the producer’s website (http://www.kremer-pigmente.com/, last accessed 6/3/2015), mainly because the provided description is confusing: Van Dyck brown is described as a lignit coal from a mine near Cassel, Germany (and in the German website the pigment is named Kasslerbraun), while Cassel earth (Saftbraun in the German website) has no direct correlation with the mines near Cassel and is generally described as sodium salts of humic acids. The G* band is asymmetric with a tail on the low wavenumber side. Its intensity maximum is relatively high (1606±10 cm-1). The disorder band (ca. 1390 cm-1) is strongly asymmetric due to the overlap with a broad feature centred on 1300 cm-1, which is already visible in the raw spectrum. After baseline correction, it appears that the disorder feature has multiple shoulders on the low wavenumber side. Iron oxides are reported among the constituents of Van Dyck brown. However, one single spectrum of Van Dyck brown showed additional bands in the low wavenumber region (212 and 272 cm-1, see Figure 4.23, e). Seen the effect of laser power on haematite (in this case 1.4 mW), these bands could be assigned to haematite α-Fe2O3 (de Faria et al. 1997). Asphaltum was also measured with the green laser and the obtained spectra are extremely similar to those of Van Dyck brown and Cassel earth. The similarity (bitumen-rich coals) among all these three materials is also observed in their Raman spectra, which show an intense fluorescence background, broad G* band and a shoulder centred around 1300 cm-1, which strongly modifies the symmetry of the D1 band. Atramentum is a particularly confusing pigment, as this name could refer to iron salts of tannic acids, soot or even ivory black (Table D.2). The spectrum of our product resembles the ones of humic earths as van Dyck brown and Cassel earth, with strongly asymmetric bands, but here the Raman band of calcite is clearly visible, at 1084 cm-1 (Figure 4.23, (Burgio and Clark 2001, Downs 2006)).

The dry black ink produced by cuttlefish was tested with both lasers. The bands maxima in the raw spectrum are at ca. 1590 and ca. 1305 cm-1 for the red excitation and at ca. 1600 and ca. 1340 cm-1 for the green one (Figure 4.24). The comparison of our spectra with the few available published data (Samokhvalov et al. 2007, Centeno and Shamir 2008) is not allowing clear conclusions. Black iron based pigments, as magnetite Fe3O4) are to be expected in this pigment. The band of magnetite can be found in the range 662•670 cm-1 (de Faria et al. 1997) and even up to 706 cm-1 (Shebanova and Lazor 2003). Also a band at 534 cm-1 is reported in literature for magnetite (de Faria et al. 1997,

179 Shebanova and Lazor 2003). A band at ca. 570 cm-1 is reported in literature to appear in Fe(III) doped Sepia (Samokhvalov et al. 2007). The presence of a black iron phase could be eventually related to a modification of the intact Sepia pigment, as Fe(III)-enriched Sepia melanin shows the appearance of bands at 570 and 1470 cm-1 (Samokhvalov et al. 2007). Moreover, the Sepia band at 460 cm-1 disappears with increasing iron content (Samokhvalov et al. 2007). It is clear that the raw spectra of carbonaceous black pigments are hard to evaluate as they are. However, some useful guidelines for discriminating among the different classes of carbonaceopus pigments can be put forward. The Raman spectrum of carbonaceous pigments, as obtained after baseline subtraction, seems more promising. At this point, it is also possible to investigate the fine structure of the Raman spectrum through deconvolution with Gaussian and/or Laurentzian curves. Such an approach is essential especially when dealing with disordered carbons, which show additional bands, and of increasing intensity with respect to ordered materials (Ulyanova et al. 2014). The expected downshift of the D1 and G* bands with increasing laser wavelength was observed in those samples measured with both lasers, which gave

Figure 4.24: Other carbon-based black pigments Raman spectra: Sepia. a) Sepia average of 3 measurements 532 nm laser, 1.4mW, 10-40x10s; b) Sepia average of 3 measurements 785 nm laser, 15.4mW, 5-25x20-35s.

180 clear Raman spectra with both excitations, for example graphite ((Everall et al. 1991, Kagi et al. 1994, Beyssac et al. 2002), Figure 4.16). Table 4.3 summarizes some features of the recorded Raman spectra. No information is given there about the bandwidth, as both the G and D bands are asymmetric and require from 2 to 4 components for deconvolution. It is interesting to note the presence of additional bands. Some are related to minerals that are present as impurities (quartz, calcite, etc.) while some others required a more detailed investigation. For most of the investigated carbon-based black pigments, the combination of G and D2 bands is expected, which affects the spectral parameters (position, bandwidth, intensity and area) of the G* band. It is then difficult to use the G* band spectral parameters in correlations, especially if they are supposed to coincide with the G band parameters (Beyssac et al. 2002). As already mentioned, a broad band was always present in the studied set of samples with a maximum between 1300 and 1400 cm-1. Different degrees of symmetry and shoulders were observable, since the disorder features of carbon (and artefacts bands related to the baseline correction, if any) all fall in this range. The so called D1 band is extremely sensitive to a variety of factors and shifts strongly, actually across the whole disorder range 1240 to 1400 cm-1 (Yoshikawa et al. 1988, Tomasini et al. 2012b, Ulyanova et al. 2014). Whenever supplementary disorder-related features arise, the correct identification of the D1 band becomes extremely difficult (see Figure 4.23). On the higher wavenumber side of the D1 band, the D3 band is found, which corresponds to impurities and has a Gaussian profile. On the lower wavenumber side, the overlap is with the D4 band, which is only described for soot samples. Moreover, additional bands at ca. 1150 and below 1100 cm-1 have been described in literature but no clear explanation and assignment is found among the scientific community. The lower recorded band position is 1040 cm-1 for the red excited spectra and 1093 cm-1 for those excited with the green laser. Again, the use of the D1 band’s spectral parameters for correlations seems questionable in the case of strongly disordered materials, due to overlapping and shifting features. In general, the red laser worked well for all the materials except those richer in tarry, bituminous materials (furnace black, atramentum, bistre, Van Dyck brown, Cassel earth and asphaltum), which were more effectively excited with the 532 nm laser. The possibility of clearly resolving the D2 band from the G one points out a higher degree of order, which is found in black chalk and ivory black in pieces (red laser) and in black earth and ivory black, when measured with the green laser. Additional bands to the expected G and D1 - D4 are found, especially

181 towards lower wavenumbers, in disordered carbons (see Table 4.3). These bands are still not assigned, but their presence can help in distinguishing among different carbon blacks. The possibility of using the obtained set of data for direct chemometric discrimination for unknown samples seems complicated by different reasons. Careful comparison of the shape of the G and D bands can especially be recommended as well as estimation of FWMH.

Table 4.3: Positions of band maxima after baseline correction, observations on the fluorescence background and presence of additional bands in the spectra of the studied pigments. In bold are the data related to the 785 nm excitation, in italic to 532 nm. *: after linear baseline correction.

ADDITIONAL BANDS -1 (CM ); DISORDER G MAX D MAX FLUORESCENCE CARBON TYPE PIGMENT -1 -1 BANDS (CARBON (CM )* (CM )* BACKGROUND RELATED - NOT -1 ASSIGNED, CM ) Flat, weak. Graphite and -; 1593±4 1308±3 Occasionally crystalline carbons 1160, 1040 Graphite intense. (see Figure 4.15 Linear, intense but -; and 4.18) 1596±7 1346±1 not overwhelming. 1100 Graphite and Linear, weak; 154, 216, 281, crystalline carbons Shungite 1587±8 1304±4 occasionally 459; (see Figure 4.15 intense. 1040 and 4.18) Graphite and Intense, almost -; 1592±1 1325±1 crystalline carbons overwhelming. 1040, 910 Black earth (see Figure 4.15 Intense but not 147, 466, 1086; 1602±3 1350±5 and 4.18) overwhelming. 1330, 1170, 1080 Graphite and crystalline carbons Black chalk 1600±1 1316±1 Linear, weak. - (see Figure 4.18) Intense but not -; 1583±3 1315±5 overwhelming. 1090, 910 Coke of animal 964, 140; origin (see Ivory black Very intense but 3 bands between Figure 4.19) 1594±5 1329±1 not overwhelming. 1500 and 1300, 1183, 1090 Coke of animal Ivory black -; origin (see 1590±3 1325±1 Linear, weak. (pieces) 1300, 1190 Figure 4.19) Coke of animal -; Bone black 1587±2 1314±2 Linear, weak. origin 1040, 920 Chars of vegetal From weak to -; origin (see Charcoal 1583±6 1326±5 intense, not 1310, 1030, 915 Figure 4.20) overwhelming.

182 ADDITIONAL BANDS -1 (CM ); DISORDER G MAX D MAX FLUORESCENCE CARBON TYPE PIGMENT -1 -1 BANDS (CARBON (CM )* (CM )* BACKGROUND RELATED - NOT -1 ASSIGNED, CM ) -; Flame carbons (see Furnace 3 bands between 1591±1 1347±1 Flat. Figure 4.22) black 1380 and 1320, 1070 Strongly Flame carbons (see increasing -; Peach black 1583±2 1313±1 Figure 4.21) towards high 1040, 920 wavenumber. Flat to intense, Flame carbons (see -; Grape black 1583±4 1317±4 not Figure 4.21) 1045, 920 overwhelming. Flat to intense, Flame carbons (see -; Cherry black 1581±5 1317±1 not Figure 4.21) 1050, 920 overwhelming. Flame carbons (see 174, 295, 1096; Bistre 1587±4 1361±5 Intense. Figure 4.22) 1180, 1090, 980 272, 360, 1083; 1565± 1315± Intense. 1410, 1080, 920 Flame carbons Atramentum 144, 258, 706, (see Figure 4.23) 1561±4 1410±10 Intense. 1026, 1083; 1360, 1080 Flame carbons (see 140, 107; Asphaltum 1597±2 1375±4 Intense. Figure 4.23) 1180, 1090 Humic earths (see Van Dyck 214, 274; 1593±3 1379±1 Intense, flat. Figure 4.23) brown 1180, 1090, 980 Humic earths (see Cassel 143, 108; 1603±1 1381±1 Intense, flat. Figure 4.23) brown 1195, 1085, 980 Others (see Figure 1588±1 1304±1 Linear. 1400 Sepia 4.24) 1597±3 1340±5 Linear, weak. 276, 1509

Carbonaceous materials are currently investigated for geological and industrial applications: the materials and analytical techniques involved in this specific literature partially overlap with archaeometrists’ interest. In this paragraph, we have presented Raman spectra obtained on carbon-based black pigment references, trying to better understand their characteristics and to establish a procedure for discrimination among different carbon-blacks on the basis of their spectral parameters.

183 Care needs to be taken when comparing Raman spectra acquired with different wavelengths, especially with different laser powers, and in the phase of band deconvolution. However, ordered carbons require less bands (3-4), and narrower, for fitting the spectrum, and often allow the separation of the G and D2 bands. On the contrary, disordered carbons require up to 5 bands for deconvolution, and the G* band is not resolvable. Finally, tarry carbons seem to be better excited with the green laser, as no bands could be recorded with the 785 nm excitation. The application of Raman spectroscopy to the study of carbon-based black pigments is a challenging topic for many reasons. The results presented here are intended to provide some references (see Table 4.3), in terms of Raman spectra and of terminology, to those dealing with works of art. Also, some guidelines based on the Raman signature of carbonaceous reference materials are provided to help in the identification of unknown carbon-based black pigments. The widespread use of the definition “carbon-black” for the whole range of carbon•based black pigments seems reductive, especially when considering the potentiality of Raman analysis, as demonstrated by its applications to the geological field. The Raman spectra of carbonaceous materials suffer from the effect of the excitation wavelength and other experimental parameter, as well as of the data processing. Moreover, the fluorescence background associated with binding media used in works of art can negatively affect the successful identification of the used pigment, as the linear baseline correction might be unapplicable. Also, thermal effects related to the use of laser sources affect the resulting Raman spectra. Heat dissipation is different for the pure pigment, for pigment grains in a paint layer, possibly even embedded in resin for cross section analyses: the comparison of spectra from a sample with reference ones requires extreme care. For the purpose of archaeometrical research, anyway, the use of Raman spectroscopy can give a deeper insight in the material history of works of art, and can help in clarifying which type of carbon black was used; always keeping in mind the complex nature of carbon-based black pigments with respect to geological definitions. In this work, reference materials were studied by means of Raman spectroscopy to provide reference spectra. All the pigments showed two broad bands of carbon, but sometimes specific excitation conditions were required to record a good quality Raman spectrum. The obtained Raman signatures are discussed, on the basis of the specificities of the pigment (natural or artificial origin; structural implications related to the raw materials used or to production processes; etc.). Therefore, on the basis of the Raman spectra of painting materials, further knowledge can be obtained on the type of carbon-based black pigments in works of art.

184 Although, in contrast to XRF based techniques, Raman spectroscopy is able to positively detect the presence of carbonaceous pigments, it needs to be pointed out that the proposed criteria for discrimination might not be applied straightforwardly to real paint samples. Moreover, additional analyses as XRD and XRF can allow a more detailed identification of the present compounds, as other black minerals based on Fe and Mn, or the phosphatic phases in cokes of animal origin, and to cross•check these data thanks to the elemental information, respectively. In addition to the identification and characterization of pigments in polychrome works of art, Raman spectroscopy is suitable for the investigation and detection of degradation processes involving pigments. The next Paragraph tackles the study of pigment degradation upon exposure to acetic acid.

The potentialities of Raman spectroscopy in the field of conservation of Cultural Heritage and archaeometry are manifold. The sensitivity of micro-Raman spectroscopy to molecular changes, and its non-destructive character make it a powerful tool in the study of degradation processes, (Edwards and Chalmers 2005, Pérez-Alonso et al. 2006, Zoppi et al. 2010, De Laet et al. 2013, Bersani and Lottici 2016; Sub-chapter 2.1). The previous paragraphs demonstrated the use of this technique to characterize materials in works of art, such as green compounds and carbon-based black pigments, which cannot be positively identified by X-ray fluorescence techniques only. In addition to the identification of the components of the paint, i.e. of the painter’s palette, Raman spectroscopy is suitable for the study of pigment degradation and of alteration processes which occur in polychrome works of art. This paragraph presents an example of micro-Raman spectroscopy applied to study the reactivity of selected pigments to volatile organic compounds that are often present in museum rooms and exhibition cases. During this thesis, the specific case of pigment interaction with acetic acid was investigated, in the framework of the European FP7 “MEMORI” project (Measurement, Effect Assessment And mitigation Of Pollutant Impact On movable Cultural Assets. Innovative Research For market Transfer).

The negative role of inorganic pollutants, such as SO2/SO3 and NOx, on Cultural Heritage objects is well known, and is often tackled by installing filters and controlling the ventilation systems in museums. To guarantee more stable conditions, enclosures are often used. The ingress of pollutants can therefore be easily controlled, but if harmful compounds are generated inside the enclosure

185 itself, they could negatively affect the conservation of artefacts (FitzHugh and Gettens 1971, Padfield et al. 1982, Ryhl-Svendsen and Glastrup 2002, Gibson and Watt 2010). Some of such harmful compounds, such as volatile organic acids, are released by wood (Padfield et al. 1982, Gibson and Watt 2010, Thickett et al. 2016), a common material used in showcases, as hemicellulose is hydrolized to release acetic and formic acid (Laine 2005). The task assigned to Ghent University in the framework of the MEMORI project concerned the study of pigment degradation upon exposure to acetic acid. It should, however, be noted that the material composition of works of art is complex, and the presence of polychromy makes the issue even more intricate, since other materials are present, which show different behaviour towards diverse aggressive factors. The other participating research groups provided information on the degradation of other materials relevant to Cultural Heritage (varnishes, leather, parchment, paper, textiles), with the aim of defining safety thresholds for preservation, and of using them to evaluate if the object is exposed to potentially harmful conditions (http://www.memori.fraunhofer.de, Grøntoft and Dahlin 2012). The first aspect is explored by performing exposure experiments, monitoring the observed changes and correlating them to the pollutants, while the second requires the development of a passive air-sampler, to be placed in the same environment as the artefact. Finally, the response of this dosimeter needs to be evaluated according to the type of material, some atmospheres being more harmful to some materials than to others (Lankester et al. 2013). In the framework of the development of the MEMORI-dosimeter, specifically as regards the definition of safety thresholds for various materials that may be found in Cultural Heritage objects, the study of pigment sensitivity and degradation is of primary importance. Threshold levels are defined, in terms of acetic acid doses (concentration of pollutants multiplied by the length of the exposure, ppm * days) for selected pigments that undergo alterations. The pigments were studied as powders, even though the influence of the binding medium should not be neglected.

In the first stages of this study, a multitude of pigments was exposed to glacial acetic acid (CH3COOH) atmospheres, and a set of five was selected for further analysis, namely malachite (CuCO3·Cu(OH)2), lead tin yellow type I (Pb2SnO4), lead white (2Pb(CO3)·Pb(OH)2), red lead (Pb3O4), sunfast orange 36

(C17H13CIN6O5). Stainless steel metal plates were used as a support for the

186 pigments, and were mounted on a modular frame fitting inside a desiccator. The sticking of the pigment powder to the support was achieved with a 3M polyester double-sided tape (type 850). The atmosphere in the desiccator is controlled by two parameters: relative humidity (RH), maintained at ca. 75% thanks to a saturated NaCl solution, and diluted acetic acid solutions, which define the amount of pollutant in the atmosphere (De Laet et al. 2013). After a first exposure to aggressive acetic acid atmosphere, where degradation products were readily formed, it was relevant to know which acetic acid doses cause an appearance of degradation products, namely a detectable amount of degradation products. Table 4.4 reports the acetic acid atmospheres used in the study, calculated according to (Tétreault et al. 1998). To obtain information about the dose, a total exposure of 5 weeks was obtained for each selected atmosphere, and the samples monitored weekly. The unexposed and exposed samples were non-destructively investigated, to evaluate the ongoing modifications, mainly focussing on the interaction with acetic acid. Calibrated digital imaging was used to assess the colour changes in the samples in weekly steps, and micro-Raman spectroscopy was chosen for characterizing the degradation products in a non-destructive way. Micro Raman spectroscopy analyses were carried out with two laboratory instruments, a Kaiser spectrometer equipped with a 40 mW 785 nm laser and a Bruker Senterra with a 15 mW 532 nm laser (see Sub-chapter 4.3). Each sample was measured over 100 points, which were then averaged to take into account variability. The discussion of the limit of detection (the lowest concentration of a product that can be detected in a certain matrix and given a certain analytical approach) for Raman spectroscopy is not trivial, and overlaps with the issue of limit of identification, but is strictly required when dealing with degradation products (Vandenabeele and Moens 2012). Moreover, in literature many studies on pigment degradation due to atmospheric pollutants and other environmental conditions (Pérez-Rodrı́guez et al. 1998, Dominguez-Vidal et al. 2014, Pereira-Pardo et al. 2016), as well as case studies where degradation products are identified (Pérez-Alonso et al. 2006, Aze et al. 2007, Svarcová et al. 2009), and reactions understood (Castro et al. 2008, Kotulanová et al. 2009, Gutman et al. 2014, Vanmeert et al. 2015), are presented. It seems relevant to gather information on pigments sensitivity towards climatic factors, in the framework of this research project. The outcome of the literature review was presented in Sub-chapter 2.1, showing a variety of processes involving pigments and paint layers, demonstrating the complexity of this research field.

187 Table 4.4: Concentrations of acetic acid (as acetic acid solution dilution, vapour phase concentration in mg/m3 and ppm), and corresponding doses used in this study.

Acetic Doses Average vapour acid phase acetic acid solution 1 week 2 weeks 3 weeks 4 weeks 5 weeks concentration conc. % mg/m3 ppm ppm * days 1 40 16 110 230 340 450 570 3 121 49 340 680 1020 1360 1700 5 202 81 570 1130 1700 2270 2830 7 283 113 790 1580 2380 3170 4000 9 368 147 1030 2060 3090 4120 5150 20 810 324 2270 4530 6800 9070 11340 33 1350 540 3780 7560 11340 15120 18890

The exposure of selected pigments (malachite. Lead white, lead tin yellow type I, and red lead) to high concentrations (9 to 33 %, Table 4.4), and to high doses, of acetic acid yielded the formation of degradation products on malachite, lead tin yellow type I, lead white, and red lead (De Laet et al. 2013). The colour change was visible by naked eye on malachite (from green to blue•green), lead tin yellow type I (from yellow to light yellow), and red lead (orange-red to black). No colour variation could be detected for lead white. The Raman characterization of the exposed pigment samples, however, detected chemical changes in all of the pigments, as some bands appeared and some others decreased in intensity until disappearance, as in the extreme case of red lead. Lead acetate (Pb(C2H3O2)2) was identified on all the lead containing pigments, but on red lead it was not the only degradation product. The formation of lead acetate was expected, but both the colour change of red lead (towards brownish black) and the Raman spectra do not account for this white compound to be formed alone (De Laet et al. 2013). The bands of tetragonal PbO, which is a light yellow material (litharge), were identified (Trettenhahn et al. 1993, Lafuente et al. 2015); and black plattnerite is hypothesized, but could not be positively identified by Raman spectroscopy, as it is a very weak Raman scatterer, and it is easily transformed to litharge upon laser irradiation, if the power is not kept sufficiently low (Burgio et al. 2001). Copper acetate was identified on malachite (Yang et al. 1989, Chaplin et al. 2006, De Laet et al. 2013). Copper acetates correspond to the medieval green•blue pigment verdigris (Chaplin et al. 2006, De Laet et al. 2013), explaining

188 the bluish hue observed on the malachite sample. These materials account for the colour change trends (De Laet et al. 2013). On the other hand, being white, lead acetate very slightly affects the appearance of lead white, while it lightens the yellow of lead tin yellow type I. It appears that Raman spectroscopy is more effective in identifying degradation products than calibrated digital imaging, as it distinguishes among compounds of the same colour, as in the case of lead white and lead acetate. However, the disadvantage of Raman spectroscopy is that a series of point analysis is needed, which is time-consuming. The relative intensity of the appearing Raman bands increases respect to the carbonate bands for malachite and lead white, and to the Pb-O band in lead tin yellow type I. For red lead, the spectra of the strongly altered pigment are completely different from the original material, indicating a complete loss of the original pigment’s structure. Table 4.5 reports the threshold levels for the studied pigments, as obtained after exposure to high concentrations of acetic acid. Lead tin yellow seems to be the most sensitive pigment, as it started to show some degradation already after 1 week of exposure to the lowest concentration (ca. 150 ppm), while the others required 2 weeks. At this stage, the reactivity of pigments was demonstrated, but still the threshold level that is responsible for the formation of a detectable amount of degradation product could be further investigated. Moreover, the reaction of red lead required further analysis. Therefore, the pigments were exposed to doses as low as ca. 100 ppm * days (1 to 7 % of acetic acid, see Table 4.4), and the vibrational spectra evaluated. These less aggressive concentrations did not affect malachite, whose spectra were always comparable to the unexposed sample, and the threshold level is the one reported in Table 4.5. For the lead pigments, it was possible to monitor the appearance of the 925 cm-1 band of lead acetate in all the lead containing pigments (lead tin yellow I shown in Figure 4.25).

Table 4.5: Overview of pigments sensitivity after the first tests with high concentrations of acetic acid (above ca. 150 ppm acetic acid corresponding to doses above 1000 ppm * days)

Pigment Lowest dose to produce observable changes Lead white Ca. 2060 ppm * days Lead tin yellow I Ca. 1030 ppm * days Red lead Ca. 2060 ppm * days Malachite Ca. 2060 ppm * days

The earliest signs of degradation of lead tin yellow are observed for a dose of ca. 1700 ppm * days (Figure 4.25), corresponding to 5 weeks of exposure of lead tin yellow to 3 % acetic acid, while the other combination (3 weeks at 5 %) seems

189 to be safe. The comparison of these results with those from the first set of experiments is contradictory for lead tin yellow. In fact, the pigment seems to be more reactive to higher concentrations of acid, which causes degradation after 1 week at 9 %, but not at 3 weeks at 3 % (same dose of ca. 1030 ppm * days). For red lead, it was already highlighted in the past that upon exposure to high concentrations of acetic acid vapours a dark material appears, while 9 % acetic acid atmosphere resulted in the formation of lead acetate. The follow up of red lead at low acetic acid concentrations shows that acetate salts of lead are formed (band at around 926 cm-1) (Bernard et al. 2009). This band appears after 1 week of exposure only in relatively high concentrations of acetic acid (20 and 33 %, not shown). As shown in Figure 4.26, this band is not visible even after 5 weeks of exposure to 1 % acetic acid atmosphere, neither after 2 weeks at 7 %, nor 3 weeks at 5 %. This last combination corresponds to a dose of 1700 ppm * days. It can be seen that the other combination yielding this dose (5 weeks, 3 %) on the contrary produces a detectable amount of lead acetate. At higher doses (> 2000 ppm * days), the degradation product is formed, no matter what the combination is (Figure 4.26). Considering the results of the exposure experiments to higher doses of acetic acid, it seems that lead acetate is reactive, and that the degradation yields other compounds (possibly as a kinetic effect related to acetic acid concentration). For the purpose of this study, it seems that the pigment begins to react at

Figure 4.25: Weekly Raman monitoring of the pigment lead tin yellow type I exposed to low concentrations of acetic acid. Each spectrum is the average of a 10x10 point mapping (5x30s), and are labeled with the corresponding dose, as ppm * days. For each graph, the spectra are recorded weekly, from 0 to 5 weeks of exposure, from top to bottom.

190 ca. 1700 ppm * days, which corresponds to the “lowest observable adverse effect limit” (LOAEL).

Figure 4.26: Selection of red lead spectra, zoomed in in the region of the main band of lead acetate (1000-800 cm-1) showing the first appearance of lead acetate, and the different parameters related to detectable degradation.

191 For lead white, the band of the acetate appears already at 4 weeks at 3 % (1360 ppm * days), probably as the carbonate structure is sensitive to acids. Anyway, in the first set of experiments, lead acetate was formed on lead white at a dose of ca. 2060 ppm * days. The prosecution of the study with lower doses clearly allowed the definition of a more accurate limit level, which will be considered for further purposes. From Figure 4.26, the appearance of degradation products can only be appreciated qualitatively (band absent vs. band present). To quantitatively account for the formation of degradation products, band intensity ratios were calculated from the newly formed compound band and one of the characteristic bands of the pigment: for lead white, the band at 180 cm-1 was used, for red lead that at 388 cm-1, for lead tin yellow that at 289 cm-1, and correlated with the dose. The graph related to this last pigment is reported in Figure 4.27. As expected, the ratio increases with time, for each acetic acid concentration, as the pigments continue to react. The same type of graph is obtained for malachite, lead white and red lead. From those graphs, the LOAEL is defined as the lowest dose inducing a detectable change in the sample (Raman band intensity ratio > 0). The safe limit called “no observable adverse effect limit” (NOAEL) is set at 80 % of the LOAEL value (Figure 4.27 and Table 4.6). Table 4.6, moreover, summarizes the suggested reaction paths of the selected

Figure 4.27: Correlation of the band intensity ratio of the bands of lead acetate and lead tin yellow type I versus the calculated dose for a selection of acetic acid concentrations.

192 pigments with acetic acid in the atmosphere. For the copper pigment malachite, the reaction produces copper acetates (i.e. verdigris). All the lead-based pigments react to form lead acetates Pb(CH3COO)2, as well as oxides, water and carbon dioxide. Especially in the case of red lead, il looks like the formed lead oxides are reactive. Lead pigments show a higher sensitivity, compared to malachite, to acetic acid loaded atmospheres. The exposure to lower doses showed that lead white and lead tin yellow are slightly more sensitive than red lead, but this last pigment undergoes a further step in the degradation, as lead oxides as litharge (detected by Raman spectroscopy for a dose of ca. 75000 ppm * days) and plattnerite (hypothesized, seen the colour of the degraded samples) are formed.

Table 4.6: Summary of the proposed reaction paths and thresholds for the studied pigments (observed LOAELs and calculated NOAELs). For malachite, the LOAEL was obtained in the first set of experiments (high concentrations/doses of acetic acid); for the lead pigments in the second, with lower doses.

NOAEL LOAEL Pigment Suggested reaction(s) ppm * days

CuCO ·Cu(OH) + 2CH COOH  Malachite 3 2 3 ca. 1650 ca. 2060 Cu(CH3COO)2·Cu(OH)2 + H2O + CO2

2 Pb(CO3)·Pb(OH)2 + 4 CH3COOH  Lead white ca. 1090 ca. 1360 2 Pb(CH3COO)2·PbO·H2O + 2 H2O + 2 CO2

Lead tin yellow Pb SnO + 4 CH COOH → 2 4 3 ca. 820 ca. 1030 type I 2 Pb(CH3COO)2·SnO2·2 H2O

Pb3O4 + 4 CH3COOH 

2 Pb(CH3COOH)2 · PbO2 · 2 H2O  2 Pb(CH COOH) + PbO + 2 H O Red lead 3 2 2 2 ca. 1360 ca. 1700

Pb3O4 + 2 CH3COOH 

Pb(CH3COO)2 + PbO + PbO2 + H2O

As these thresholds have the specific aim to protect and preserve our Cultural Heritage, it is evident that, when different materials are present at the same time, the dose is defined by the most sensitive material, unless degradation products of one material affect the other compounds (synergistic effects). On the other hand, as polychrome objects are extremely complex in build-up and composition, care should be taken not to underestimate the interactions between materials, and of the whole system with the environment. Typically, in wood based showcases, acetic acid concentrations reported in literature range between 1 and 2 ppm (Ryhl-Svendsen and Glastrup 2002, Gibson

193 and Watt 2010). Lower concentrations are observed for the museum environment (ca. 10 to 5 times less) (Gibson and Watt 2010). The initial acetic acid concentrations used in this study are larger than what is reported in literature (above 16 ppm). However, the dose calculations allow for a better understanding of the potential damage caused by the atmosphere inside a showcase on pure pigment powders displayed in it. In fact, if we consider a realistic concentration of 1 ppm inside the , based on the calculated doses, we expect to see degradation (LOAEL) in less than 3 years on lead tin yellow type I, in ca. 3 years and a half on lead white, in ca. 4 years and a half on red lead, and in ca. 5 years and a half on malachite, assuming concentrations, relative humidity and temperature are stable. It is however important to note that pure pigment powders, without any binder or varnish, are rarely found in museum objects. Anyway, the availability of information on sensitivity of materials in terms of dose seems relevant, especially as it corresponds to realistic museum exposures of just a few years.

In this paragraph, the application of Raman spectroscopy to follow up degradation processes of pigments exposed to volatile organic compounds of relevance in museum displays, such as acetic acid, was illusrated. The capability of the technique to characterize degradation products, which might not be visually apparent, shows the advantages of Raman spectroscopy.

On the other hand, some materials, such as plattnerite (PbO2), are very weak Raman scatterers, so that no positive identification could be achieved. After understanding the issue of the definition of the Raman spectroscopic detection limit, it is of primary importance to be able to provide curators and conservation scientists with threshold values that guarantee the safeguard of Cultural Heritage objects on exhibition. As these objects are complex systems, their components might respond differently to the same environmental conditions, and the safe threshold corresponds to the most sensitive material. So far, a laboratory micro-Raman spectrometer was used to record reference spectra of green compounds (Paragraph 4.3.1), to evaluate the differences among different types of carbon-based black pigments (Paragraph 4.3.2), and to monitor the appearance of degradation products on pigments exposed to acetic acid (Paragraph 4.3.3). In the next Sub-chapter, the use of a portable Raman spectrometer for direct analysis of works of art will be described.

194 For direct analysis of art objects, in this thesis an EZRAMAN-I-DUAL dual laser portable instrument (TSI Inc., Irvine, USA) was used. The whole system, schematized in Figure 4.28, is included in a trolley (43 x 33 x 18 cm, 17 kg) (Lauwers et al. 2014c, Vandenabeele and Donais 2016, Lauwers 2017). It can work on 220 V AC, as well as on batteries (Lauwers et al. 2014c). The two lasers (532 and 785 nm) have adjustable laser powers (maximum 20 and 230 mW respectively). The spectral windows are 100-2350 cm−1 for the red laser, and 100-3200 cm−1 for the green one. The spectral resolution is of 6 or 7 cm−1, for the 785 and 532 nm lasers, respectively. The calibration was checked at every measurement session by testing five different reference materials, showing bands between 150 and 2300 cm-1 (Lauwers et al. 2014c). The instrument is provided with three different lenses, a standard one, a high numeric aperture and a long working distance lens. For the purpose of this thesis, the standard lens was preferred, as it has a focal distance of 7 and 8 mm for the 785 nm and 532 nm laser, respectively (Lauwers et al. 2014c, Lauwers 2017). The interference of environmental light, and the issue of stable, yet flexible positioning are major challenges. The probehead holder can be mounted on a variety of elements that allow easy macro and micro positioning. An articulated arm (Figure 4.29), or a tripod and cross•mounted sliding elements (Figure 4.30), account for the rough positioning, while the accurate one, as well as the focussing, is achieved thanks to micrometric screws. Each system can have specific advantages for measurement situations: for example the arm is preferred to measure a sample lying horizontal, while the

Optic fiber collection Optic fiber excitation Grating 1

Solid state laser CCD Sample Diode laser

Grating 2

Figure 4.28: Schematic of the portable instrument EZRAMAN-I-DUAL by TSI Inc., Irvine,, USA. Image after (Lauwers 2017).

195 tripod system is normally used for paintings standing vertically, and is transported more easily (more compact, easy (dis)assemblage, less weight - counterweight elements are needed, but can be easily found on site).

Figure 4.29: The articulating arm. a) Positioning of the probehead by using the articulated arm. Photo UGent (Sint-Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW); b) detail of the probehead mounted; c) Raman spectroscopic study of a supposedly Clara Peeters painting. Due to its size, the analyses were performed by using the mobile instrument available in the laboratory. Photos UGent (Raman Spectroscopy Research Group).

196 Finally, if contact with the painted surface is allowed, the operator can hold the probehead by hand (Figure 4.31). In these circumstances, the measurement has to be limited in time, due to the physical impossibility of maintaining the same positioning for longer than a couple of minutes.

Figure 4.30: The tripod mount. a) and b) Scheme and picture of the tripod setup; c) fine positioning (focussing) by means of the micrometric screw. Photo UGent (Sint-Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW). d) the setup being used at the Museo Regionale di Messina, Italy. Photo UGent (Raman Spectroscopy Research Group).

197 This approach is often preferred when measuring on scaffoldings (as in Funchal, Madeira (Paragraph 4.4.1), and in Catania, Italy (Barone et al. 2016)), or on ladders (Lauwers et al. 2014c), due to vibrations that modify the positioning obtained by a rigid system, such as with the tripod.

Figure 4.31: Examples of holding the probehead by hand. a) and b): Measurements on the Altarpiece in Funchal, Madeira, Portugal; c) and d): measurements on the painted vault of Sala Vaccarini, Catania, Italy. Photos UGent (Raman Spectroscopy Research Group).

198 In this paragraph, a brief introduction on the Funchal’s Altarpiece will be given, and then some results will be discussed, with specific attention to the challenges of in situ Raman spectroscopic analyses. The overall dimensions of this Altarpiece are 8.79x6.73 m. The polyptych (paintings mounted into a complex oak-structure, including statues and decorative elements as well (Salteiro 2014, Ferreira et al. 2016)) maintained its original location for centuries, with limited interventions on the structure, which is an exceptional case in Portugal (Mesquita et al. 2012, “Conservation And Restoration Of The Altarpiece And Choir-Stalls In The Chancel Of Funchal´s Cathedral 2013 - 2014” 2014) (Figure 4.32).

Figure 4.32: The Altarpiece in Funchal’s Cathedral (Madeira, Portugal). Photo Direcção Regional dos Assuntos Culturais da Madeira.

The paintings represent scenes of the life of Jesus and of Mary (Annunciation, Nativity, Last supper; scenes from the Passion, Pentecost, Assumption of the Virgin), as well as episodes from the Old Testament (Abraham and Melchisedek, Mana picking in the desert), and the miracle of St. Gregor. The initial church was built starting from 1493, and converted into a cathedral at the beginning of the 16th century (Oliveira 2003, Ferreira et al. 2016). An archival document dating to 1512 testifies the payment for an altarpiece painted

199 “à maneira flaminga” (Mesquita et al. 2012), but the painter remains unknown (Ferreira et al. 2016). Art historians attribute this polyptych to Francisco Henriques, the Master of Lourinhã, Olivier de Gand or Jorge Afonso, or even to the cooperation of two or more painters working together (Mesquita et al. 2012, Ferreira et al. 2016). A multi-disciplinary preliminary study revealed that the general conservation state of the polyptych was not optimal, which led to the conservation treatment funded by the World Monuments Fund, the José de Figueiredo Conservation and Restoration Laboratory, the Department of Conservation and Restoration of Instituto dos Museus, and the Hercules laboratory of University of Évora (Mesquita et al. 2012, Ferreira et al. 2016). This was the starting point to establish the most appropriate strategy to follow in the next phases of the conservation treatment (Ferreira et al. 2016). Moreover, in 2013 a new in situ campaign was conducted, during the final stages of the conservation treatment (retouching and final varnishing). In fact, scaffoldings were installed in the cathedral, to allow the conservation team to work on the paintings. A multi-technique approach was chosen for the purpose of non-destructive characterization of materials (Ferreira et al. 2016). Raman and XRF spectroscopic analyses were performed in situ (UGent and UÉvora, respectively), and in the laboratory on cross sections (in Évora: micro-FTIR, SEM•EDS, in Ghent: micro-Raman spectroscopy). In the following, the results of the portable Raman study are discussed. Some preliminary results were presented at international conferences as posters (Lauwers et al. 2014b, Gomes et al. 2016).

Due to the conservation treatment, most of the paintings were already completely retouched and varnished. In this paragraph we focus on the Raman in situ analyses of the “Pentecost” panel (Figure 4.32, painting G; Figure 4.33). The dual-laser EZ-RAMAN system described in Sub-chapter 4.4 was transported on site and installed on the scaffolding. As a result of the scaffolding construction, the tripod set-up illustrated in Figure 4.30 could not be used safely, due to vibrations and to the distance of the structure from the painted surfaces (Figure 4.31, a and b). The probehead was held by hand in contact with the painted surface to block the environmental light from interfering with the spectra acquisition. Moreover, the used light blockers allowed the operator to optimize the focussing. In fact, due to the presence of a layer of varnish, it is necessary to try to focus the laser on the pigmented layer underneath it: the plastic tube used to

200 Figure 4.33: Pentecost painting. a) Before treatment. Photo Direcção Regional dos Assuntos Culturais da Madeira; b) after cleaning. Photo UGent (Raman Spectroscopy Research Group). block the light can easily slide over the lens, allowing a more accurate focussing compared to the use of a fixed-length cap. The points selected for direct Raman measurements correspond as much as possible (accessibility, presence of varnish) to those analysed by handheld X-ray fluorescence analysis by the colleagues of HERCULES/UÉvora, so that data can be cross-checked, although the combination of a surface technique (Raman spectroscopy), with a bulk one (hXRF), requires care (see next Chapter 5). Finally, samples were acquired to answer specific conservation and art historical questions, and analysed in the laboratory. Cross sections were taken to clarify the stratigraphy of the paintings, and studied by means of complementary techniques (µFTIR, SEM-EDS), and in selected cases by means of micro-Raman spectroscopy. Micro-Raman spectroscopy and TXRF analyses were as well carried out on a sample taken with the cotton swab method (described in Paragraph 3.4.1), as no conclusive results could be obtained in situ. To allow the collection of minute amounts of pigment particles from the pigmented layer, the conservators agreed on removing the varnish from a small area (Figure 3.12).

201 The selection of points for Raman analysis was based on the characterization of the palette used by the painter of the “Pentecost”. The colours white, yellow, orange, red, skin tone, green, and blue were investigated. The white veil of the Virgin Mary showed the presence of both lead white

(2PbCO3·Pb(OH)2) and anhydrite (CaSO4), based on the bands at 1050 and 1016 cm-1 (Bell et al. 1997) (Figure 4.34, a). Anhydrite often occurs together with gypsum in the preparation of the ground layers in paintings on wood in Italian and Southern European paintings. This could mean that the paint layer is thin enough to allow the detection of the underlying materials, or that anhydrite was used as a pigmenting agent/additive in the paint. Both lead white and lead tin yellow type I (Pb2SnO4) were identified in

Figure 4.34: Analysed points on the painting “Pentecost”. Raman spectra of white and yellow shades: a) white veil of Virgin Mary: λ=785nm, STD lens, 10x5s, 35mW, ext. power supply, baseline corrected; b) white dove: λ=785nm, STD lens, 4x3s, 35mW, ext. power supply, baseline corrected; c) halo of the dove: λ=785nm, STD lens, 10x2s, 35mW, ext. power supply, baseline corrected, average. Reference spectra (dotted lines):

d) lead white (2PbCO3·Pb(OH)2); e) anhydrite (CaSO4); f) lead tin yellow type I

(Pb2SnO4).

202 the flames surrounding the dove, in a white and golden-yellow area, respectively (bands at 1050, and 194, 130 cm-1 (Bell et al. 1997), Figure 4.34, b and c). Lead tin yellow type I is present as well in the yellow sleeve of one of the apostles. There, it appears mixed with white compounds, such as lead white and −1 dolomite (CaMg(CO3)2). This carbonate is identified by the band at 1093 cm (Edwards and Villar 2005). The red pigment vermillion (HgS) was detected in red areas, such as the vest of the figure on the right, the flame above Mary’s head, and in the orangish vest of the second figure from the left (Figure 4.35, a). The characteristic Raman bands of vermillion appear at 343 and 252 cm-1 (Bell et al. 1997). The good Raman scattering properties of vermillion allowed the recording of good quality Raman spectra in maximum 30 seconds in total. Vermillion was as well used to obtain the skin and lip colour, together with anhydrite and lead white (Figure 4.35, b).

Figure 4.35: Analysed points on the painting “Pentecost”. Raman spectra of orange and skin tone. a) Orangish shirt of man in background: λ=785nm, STD lens, 10x2s, 35mW, ext. power supply, baseline corrected; b) skin tone of Virgin Mary: λ=785nm, STD lens, 4x10s, 35mW, ext. power supply, baseline corrected, average. Reference spectra

(dotted lines): c) lead white (2PbCO3·Pb(OH)2); d) anhydrite (CaSO4); e) vermillion (HgS).

203 Figure 4.36: Analysed points on the painting “Pentecost” and “Nativity”. Raman spectra of green and blue shades. a) Greenish highlight in green vest. λ=785nm, STD lens, 20x2s, 35mW, ext. power supply, baseline corrected; b) bluish sleeve. λ=785nm, STD lens, 10x5s, 35mW, ext. power supply, baseline corrected. Reference spectra (dotted lines):

c) carbon black (C); d) lead white (2PbCO3·Pb(OH)2). e) Blue wing of angel in Nativity scene (painting F): λ=785nm, STD lens, 6x10s, 35mW, ext. power supply, baseline

corrected. Reference spectra (dotted lines): f) azurite Cu3(CO3)2(OH)2.

The green vest of the first man from the left was tested by means of direct Raman spectroscopy: in the lighter area the signal of lead white was recorded, but no signal from a green compound could be obtained (Figure 4.36, a). This is a common problem for in situ Raman analysis of oil paintings: in fact, the red laser is not always the best choice for the study of green pigments (especially those based on copper, see Paragraph 4.3.1), and the green laser, although preferable, interferes strongly with the organic materials present, producing intense fluorescence that hampers the recording of any Raman bands (see Paragraph 4.3.1 and Table C.2, (Coccato et al. 2016). When analysing the blue shade of the sleeve of the first man standing on the right, only carbon black could be detected, thanks to the broad, weak band at 1580 cm-1 (Bell et al. 1997, Coccato et al. 2015, and Paragraph 4.3.2)

204 (Figure 4.36, b). This is an example of the highlighted difficulties in applying the methodology proposed in Paragraph 4.3.2 to carbon-based black pigments in real paintings. The blue vest of the Virgin was tested as well, but only the signal of carbon black was recorded, due to the presence of varnish and to the weak scattering properties of the used blue pigment.

The blue pigment azurite (Cu3(CO3)2(OH)2) was anyway identified in situ on another painting of the polyptych (wing of the angel, Nativity, painting F, Figure 4.36, e), by the bands at 400 and 1095 cm−1 (Bell et al. 1997). Thanks to the presence of conservators on site, it was possible to ask for the removal of the varnish to try to directly characterize the pigment. Carbon black was detected again, and very weak bands possibly related to azurite appeared, but this analysis could not be considered conclusive. The last option to characterize the blue pigment was the collection of a minute

Figure 4.37: Micro-Raman study of cotton swab sample (blue mantle of Virgin Mary). a) Black particle in dark blue area: λ=532nm, STD lens, 60-2640 cm-1, 3-5 cm-1, 50x1000aperture, 20× objective, 60x5s, 6.9 mW, average, baseline corrected. Reference spectra (dotted lines): b) carbon black (C); c) cotton. d) Bright blue particle in light blue area. λ=532nm, STD lens, 50-1550 cm-1, 3-5 cm-1, 50x1000aperture, 50× objective, 10x10s, 0.8 mW, average, baseline corrected. Reference spectra (dotted lines): e)

azurite Cu3(CO3)2(OH)2.

205 sample. As the question regarded only the surface layer, and no cross section was foreseen in the area of interest, the non-invasive cotton swab methodology was followed (Sub-chapters 1.5 and 3.4 (Devos et al. 1995, Moens et al. 1995, Wehling et al. 1999, von Bohlen 2004)). The sample was then analysed by means of total-reflection X-ray fluorescence (TXRF, Sub-chapter 3.4) and micro-Raman spectroscopy (Bruker Senterra Raman spectrometer, Sub-chapter 4.3) in the laboratory, which allowed the detection of copper, and the identification of particles of carbon black (1580 and 1317 cm-1, Figure 4.37, a), and of azurite (1095, 399, 247, 178, 153, 139, 83 cm-1 (Bell et al. 1997), Figure 4.37, d) trapped between the cotton fibers. The Raman spectrum of cotton is given as well, for comparison (Figure 4.37, c).

In this Chapter, applications of micro-Raman spectroscopy to the study of pigment powders by using laboratory instruments were discussed first. The importance of databases, and of a critical comparison with the reference spectra, as well as the existence of nomenclature issues were highlighted in Paragraphs 4.3.1 and 4.3.2. Moreover, Raman spectroscopy was confirmed, once again, as a powerful technique in monitoring pigment degradation in museum-like environments, thanks to its molecular specificity, sensitivity, non-destructiveness. Thanks to these factors, it can be used to detect ongoing degradation even when it does not alter the chromatic appearance of the objects (e.g. lead white vs. lead acetate, Paragraph 4.3.3). In the case of painting analyses, one major restriction is related to the measurement stage/chamber size. This, in fact, can strongly limit the feasibility of micro-Raman analyses, unless the object is small, or a sample is available, or if microscope adaptation systems are present (Edwards and Chalmers 2005, pages 5•6). On the other hand, a portable instrument can be used inside the laboratory for bigger objects (Figure 4.3, c), as well as on the field (Paragraph 4.4.1, Figures 4.29, 4.30 and 4.31), but the achievable spatial and spectral resolutions are, in general, worse (Vandenabeele and Moens 2012, Vandenabeele and Donais 2016). Also, the in situ detection of weak Raman scatterers, such as green and blue pigments, remains challenging. In the case of Funchal’s Altarpiece, the detection

206 of azurite was achieved thanks to the removal of the varnish (Figure 4.35), which might not always be possible. As it concerns the green pigments, direct Raman analysis might be unsuccessful. The contribution of Raman spectroscopic identification of materials in works of art to answering the questions highlighted in Chapter 1 is subjected to the availability of reference spectra for comparison, and can have an impact on authentication, definition of a painter’s palette (material selection/ creative process), chronological issues, material history, and preservation (as a consequence of detection of degradation phaenomena). Moreover, as a molecular technique, it is a powerful tool to cross-check the elemental information acquired for example by means of XRF, LIBS, PIXE, etc. Raman spectroscopy proves to be sensitive, specific and non-destructive, and it allows for spatial resolution down to 1 μm. It anyways suffers from fluorescence from the binders, and from environmental light interference. Absorption of the laser light can make the acquisition of the Raman signal challenging. Measurements, as well as the positioning for in situ point analysis, can be time consuming. Compared to other molecular (structural) techniques, such as XRD, the positioning is easier, especially for in situ applications, as it does not require alignment of the source and detector. Moreover, as visible light from lasers has lower penetrating power than X-rays, the measured volume is limited to the surface layer, making the data interpretation more straightforward. Also, XRD is not sensitive to short-ordered and amorphous phases, which makes the technique not suitable for the identification of some pigments (e.g. smalt, disordered carbon blacks). On the contrary, it allows for quantification of the present phases, which is not as straightforward in Raman spectroscopic studies. The next Chapter is entirely dedicated to the use of mobile instrumentation for the direct, in situ study of the frames of the Ghent Altarpiece. There, a multi•technique approach, namely using both XRF and Raman spectroscopies, is proposed in order to maximize the information extracted non-invasively and non•destructively from the object under investigation.

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226 As highlighted in Chapters 3 and 4, both XRF and Raman spectroscopic techniques have their own advantages and disadvantages in archaeometry. The possibility of non-destructive in situ measurements is particularly appreciated by conservators and curators. On the other hand, neither of the techniques can grant a complete and detailed characterization of the object under study. Therefore, it appears that the application of complementary techniques can successfully optimize the study of materials of our Cultural Heritage. As a final step, the chemical characterization of the materials, achieved non•invasively and through a multi-technique approach, needs to be included in the interdisciplinary study of the artefact, as discussed in Sub-chapter 1.5. The combination of an elemental technique with a molecular one for the study of polychromies and pigments is widely exploited (Clark 2005, Vandenabeele et al. 2007, 2014, Madariaga 2015, Bersani et al. 2016, Carò et al. 2016, Vandenabeele and Donais 2016). For example, XRF and XANES (Cotte et al. 2011), XRF and XRD (Hochleitner et al. 2003, Uda 2004), LIBS and Raman spectroscopy (Burgio et al. 2001, Bruder et al. 2007), PIXE and Raman spectroscopy (Bussotti et al. 1997, Bicchieri et al. 2001), XRF and Raman spectroscopy (Ramos and Ruisánchez 2006, Castro et al. 2007, Akyuz et al. 2008, Appolonia et al. 2009, Sawczak et al. 2009, Deneckere et al. 2010, Chaplin et al. 2010, Deneckere 2011, Hamdan et al. 2012, Van De Voorde et al. 2014). The latter combination is used in this research as well, and proved successful for the in situ characterization of the decorated frames of the Ghent Altarpiece (Sub-chapter 5.1).

227 The UGent GOA project on the archaeometrical study of the Ghent Altarpiece provided the conservators and art historians with some analytical data on the frames of the painting. The importance of such a study relies in the fact that the frames were always a physical and conceptual part of the painting itself, and they share the same material history (Coremans 1953, Glatigny et al. 2010; Figure 5.1). Also, the frames report the notorious quatrain, indicating the names of the painters (Jan and Hubert van Eyck), of the commissioner (Joos Vijd), and the supposed date of completion of the masterpiece in the form of a chronogram (6th of May 1432). The discussion on the authenticity of the quatrain is still ongoing, and involving many experts in the fields of art history and inscriptions (van der Velden 2011, Herzner 2013, Heyder 2015). The original supports and frames of the Ghent Altarpiece are made of Baltic oak wood (Coremans 1953, Glatigny et al. 2010; Figure 5.2). They were prepared and painted together, as testified by the lack of preparation and paint under the frame, as well as the presence of a barbe at the junction between the panel and its frame (Coremans 1953, Glatigny et al. 2010). However, panels and frames were detached from each other at some point (van Grevenstein et al. 2011), and the frames of the wings of the Ghent Altarpiece were sawn in Berlin in 1894, as the panels themselves (Coremans 1953, Glatigny et al. 2010, van Grevenstein et al. 2011). Pine wood additions were incorporated (Glatigny et al. 2010), and the polychromy was reconstructed (van Grevenstein et al. 2011). The frames of the open Altarpiece are gilded, partially restored with bronze-like paint (van Grevenstein et al. 2011), while the outer frames are heavily damaged and overpainted, so that during the treatment campaign of 1950-51 it was reported that “il est difficile d’en retracer l’histoire” (Coremans 1953, p. 87). More recently, various studies focused on the investigation of the polychromy of the outer wings frames, which appeared to be a grey stone imitation (Glatigny et al. 2010), painted on top of silver foil (van Grevenstein and Spronk 2011, van Grevenstein et al. 2011, Vekemans et al. 2012) (Figure 5.3). This discovery was supported both by the study of samples and by in situ non-invasive non•destructive analysis (handheld XRF) (van Grevenstein et al. 2011, Vekemans et al. 2012). The combined study allowed as well to identify two different sorts of overpaint, one based on bronze and one on tin (van Grevenstein et al. 2011).

228 Figure 5.1: Overview of the Altarpiece (outer wings panels) after the conservation treatment. Photo Dominique Provost - Lukas - Art in Flanders VZW.

229 Figure 5.2: Scheme of the planks and the frames of the outer wings of the Ghent altarpiece. Drawing taken from (Coremans 1953, p. 12).

230 Following the various interventions on the frames, like the replacement of hinges, wooden parts needed to be arranged to fill the gaps, and the polychromy was restored by means of overpaint . During this PhD research, the investigation of both restored and original parts of the frames was performed by means of a combination of in situ non-invasive techniques, to obtain information on the different materials used in rendering the stone-like appearance typical of the original frames. Moreover, mock-ups were used to evaluate the protective power of different varnishes suggested by the conservation team ((Rousaki et al. 2016), part of the PhD research project of Anastasia Rousaki).

The comparison between original and overpaints regarded the background, the painted gemstones, and the lines marking the stones. A multi-technique approach was chosen as the preliminary results demonstrated a complex composition of the polychromy. This included the original silver leaf, glazed and decorated to imitate polished stone, and bronze• and tin- paint based restorations, to match the precious stones appearance. The presence of metallic foils/paints requires the application of elemental analytical techniques, while the characterization of the pigments used for imitating gemstones and to create the three-dimensional appearance of the stone blocks was achieved by means of Raman spectroscopy.

Figure 5.3: Detail of the applied polychromy. Each stone-like section is decorated with painted gemstones in alternating colours (pink/yellow/green). Photo UGent (Sint•Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW).

231 Original Original Restored Restored

Figure 5.4: a) The painting Saint John the Evangelist with its frame before the conservation treatment. Photo KIK-IRPA (Sint-Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW). b) Detail of the bottom right corner of the frame during the conservation treatment. Photo UGent (Sint• Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW).

232 Restored Original

Figure 5.5: Detail of the area investigated by means of Raman spectroscopy. Photo UGent (Sint-Baafskathedraal Ghent, copyright Lukasweb.be - Art in Flanders VZW).

The combination of Raman and XRF spectroscopies to study paintings non• invasively is well established in literature (Ramos and Ruisánchez 2006, Castro et al. 2007, Akyuz et al. 2008, Appolonia et al. 2009, Sawczak et al. 2009, Deneckere et al. 2010, Chaplin et al. 2010, Deneckere 2011, Hamdan et al. 2012, Van De Voorde et al. 2014). Non-depth specific elemental information was obtained by means of portable XRF analysis, while the used pigments for the surface decoration were studied by portable Raman spectroscopy, which allows a better spatial resolution than what can be achieved by hXRF. The combination of the two techniques was meant to analytically (spectroscopically) confirm the complex material history of the Altarpiece, including the frames. For the purpose of this study, our investigation was based on direct analysis of the frame of John the Evangelist (Figures 5.4 and 5.5). The used portable XRF is the InnovX Delta instrument, already described in Sub- chapter 3.3. As it concerns the used portable Raman spectrometer, its characteristics have been provided in Sub-chapter 4.4 (Lauwers et al. 2014b, Lauwers 2017).

The in situ direct analysis of the frames based on portable (handheld) XRF and portable Raman spectroscopy requires specific measures to be taken. In fact, the positioning of the probehead in a stable, yet flexible, way, in order to minimize the risk for the painting and to ensure the collection of reproducible data, is achieved by means of in-house developed positioning systems (Sub-chapters 3.3 and 4.4).

233 In the case of hXRF analysis, two more aspects need to be taken into account. First, the spotsize being approximately 5 mm in diameter and the optimal distance from the painted surface of just a few mm, the positioning is simply achieved by carefully sliding the probehead holder by hand. This allows for sufficient precision for such an approach. Moreover, safety concerns for the operators and the other persons in the area surrounding the instrument necessitate the definition of a safety distance around the instrument, and the use of USB cables for remote operation of the instrument via a laptop (see Figures 3.7 and 3.8). On the other hand, the used portable Raman instrument has a much smaller spot size (ca. 0.7 mm), allowing for the analysis of smaller details, so that the positioning needs to be more precise (Figure 5.6). Moreover, the focussing is crucial: the operator can observe an improvement of the Raman spectra when getting closer to the focal distance (approximately 7 mm, see Sub-chapter 4.4). This empiric focussing process ensures for a good quality of recorded spectra, as well as for a precise determination of the measured point, as the laser can be seen on the painted surface (Figure 5.6). In this case, the obstacle is related to the detector’s sensitivity to environmental stray light. Raman measurement should be performed in the dark, or at least the point of analysis should not receive any light from the surroundings. In the case of the direct study of the Ghent Altarpiece, the surface could not be touched at any moment (contrarily to the cases of Pianazzola (Lauwers et al. 2014b), Sala Vaccarini (Barone et al. 2016), and Paragraph 4.4.1). However, inside the restoration atelier at the MSK in Ghent, a black fabric tent for UV light observation was present, and was used for Raman in situ analysis as well. Still, the level of Figure 5.6: Dinolite picture taken during positioning for in situ Raman analysis. The area under study darkness provided by the tent is one of the lines marking the stones. The was not sufficient to completely low•power laser can be seen shining on the remove environmental light white detail (brushstroke thickness ca. 2 mm). interferences, which triggered the Photo UGent (Sint•Baafskathedraal Ghent, substitution of the black fabric copyright Lukasweb.be - Art in Flanders VZW). with thicker velvet, which showed

234 better shielding from the undesired light in the next stages of the Raman in situ study of the Altarpiece (see further). First, hXRF was used to compare the plain stone-like surface, avoiding any interference from added pigments. Then, the same hXRF technique was used to try to characterize the painted gemstones and separations of the stones, although the spotsize of the instrument is not optimal for such purpose (5 mm diameter vs. ca. 1 mm for the lines, ca. 3-4 mm for the gemstones). After evaluating these results, Raman spectroscopy was used to complete the characterization of the used pigments both in the original and restored parts. The comparison of the original area with the restored one shows clear differences in the elemental pattern (Figure 5.7): in fact, in the latter, manganese, copper, zinc and tin (as well as minor amounts of other metals such as Ti, V, Cr) are clearly visible, suggesting the use of a cheaper metallic paint, based on some sort of bronze-like alloy (Cu > Zn > Sn). On the contrary, silver was used in the original. The presence of a weaker peak at ca. 22 keV in the uncoated wood needs to be pointed out. In fact, already during the 2010 hXRF measurement campaign, the spectrum of a resinous wood, acquired for comparison, showed that the scattering profile of the wood presented a feature in this area (Vekemans et al.

Figure 5.7: HXRF spectra of the stone-like imitation. Comparison of the original (full line) and restored (dotted line) parts: in the first the signals of Ca, Fe and Pb are present, and related to the wood (in grey). The signal of Ag is also visible. In the latter Mn, Cu, Zn and Sn signals are evident, which confirms the presence of a cheaper metallic-looking layer.

235 2012). Therefore, care needs to be taken in interpreting the spectra, and comparison with the wood signal is necessary (Figure 5.7, grey). As it concerns the analysis of the applied polychromy (Figure 5.8), the signal of the surroundings is always present due to the penetrating power of X-rays and to the spotsize of the instrument compared with the studied details. In fact, the signal of silver in the original can still be seen in the XRF spectrum (Figure 5.8, full line). In the pink areas, studied both on the original and on the restored part of the frame of John the Evangelist, the signal of mercury is present, pointing out the use of vermillion to make the pink colour (Figure 5.8). However, the white pigment used to create such a pink shade could not be identified by means of hXRF analysis. Therefore, in situ Raman spectroscopy was performed on a cleansed area (Figure 5.3), and the acquired spectra are shown in Figure 5.9 and Figure 5.10. The environmental signal is given for reference, as some features in the collected spectra are not Raman bands of materials in the painting, but interferences (Figure 5.9, d, and Figure 5.10, c). The comparison of the data with reference spectra (Figure 5.9, b and c, and Figure 5.10, b) and with published values (Bell et al. 1997, Burgio and Clark 2001) confirmed the presence of vermillion (bands at 383(m), 343(m), 253(s) cm-1 (Bell et al. 1997)).

Figure 5.8: HXRF spectra of the pink gemstones. Comparison of the original (full line) and restored (dotted line) parts: in the first silver is visible, in the latter Mn, Cu, Zn and Sn signals are evident, which confirms the presence of a cheaper metallic-looking layer. In both areas the lines of mercury are detected.

236 Figure 5.9: Raman spectra on the pink gemstones in the original part of the frame of John the Evangelist. The environmental signal, and the reference spectra of lead white and vermillion are given for comparison: a) spectrum (λ=785nm, 30x15s,

STD lens, 0.06mW, external power source); b) Lead white (2PbCO3·Pb(OH)2); c) Vermillion (HgS); d) Environmental signal.

However, two different white materials were identified, according to their belonging to an original or restored area. In the original part (Figure 5.5), lead white was identified thanks to the band at 1052 cm-1. In the restored part, chalk was identified by the band at 1085 cm-1. The similar appearance of the retouchings to the original pink polychromy was therefore achieved by using different mixtures of red and white materials. As chalk was used to prepare ground layers and fillings, its presence can be related to such mixtures for re•creating the polychromy.

In this Sub-chapter, the application of a non-invasive multi-technique approach was investigated for the study of the frames of the Ghent Altarpiece. The characterization of the materials had to be performed non-invasively, and it was successfully achieved thanks to the combination of complementary

237 Figure 5.10: Raman spectra on the pink gemstones in the restored part of the frame of John the Evangelist. The environmental signal, and the reference spectra of chalk are given for comparison: a) spectrum λ=785nm, 30x10s, STD lens, 0.06mW, external power source,

b) Chalk (CaCO3); c) Environmental signal. techniques. The elemental, bulk information provided by portable XRF analyses was combined with the molecular characterization of materials in the uppermost layers obtained by mobile Raman spectroscopy. The present non-invasive, in situ characterization of the polychrome frames of the Ghent Altarpiece is a good example of the potential and drawbacks of such an approach for studying works of art. On the positive note, it is important to highlight that Raman spectroscopy and XRF are complementary techniques both as regards the type of information obtained (molecular / elemental), and as regards the analysed spot. In fact, XRF provides the qualitative elemental composition of a volume, considering the instrumental spotsize and penetration depth of X-rays, while Raman spectroscopy is a surface technique with a much smaller spotsize, which allows the analysis of details in the uppermost layer. The combination of these two techniques, specifically using the two available instruments, is promising, although some limits might be encountered. In this present case, it was possible to ascertain the presence of silver foil in the original frames. Metals are not Raman-active, while they are easily detected by XRF

238 analysis. In this case, the detection of silver is subject to interference due to the scattering of the plain wood in the region of its K-lines, while L-lines overlap with Ar-K (from the air) and Rh-L (from the source). Silver was already successfully identified in situ (Vekemans et al. 2012), and here it was identified in all of the studied original areas. Moreover, the proposed methodology confirmed the presence of cheaper materials to imitate the precious metal of the original, and different compounds used to render the polychromy during restoration. However, these analyses cannot clarify the sequence of different overpaint layers, which can be achieved only by means of cross section analysis, which is a minimally-invasive approach. The combined knowledge of the analytical techniques (including physico•chemical principles and instrumental features) and of the materials used in making works of art appears to be fundamental in providing the art historians and conservators high quality information for the purpose of conservation treatments and general characterization of the materials. The next Chapter will provide some general, conclusive considerations on the potential of archaeometrical investigation of Cultural Heritage materials by means of the proposed techniques, XRF and Raman spectroscopies, alone or combined.

Akyuz, S., T. Akyuz, S. Basaran, C. Bolcal, and A. Gulec. 2008. Analysis of ancient potteries using FT-IR, micro-Raman and EDXRF spectrometry. Vibrational Spectroscopy 48:276-280. Appolonia, L., D. Vaudan, V. Chatel, M. Aceto, and P. Mirti. 2009. Combined use of FORS, XRF and Raman spectroscopy in the study of mural paintings in the Aosta Valley (Italy). Analytical and bioanalytical chemistry 395:2005-2013. Barone, G., D. Bersani, A. Coccato, D. Lauwers, P. Mazzoleni, S. Raneri, P. Vandenabeele, D. Manzini, G. Agostino, and N. F. Neri. 2016. Nondestructive Raman investigation on wall paintings at Sala Vaccarini in Catania (Sicily). Applied Physics A 122:838. Bell, I. M., R. J. H. Clark, and P. J. Gibbs. 1997. Raman spectroscopic library of natural and synthetic pigments (pre- ≈ 1850 AD). Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 53:2159-2179. Bersani, D., C. Conti, P. Matousek, F. Pozzi, and P. Vandenabeele. 2016. Methodological evolutions of Raman spectroscopy in art and archaeology. Analytical methods 8:8395-8409. Bicchieri, M., M. Nardone, P. A. Russo, A. Sodo, M. Corsi, G. Cristoforetti, V. Palleschi, A. Salvetti, and E. Tognoni. 2001. Characterization of azurite and lazurite based pigments by laser induced breakdown spectroscopy and micro- Raman spectroscopy. Spectrochimica Acta - Part B Atomic Spectroscopy 56:915-922. Bruder, R., V. Detalle, and C. Coupry. 2007. An example of the complementarity of

239 laser-induced breakdown spectroscopy and Raman microscopy for wall painting pigments analysis. Journal of Raman Spectroscopy 38:909-915. Burgio, L., and R. J. H. Clark. 2001. Library of FT-Raman spectra of pigments, minerals, pigment media and varnishes, and supplement to existing library of Raman spectra of pigments with visible excitation. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 57:1491-1521. Burgio, L., K. Melessanaki, M. Doulgeridis, R. J. H. Clark, and D. Anglos. 2001. Pigment identification in paintings employing laser induced breakdown spectroscopy and Raman microscopy. Spectrochimica Acta - Part B Atomic Spectroscopy 56:905-913. Bussotti, L., M. P. Carboncini, E. Castellucci, L. Giuntini, and P. A. Mandò. 1997. Identification of pigments in a fourteenth-century miniature by combined micro- Raman and PIXE spectroscopic techniques. Studies in Conservation 42:83-92. Carò, F., E. Basso, and M. Leona. 2016. The Earth Sciences from the Perspective of an Art Museum. Elements 12:33-38. Castro, K., M. Pérez-Alonso, M. D. Rodríguez-Laso, N. Etxebarria, and J. M. Madariaga. 2007. Non-invasive and non-destructive micro-XRF and micro- Raman analysis of a decorative wallpaper from the beginning of the 19th century. Analytical and bioanalytical chemistry 387:847-860. Chaplin, T. D., R. J. H. Clark, and M. Martinón-Torres. 2010. A combined Raman microscopy, XRF and SEM-EDX study of three valuable objects - A large painted leather screen and two illuminated title pages in 17th century books of ordinances of the Worshipful Company of Barbers, London. Journal of Molecular Structure 976:350-359. Clark, R. 2005. Raman Microscopy in the Identification of Pigments on Manuscripts and Other Artwork. Pages 162-185. (Sackler NAS Colloquium) Scientific Examination of Art. National Academies Press, Washington, D.C., USA. Coremans, P. 1953. Les Primitifs Flamands. III. Contribution a l’etude des Primitifs Flamands 2. L’Agneau Mystique au Laboratoire. De Sikkel, Antwerp, Belgium. Cotte, M., J. Szlachetko, S. Lahlil, M. Salome, V. Solé, I. Biron, and J. Susini. 2011. Coupling a wavelength dispersive spectrometer with a synchrotron-based X-ray microscope: A winning combination for micro-X-ray fluorescence and micro- XANES analyses of complex artistic materials. Journal of Analytical Atomic Spectrometry 26:1051-1059. Deneckere, A. 2011. Development, optimisation and application of Raman spectroscopic and X-ray spectroscopic methodology in the field of cultural heritage. Ghent University. PhD thesis. Deneckere, A., W. Schudel, M. Van Bos, H. Wouters, A. Bergmans, P. Vandenabeele, and L. Moens. 2010. In situ investigations of vault paintings in the Antwerp cathedral. Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy 75:511-519. Glatigny, J.-A., A. Genbrugge, J. Roeders, and R. Meurs. 2010. Report in progress: Observations on the supports and frames of the Ghent Altarpiece. van Grevenstein, A., H. Dubois, M. Postec, A.-S. Augustinyak, J. Sanyova, I. Geelen, S. Saverwyns, G. Steyaert, J.-A. Glatigny, A. Genbrugge, J. Roeders, R. Meurs, B. Vekemans, H. van Keulen, and G. Fife. 2011. Conservatie en materieel onderzoek van het Retabel van het Lam Gods Ghent, Sint-Baafskathedraal. van Grevenstein, A., and R. Spronk. 2011. Lasting Support - An interdisciplinary research project to assess the structural condition of the Ghent Altarpiece. Hamdan, N. M., H. Alawadhi, and N. Jisrawi. 2012. Integration of μ-XRF, and u-

240 Raman techniques to study ancient Islamic manuscripts. IOP Conference Series: Materials Science and Engineering 37:12006. Herzner, V. 2013. The quatrain of “The Ghent Altarpiece”, again. Simiolus: Netherlands Quarterly for the History of Art 37:95-99. Heyder, J. 2015. Further to the discussion of the highlighted chronogram on The Ghent Altarpiece. Simiolus. Netherlands quarterly for the history of art 38:5-16. Hochleitner, B., V. Desnica, M. Mantler, and M. Schreiner. 2003. Historical pigments: A collection analyzed with X-ray diffraction analysis and X-ray fluorescence analysis in order to create a database. Spectrochimica Acta - Part B Atomic Spectroscopy 58:641-649. Lauwers, D. 2017. Characterisation and optimisation of mobile Raman spectroscopy for art analysis. Ghent University. PhD thesis. Lauwers, D., A. Hutado, V. Tanevska, L. Moens, D. Bersani, and P. Vandenabeele. 2014. Characterisation of a portable Raman spectrometer for in situ analysis of art objects. Spectrochimica Acta Part A-Molecular and Biomolecular Spectroscopy 118:294-301. Madariaga, J. M. 2015. Analytical chemistry in the field of cultural heritage. Analytical methods 7:4848-4876. Ramos, P. P. M., and I. Ruisánchez. 2006. Data fusion and dual-domain classification analysis of pigments studied in works of art. Analytica Chimica Acta 558:274-282. Rousaki, A., P. Vandenabeele, J. Sanyova, H. Dubois, and L. Moens. 2016. A protocol to evaluate how good varnishes protect silver gilding against atmospheric pollutants. Page 81. 4th International Congress Chemistry for Cultural Heritage, Brussels, Belgium. Sawczak, M., A. Kamińska, G. Rabczuk, M. Ferretti, R. Jendrzejewski, and G. Śliwiński. 2009. Complementary use of the Raman and XRF techniques for non-destructive analysis of historical paint layers. Applied Surface Science 255:5542-5545. Uda, M. 2004. In situ characterization of ancient plaster and pigments on tomb walls in Egypt using energy dispersive X-ray diffraction and fluorescence. Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 226:75-82. Vandenabeele, P., H. G. M. Edwards, and L. Moens. 2007. A Decade of Raman Spectroscopy in Art and Archaeology. Chemical Reviews 107:675-686. Vandenabeele, P., H. G. M. Edwards, and J. Jehlička. 2014. The role of mobile instrumentation in novel applications of Raman spectroscopy: archaeometry, geosciences, and forensics. Chemical Society Reviews 43:2628-2649. Vandenabeele, P., and M. K. Donais. 2016. Mobile Spectroscopic Instrumentation in Archaeometry Research. Applied spectroscopy 70:27-41. Vekemans, B., J. Sanyova, A.-S. Augustinyak, P. Vandenabeele, and L. Vincze. 2012. A study of Panels of the “Adoration of the Mystic Lamb” in the St Bavo Cathedral in Ghent with the InnovX Delta Handheld X-ray Fluorescence Analyzer: Further Comments on Some Mirrored Images in Jan van Eyck’s Paintings. Van Eyck Studies Colloquium. Brussels, Belgium. van der Velden, H. 2011. The quatrain of “The Ghent Altarpiece.” Simiolus: Netherlands Quarterly for the History of Art 35:5-40. Van De Voorde, L., J. Van Pevenage, K. De Langhe, R. De Wolf, B. Vekemans, L. Vincze, P. Vandenabeele, and M. P. J. Martens. 2014. Non-destructive in situ study of “Mad Meg” by Pieter Bruegel the Elder using mobile X-ray fluorescence, X-ray diffraction and Raman spectrometers. Spectrochimica Acta - Part B Atomic Spectroscopy 97:1-6.

241

This thesis focusses on the analytical investigation of the material aspects of polychrome objects from our Cultural Heritage. Pigments are the main objective of this study, for many reasons, which will be summarized in the next paragraphs. First of all, polychrome objects, such as paintings, constitute a great deal of our Cultural Heritage. While carrying symbolic, aesthetic, cultural values, these objects are made of matter. This aspect is often underestimated, as paintings are regarded mainly as two-dimensional images. However, the material composition of polychrome artefacts, which can be simplified in support/binders and varnishes/pigmenting agents, depends on cultural, social, political and economical factors. In fact, the commissioner’s ideas and wealth, cultural environment, traditional artisanal practices, and the painter’s skills are all affecting and shaping the appearance of the object. The choice and handling of pigments has art historical, technical, social, cultural, etc. implications (Chapter 1). The understanding of the material aspects of polychromies, including a precise identification of the used pigments, based on chemical characterization, here achieved non•destructively, is a powerful tool to appreciate social, economical and cultural assets in the past. Moreover, for the purpose of preservation and conservation of our Cultural Heritage, it is important to study degradation processes (Chapter 2), so that they can be understood, and actions can be taken to limit, stop, and prevent further damage, in order to preserve the masterpieces for future generations. It appears that, after these considerations, the boundary between material science, conservation and art history is dissolving, and offers the chance to contribute to answer questions about chronology, authentication, creative process, material history and preservation of our Cultural Heritage (Table 1.1). It is important to note that a clear statement of the research question is fundamental in defining and guiding the analytical approach. For example, elemental techniques cannot be used to distinguish between the two titanium whites anatase (after 1923) and rutile (after 1947) (see Table 3.2), while they can successfully detect titanium, and point out the presence of a modern overpaint

243 (Sub-chapter 3.3). Another example can regard the need for the characterization of the top-layer (including overpaints), in contrast to the investigation of hidden paint layers (support, underdrawing, pentimenti, original layer under overpaints (Sub-chapter 3.3)). Moreover, as our Cultural Heritage is irreplaceable, the range of analytical approaches that can be exploited is limited. In fact, sampling is not always permitted, and transportation of the object to a laboratory is not always feasible, or allowed. According to the type of information needed to answer the research question, non-invasive and non-destructive analysis have to be preferred. Moreover, thanks to the development of mobile instrumentation, direct analysis becomes possible inside the laboratory, and even in situ. Non-destructive analysis, although sometimes requiring a minute sample, offer the possibility of a multi-technique study, that allows cross-checking the results, and the optimization of the extracted information. Non-invasive analysis is performed without sampling and without measurement-related damage, therefore the object’s integrity is fully safeguarded. Direct analysis, finally, is in principle non-invasive, with the added values of minimizing the risks related to the transportation and handling of precious objects, and of allowing the study of unmoveable objects (e.g. wall paintings). A multi-technique approach is also always recommended, to optimize the characterization. Nevertheless, the material characterization alone cannot be sufficient, as the chemical information needs to be combined and included in the bigger picture, together with the knowledge from humanities, history and social sciences. Communication among experts from these different fields is a critical point. The required commitment is noteworthy from all sides, but the achievable results, in terms of interdisciplinary study of a work of art, seem worth the effort. The reasons why paintings should be regarded and studied as unique and irreplaceable material entities by using an interdisciplinary approach were discussed in Chapter 1, while Chapter 2 focussed on the materials under investigation in this thesis, which are pigments in use during the Middle Ages, with specific attention to their sensitivity to degradation. Chapters 3, 4 and 5, on the other hand, give some insights on how to perform minimally-invasive non-destructive, non-invasive and direct analysis to achieve the characterization of pigments. The techniques of choice (XRF and Raman spectroscopies, Chapters 3 and 4, respectively) are well known in the field of archaeometry, for their effectiveness in providing useful results, their versatility in tackling a variety of research questions, and the possibility of performing direct analysis by means of mobile instrumentation. They are used for studying pigments and paintings, alone (Chapters 3 and 4) or in combination (Chapter 5), and specific advantages and pitfalls can be highlighted in this thesis.

244 To begin with, handheld X-ray fluorescence (hXRF) analyses are performed on supposedly overpainted areas of the outer wings of the Ghent Altarpiece (Sub- chapter 3.3). The detection of elements such as Ti, Cr, Zn confirmed the presence of overpaint. However, this information was not sufficient for the conservators, who needed to know as well the spatial distribution of the overpaints, as well as the conservation state of the underlying paint layers. HXRF can provide quickly non depth-specific elemental results, but it is not sufficient in the framework of a conservation treatment. In that case, the participation of University of Antwerp, with their macro-XRF scanning system, allowed the successful implementation of the preliminary results into the conservation practice. A different application of elemental techniques for the study of pigments is based on total-reflection XRF (TXRF, Sub-chapter 3.4). Although this approach requires a (minute) sample, it allows for quantification of major, minor and trace elements. However, the quantification technique is optimised for liquid samples, which is not the case of pigment grains sampled on precious polychrome objects. These non-conventional samples require specific measures in order to achieve results. Moreover, the quantification step is investigated by comparing the results of commercial software with those from calculations. This work provides preliminary observations for the successful application of TXRF for (quantitative) pigment analysis. These two examples of use of XRF analysis for the study of Cultural Heritage materials prove the effectiveness of these techniques for answering specific research questions, like the identification of pigments based on key elements. However, they might not be sufficient, as in the characterization of green and black pigments, and of degradation processes. In these cases, molecular and structural aspects need to be considered as well, as the elemental information obtained by means of XRF is not always adequate to identify the materials positively, and additional analyses are necessary (Sub-chapter 3.5). In some of these cases, Raman spectroscopy proves to be a much better tool to tackle pigments characterization; however, it is important to have access to databases of reference spectra for comparison. During this research project, it was observed that, on the one hand, the variety of green pigments available to artists through time was much wider than what expected (i.e. green earths, malachite, verdigris, see Chapter 2) based on scientific analysis of paintings. On the other hand, although Raman spectroscopy is well suited for discriminating among different green salts, a coherent database for the archaeometrist was still missing (Paragraph 4.3.1). The collected Raman spectra (including the instrumental parameters used) are made available to the community, for facilitating the identification of green materials. Moreover, we show, through the study of a paint sample, how the identification of pigments by comparison with

245 reference spectra needs to be tackled cautiously, as different instrumental set-ups and parameters affect the recorded spectra. Next to the creation of a database of green materials, a wide range of carbon•based black pigments is studied (Paragraph 4.3.2), to evaluate the use of Raman spectroscopy to obtain a greater extent of information about the nature of the used black pigment (raw material, degree of disorder, etc.). In this case, geological literature is the starting point, but nomenclature issues are tackled with respect to the archaeometrical context of this research. Some guidelines to better understand the type of carbon under investigation are provided. However, often fluorescence due to the presence of organic binders hampers the interpretation. Finally, thanks to the molecular sensitivity of Raman spectroscopy, it is possible to confirm the suggested reaction pathways, and to identify the occurrence of degradation, even if it does not affect the optical properties (i.e. colour) of the materials (Paragraph 4.3.3). The study of pigment degradation upon exposure to acetic acid is completed, and the lowest doses of acetic acid to cause the appearance of detectable amounts of degradation products are converted to safety thresholds for the studied pigments. It is however important to note that pigments are rarely directly exposed to the atmosphere, due to the presence of binders and varnishes. These materials, as well as others of relevance for Cultural Heritage, were also studied in terms of acetic acid sensitivity during the MEMORI project, to help museum curators in identifying potentially harmful conditions for complex objects, based on the most sensitive material. Next to the use of laboratory instruments, a portable Raman spectrometer was used to study the Altarpiece in Funchal, Portugal. Direct access to the paintings was ensured by scaffoldings. Positioning and focusing was achieved by holding the probehead by hand, and the environmental light interference was minimized thanks to the use of light blockers. The measurements were performed during the conservation campaign, which included the possibility of analysing unvarnished areas. Direct Raman analysis proved successful in the study of the painter’s palette, with the exception of the green pigment. Although green copper-based pigments give better results when excited with a green laser compared to a red one (see Paragraph 4.3.1), the fluorescence due to the organic binder overwhelms any present Raman band. The use of the red laser, although reducing the fluorescence, is also not effective for this task, which remains a challenge for in situ Raman spectroscopy. XRF analyses performed on the same spot can help in identifying the used materials, but the full characterization of the pigments, as well as of the stratigraphic structure, was finally achieved on cross sections. So far, it appears that hXRF is a fast technique, suitable for in situ measurements, which supports the identification of pigments based on the presence of key elements, while it cannot provide information about the composition of single layers, due to the penetrating power of this radiation

246 (Sub•chapter 3.3). On the other hand, TXRF can allow for quantification of major, minor and trace elements, but it requires a sample. XRF techniques are not sensitive to low Z elements, like carbon, and do not allow for the identification of the compound (Sub-chapter 3.4). Then again, Raman spectroscopy is successfully used to characterize a variety of compounds used in works of art, including pigments but excluding metals. The availability of databases is crucial to successfully identify the materials (Paragraph 4.3.1 and 4.3.2). Raman spectroscopy is a sensitive, specific, and versatile technique, which can be used as well to monitor and understand degradation processes (Paragraph 4.3.3), but it suffers from fluorescence from the binders and varnishes, and from interferences from environmental light, in case of direct analysis. These two aspects need to be kept in mind, as better results are obtained if the varnish is removed (or thinned, as during conservation treatments), and when stray light is somehow blocked (Paragraph 4.4.1 and Sub- chapter 5.1). Moreover, the combination of the molecular information obtained by means of Raman spectroscopy with complementary data (e.g. elemental) improves significantly the success rate of material identification. XRF and Raman spectroscopic techniques can be considered as complementary from different points of view: they provide, respectively, elemental and molecular information, and information from a certain volume (including underlaying layers) and from the surface only; one is sensitive to heavier elements (Z>12), while the other to compounds (Chapter 5). The combined use of the two techniques is, therefore, even more powerful in the characterization of materials present in Cultural Heritage materials. In fact, this is exemplified by the study of painted frames of the Ghent Altarpiece (Sub•chapter 5.1). HXRF analyses confirmed the presence of precious metals (Ag) in the original frames of the Ghent Altarpiece, and the use of a cheaper bronze-like paint in the restored parts. By means of Raman spectroscopy, the used pigments are characterized in both, the original and restored areas. As a final consideration, the possibility of non-invasively obtaining information on different aspects of the polychromy, by means of XRF and Raman spectroscopies, is extremely valuable before, during, and after a conservation treatment. The characterization of the materials used for obtaining the illusionistic effects that leave us speechless in front of a masterpiece is a valuable tool to appreciate the object in its material aspects as well. On the one hand, as it concerns the progress in the analytical techniques, and their application to Cultural Heritage materials, it seems that direct analysis can be of great help in this field of research. The availability of macro-XRF scanners, which allow for obtaining the elemental distribution images, is pushing forward to the development of analogous systems for Raman spectroscopy. However, the issues of binder/varnish fluorescence, the need for accurate positioning and

247 focussing, and the time needed for the analysis are all major aspects to tackle before in situ Raman mapping can become an established technique. Some devices and concepts are already available (longer wavelength lasers to reduce fluorescence, and sensitive detectors in the infrared region; spatially-offset Raman spectroscopy (SORS) to investigate layers underneath the surface), and will likely support this advancement in the future. SORS could also help in investigating the underdrawings, but as the pigmented layers on top of it are inhomogeneous, and the layered structure complex (in terms of materials, number and thickness of layers), this might pose additional challenges, which require further research. Finally, as it regards pigments identification, accessible databases of reference spectra will help the archaeometrist in correctly identifying the materials. However, it will be fundamental to provide spectra recorded with a variety of laser excitations, as well as the instrumental parameters and measurement details, as these are likely to affect the Raman spectrum, as shown in Chapter 4. Also, considering the increase in number of reference spectra, to facilitate identification, the creation of searching algorithms and auto-identification softwares seems an interesting aspect to consider. The cooperation with engineers and computer science experts will expand once again the range of subjects involved in this interdisciplinary approach, advancing the chances of successfully identifying the materials, and limiting the time needed for it. On the other hand, for the study of pigments degradation, Raman spectroscopy can be successfully applied to explore a variety of processes involving molecular and structural changes. In all cases, the possibility of in situ analyses can enhance the impact of the chemical information on the overall understanding of a work of art, including conservation approaches. On the other hand, the understanding of artisanal practices, technological skills, and painter’s mastery in handling materials, is a fascinating, complex, yet underestimated field of research. It requires a multifaceted, interdisciplinary approach that includes the knowledge of painting materials, their properties and reactivity, of technical art history, of the type of questions that might arise when investigating our Cultural Heritage, and of the analytical techniques that are suitable to tackle a specific subject. Again, the communication among the different fields of expertise is necessary, in order to achieve a complete and detailed understanding of the masterpiece as a whole. I am convinced that any further development in archaeometry and conservation science will only be possible if all parties agree on making the effort to understand each other, and share the knowledge, as this can only improve the depth and completeness of our understanding of works of art.

248 Deze thesis richt zich op het analytisch onderzoek van de materiële aspecten van polychrome erfgoedobjecten. Deze studie belicht vooral pigmenten, omwille van verschillende redenen, die in de volgende paragrafen opgesomd zijn. Vooreerst vertegenwoordigen polychrome objecten een belangrijk deel van ons cultureel erfgoed. Ze zijn dragers van symbolische, esthetische en culturele waarden, maar zijn uit materie opgebouwd. Dit aspect wordt vaak onderschat, omdat schilderijen als 2-dimensionele afbeeldingen beschouwd worden. De materiële samenstelling van de polychrome kunstvoorwerpen, kan vereenvoudigd beschouwd worden als drager-bindmiddel en vernis-pigment systemen en hangt af van culturele, politieke, sociale en economische factoren. De rijkdom en het gedachtengoed van de opdrachtgever, de culturele omgeving, de kunstenaarstraditie, en de vaardigheden van de schilder beïnvloeden allemaal de uiteindelijke vorm en het uitzicht van het object. De keuze en verwerking van de pigmenten heeft kunsthistorische, technische, sociale, culturele, etc. gevolgen (Hoofdstuk 1). Als we met niet-destructieve spectroscopische technieken nauwkeurige pigmentanalyse kunnen uitvoeren, geeft dat een goed basisbegrip van de materiële aspecten van de polychromie. Zo vormt dit onderzoek een krachtig instrument om sociale, economische en culturele bijdragen in het verleden, te waarderen. Met het oog op de conservatie van ons cultureel erfgoed, is het bovendien belangrijk om degradatieprocessen te bestuderen (Hoofdstuk 2), en er eventueel acties ondernomen kunnen worden om verdere schade te beperken, stoppen of vermijden, en zo de meesterwerken voor toekomstige generaties te bewaren. Het lijkt erop dat de grens tussen materiaalwetenschappen, conservatie en kunstwetenschappen aan het oplossen is. Dit geeft ons de kans om een bijdrage te leveren tot vragen over chronologie, authenticiteit, het creatief proces, materiële geschiedenis en de bewaring van cultureel erfgoed (Tabel 1.1). Een duidelijke formulering van de onderzoeksvraag is uitermate belangrijk om de

249 analytische benadering te sturen. Zo kunnen element-specifieke technieken niet gebruikt worden om bijvoorbeeld de twee vormen van titaanwit – anataas (vanaf 1923) en rutiel (vanaf 1947) (zie Tabel 3.2) – te onderscheiden, ondanks dat titaan wel kan gedetecteerd worden om zo een moderne overschildering te identificeren (Hoofdstuk 3). Een ander voorbeeld illustreert de nood om de bovenste verflaag (inclusief overschilderingen) te identificeren, in tegenstelling tot het onderzoek van verborgen lagen (drager, ondertekening, pentimenti, oorspronkelijke verflaag onder overschilderingen (Hoofdtuk 3)). Bovendien, omdat ons cultureel erfgoed onvervangbaar is, kunnen maar een beperkte groep analytische technieken ingezet worden. Staalname is niet altijd toegestaan en transport van het object naar het laboratorium is niet altijd mogelijk. Afhankelijk van de soort informatie die nodig is om de onderzoeksvraag te beantwoorden, verkiest men niet-destructieve of niet-invasieve analyses. Dankzij de ontwikkeling van mobiele apparatuur is rechtstreekse analyse mogelijk in het lab, en zelfs in situ. Hoewel niet-destructieve analysetechnieken soms een minuscuul staaltje nodig hebben, kan op die manier een multi-techniek onderzoek gevoerd worden. Dit heeft als voordeel dat de resultaten kunnen gecrosschecked worden en dat de bekomen informatie kan geoptimaliseerd worden. Niet-invasieve analyse gebeurt zonder staalname en zonder beschadiging van het object en zo blijft de volledige integriteit van het object bewaard. Tot slot, rechtstreekse analyse is in principe niet-invasief, met als bijkomend voordeel dat de risico’s die met het transport en manipulatie van kostbare voorwerpen gepaard gaan, geminimaliseerd worden. Ook objecten die niet verplaatsbaar zijn (vb. muurschilderingen) kunnen onderzocht worden. Een multi-techniek benadering wordt altijd aangeraden, om gedetailleerde informatie verkregen over de samenstelling van de objecten. Desalniettemin is de karakterisering van de materialen onvoldoende: chemische informatie moet in het bredere verhaal geïncorporeerd worden, samen met kennis van de humane wetenschappen, geschiedenis en sociale wetenschappen. Communicatie tussen experten in deze verschillende disciplines is een kritisch punt. Van elk van deze disciplines wordt een inspanning gevraagd, maar het resultaat, binnen een interdisciplinaire studie van een kunstwerk, is de moeite waard. De redenen waarom schilderijen als unieke en onvervangbare materiële objecten moeten beschouwd worden, werden in hoofdstuk 1 bediscussieerd. Hoofdstuk 2 focust op de materialen die in deze scriptie onderzocht werden: pigmenten uit de Middeleeuwen, waarbij er specifiek aandacht besteed werd aan hun gevoeligheid voor degradatie.

250 Hoofdstukken 3, 4, en 5 daarentegen, bieden enkele inzichten over hoe minimaal-invasieve niet-destructieve, niet-invasieve en rechtstreekse analyses ingezet kunnen worden om tot de karakterisering van de pigmenten te komen. De geselecteerde technieken (XRF- en Ramanspectroscopie, Hoofdstukken 3 en 4, resp.) zijn wel bekend binnen de archeometrie omdat ze nuttige resultaten opleveren, omdat ze bruikbaar zijn om een breed gamma onderzoeksvragen te beantwoorden en omdat het mogelijk is om objecten rechtstreeks te onderzoeken met behulp van mobiele apparatuur. Ze worden ingezet voor de studie van pigmenten en schilderijen, elk apart (Hoofdstukken 3 en 4) of gecombineerd (Hoofdstuk 5), en specifieke voordelen en valkuilen worden in deze scriptie besproken. In eerste instantie werden handbediende X-stralen fluorescentie (hXRF) analyses uitgevoerd op overschilderde zones van de buitenste vleugels van het Lam Gods (Hoofdstuk 3). De detectie van elementen als Ti, Cr, en Zn bevestigde de aanwezigheid van overschilderingen. Deze informatie was echter onvoldoende voor de restaurateurs, voor wie het ook noodzakelijk was om de exacte ruimtelijke verdeling van de overschilderingen te kennen, en ook de conservatietoestand van de onderliggende lagen. hXRF kan vrij snel de elementsamenstelling bepalen. Zonder onderscheid tussen de verschillende verflagen is dit onvoldoende in het licht van een conservatiebehandeling. Hiervoor werd de hulp ingeroepen van het team van de Universiteit Antwerpen, met hun macro-XRF scanning apparatuur. Dit zorgde voor een succesvolle integratie van de preliminaire resultaten in de conservatiepraktijk. Een andere toepassing van elementanalyse voor pigmentanalyse is gebaseerd op totale-reflectie X-stralen fluorescentie (TXRF, Hoofdstuk 3). Deze benadering heeft weliswaar een erg klein staaltje nodig, maar ze laat toe om zowel hoofd-, neven-, en sporenelementen kwantitatief te bepalen. De kwantificatie van de techniek is geoptimaliseerd voor vloeibare staaltjes, waardoor pigmentkorrels van kostbare polychrome kunstobjecten traditioneel niet in aanmerking komen. Deze niet-conventionele stalen hebben specifieke maatregelen nodig om goede resultaten te bereiken. Daarbij werd de kwantificatiestap onderzocht door de resultaten van commerciële software te vergelijken met eigen berekeningen. Dit onderzoek levert voorlopige observaties over de succesvolle toepassing van TXRF voor (kwantitatieve) pigmentanalyse. Deze twee voorbeelden van het gebruik van XRF analyses voor de studie van cultureel erfgoed bewijzen de efficiëntie van deze technieken voor specifieke onderzoeksvragen, zoals de identificatie van pigmenten op basis van sleutelelementen. Deze technieken kunnen echter niet altijd voldoen aan de verwachtingen, zoals bij de identificatie van groene en zwarte pigmenten, en bij de studie van degradatieprocessen. In deze gevallen moeten ook moleculaire en

251 structurele aspecten beschouwd worden, gezien de elementaire informatie die met XRF bekomen wordt niet altijd voldoende is om de materialen positief te identificeren (Hoofdstuk 3). In sommige gevallen blijkt Raman spectroscopie een betere analysemethode te zijn om pigmenten te karakteriseren. Het is belangrijk om toegang te hebben tot databanken met referentiespectra. Tijdens dit onderzoeksproject observeerden we enerzijds dat de variatie in groene pigmenten die beschikbaar waren veel breder is, dan wat initieel gedacht (vb. groene aarde, malachiet, verdigris, zie Hoofdstuk 2). Anderzijds bleek dat, hoewel Raman spectroscopie geschikt is om een onderscheid te maken tussen verschillende groene zouten, er geen coherente databank beschikbaar was voor de archeometrist (Hoofdstuk 4). De verzamelde Raman spectra, met de gebruikte instrumentele parameters, worden ter beschikking gesteld van de onderzoeksgemeenschap. Door de studie van een authentiek staal, tonen we hoe de identificatie van pigmenten door vergelijking met referentiespectra voorzichtig dient aangepakt te worden, gezien verschillende instrumentele opstellingen en parameters de spectra beïnvloeden. Naast de creatie van een databank van groene materialen, wordt ook een breed gamma aan koolstof-bevattende zwarte pigmenten gebruikt (Hoofdstuk 4). Deze werden geanalyseerd met Ramanspectroscopie, om zo gedetailleerde informatie te bekomen over het type zwart pigment dat gebruikt werd (ruw materiaal, graad van orde, enz.). Hier was de geologische literatuur het startpunt, problemen met nomenclatuur werden behandeld, voor wat de archeometrische context van dit onderzoek betreft. Er worden richtlijnen gegeven om zo beter het type onderzocht koolstof te kunnen identificeren. Jammer genoeg wordt de interpretatie vaak ook bemoeilijkt door fluorescentie, veroorzaakt door het organisch bindmiddel. Doordat Raman spectroscopie een moleculair spectroscopische techniek is, is het mogelijk om bepaalde reactieschema’s te onderzoeken. Zo kan degradatie onderzocht worden, zelfs als het de visuele verschijning (kleur) van het object niet beïnvloedt (Hoofdstuk 4). Pigmentdegradatie na blootstelling aan azijnzuur werd onderzocht, en de laagste dosis die aanleiding gaf tot degradatie, werd omgezet naar veiligheidslimieten voor de bestudeerde pigmenten. Er moet weliswaar opgemerkt worden dat pigmenten meestal niet rechtstreeks blootgesteld worden aan de atmosfeer, omwille van de aanwezigheid van bindmiddelen en vernissen. Deze materialen, samen met andere producten die relevant zijn voor erfgoedstudies, werden getest op hun gevoeligheid ten opzichte van azijnzuur- dampen. Zo worden conservators bijgestaan om potentieel schadelijke condities te vermijden voor complexe, samengestelde objecten, gebaseerd op de gevoeligheid van het meest kwetsbare materiaal.

252 Naast het gebruik van laboratoriumtoestellen werd een draagbaar Ramantoestel gebruikt voor de studie van het altaarstuk in Funchal (Madeira, Portugal). De schilderijen waren toegankelijk via stellingen. De meetsonde werd gepositioneerd en gefocust door de meetsonde met de hand voor het schilderij te houden. Interferentie door omgevingslicht werd geminimaliseerd door afschermkapjes. De metingen werden tijdens de restauratiecampagne uitgevoerd, waardoor er ook niet-verniste zones konden onderzocht worden. Met rechtstreekse Raman-analyse konden we het schilderspalet identificeren, behalve voor het groene pigment. Hoewel groene koperhoudende pigmenten over het algemeen betere resultaten opleveren als ze met de groene laser bestudeerd worden, in tegenstelling tot de rode laser, overstemt de fluorescentie in dat geval elke mogelijke Ramanband. Het gebruik van de rode laser vermindert dan wel de fluorescentie, maar ze laat niet toe om het Ramanspectrum op te nemen. Dit onderzoek blijft een uitdaging voor in situ Raman analyse. Het gebruik van XRF op dezelfde plaats kan helpen om de materialen te identificeren, maar de volledige karakterisering van groene pigmenten en hun lagenopbouw, werd uiteindelijk gerealiseerd aan de hand van een ingebed staaltje. Tot nu toe lijkt hXRF een snelle techniek te zijn, die geschikt is voor in situ metingen. Pigmenten kunnen geïdentificeerd worden aan de hand van sleutelelementen, maar geen informatie kan bekomen worden over de laagopbouw (Hoofdstuk 3). Anderzijds kan TXRF hoofd-, neven- en sporenelementen kwantitatief bepalen, maar dan is een staal noodzakelijk. XRF- gebaseerde technieken zijn niet gevoelig voor lichte elementen, zoals koolstof, en laten de precieze identificatie van het product toe (Hoofdstuk 3). Ook hier is Ramanspectroscopie succesvol ingezet voor de analyse van een veelheid aan componenten in kunstwerken. Metalen kunnen met Ramanspectroscopie niet geïdentificeerd worden. Het gebruik van databanken is essentieel voor de positieve identificatie van een aantal materialen (Hoofdstuk 4). Raman spectroscopie is een gevoelige, specifieke en veelzijdige techniek. De methode kan ook gebruikt worden voor de studie van degradatieprocessen (hoofdstuk 4), maar fluorescentie veroorzaakt door bindmiddelen en vernissen, bemoeilijken de techniek. Omgevingslicht kan interferentie veroorzaken, vooral bij directe analyse. Deze twee beperkingen moeten in gedachten gehouden worden, en daarom worden betere resultaten bekomen als de vernislaag weggenomen is (of afgedund), bijvoorbeeld tijdens een conservatiecampagne, of als omgevingslicht wordt tegengehouden. Bovendien kunnen meer materialen geïdentificeerd worden als moleculaire informatie (Raman spectroscopie) met complementaire gegevens (vb. elementspectroscopie) gecombineerd wordt. XRF en Raman analyse kunnen op verschillende vlakken als complementair beschouwd worden: ze leveren respectievelijk elementaire en moleculaire

253 informatie en informatie van een zeker meetvolume (inclusief onderliggende lagen) of enkel van de oppervlaktelaag. De ene techniek is ook gevoeliger voor zware elementen (Z>12), terwijl de andere methode ook (organische) verbindingen kan meten (Hoofdstuk 5). De combinatie van beide technieken is zelfs nog krachtiger in de identificatie van erfgoedmaterialen. Dit wordt geïllustreerd aan de hand van de analyse van de lijsten van het Lam Gods (Hoofdstuk 5). hXRF analyse bevestigde de aanwezigheid van Ag in de originele lijsten, en het gebruik van een goedkopere Cu-legering in de gerestaureerde zones. Met Ramanspectroscopie konden de pigmenten geïdentificeerd worden in de originele en gerestaureerde zones. Concluderend kunnen we ook stellen dat de mogelijkheid om met XRF en Ramanspectroscopie op een niet-invasieve manier informatie te bekomen over verschillende aspecten van de polychromie erg nuttig is, zowel voor, tijdens als na een conservatiecampagne. Als we de methodeontwikkeling en de toepassing voor kunstanalyse beschouwen, lijkt het dat directe analyse erg waardevol kan zijn in dit onderzoeksdomein. De beschikbaarheid van macro-XRF scanners, die toelaten om beelden van de elementdistributie te bekomen, zet aan tot verdere ontwikkelingen van soortgelijke systemen voor Ramanspectroscopie. Problemen met bindmiddel/vernis-geïnduceerde fluorescentie, de noodzaak van accurate positionering en scherpstellen, en de lange meettijd, zijn allemaal belangrijke aspecten die behandeld moeten worden, voordat in situ Raman mapping een gevestigde techniek wordt. Sommige toestellen en concepten bestaan al (lange laser golflengtes om fluorescentie te vermijden, gevoelige detectoren in het infrarood gebied, Spatially Offset Raman Spectroscopy om lagen onder het oppervlak te onderzoeken) en zullen wellicht bijdragen tot verdere ontwikkelingen in de toekomst. SORS zou in principe kunnen bijdragen tot het onderzoek van ondertekeningen, maar gezien de bovenliggende pigmentlagen inhomogeen zijn (verschillende materialen, verschillend aantal lagen en verschillende laagdikte), stelt dit bijkomende uitdagingen, en is dit voer voor verder onderzoek. Wat pigmentidentificatie betreft, zullen toegankelijke databanken met referentiespectra de archeometrist helpen om materialen correct te identificeren. Het zal echter fundamenteel belangrijk zijn om materialen met verschillende lasers te onderzoeken en de instrumentele parameters en meetgegevens in de analyse te betrekken. Deze zullen wellicht in sommige gevallen het spectrum beïnvloeden, zoals in Hoofdstuk 4 getoond werd. Doordat er steeds meer spectra in zo’n databank aanwezig zijn, dringt zich een ontwikkeling en verbetering van zoekalgorithmes op. De samenwerking met ingenieurs en IT specialisten zullen op hun beurt de onderwerpen in dit interdisciplinair onderzoek verder uitbreiden,

254 waarbij de identificatie van de materialen steeds sneller en accurater afgewerkt kan worden. In elk geval kan de mogelijkheid tot in situ analyse bijdragen aan de algehele interpretatie van het kunstwerk en de conservatie ervan. Anderzijds is ons begrip van het vakmanschap van de kunstenaar, zijn technische vaardigheden en zijn materiaalgebruik, een fascinerend en complex - maar onderschat - onderzoeksdomein. Hiervoor is een interdisciplinaire benadering noodzakelijk, met kennis van kunstenaarsmaterialen, hun eigenschappen en reactiviteit, maar ook kennis van technische kunstwetenschappen. Finaal is het ook belangrijk om goed te weten welke analysetechnieken geschikt zijn om specifieke vragen op te lossen. Ook hier is communicatie tussen de verschillende expertisedomeinen uitermate belangrijk om een volledig begrip van het kunstwerk te bekomen. Ik ben ervan overtuigd dat elke verdere ontwikkeling in de archeometrie en conservatiewetenschap enkel mogelijk zal zijn als alle partijen samen de moeite willen doen om elkaar te begrijpen en kennis te delen. Dit kan enkel helpen om kunstwerken meer volledig en in de diepte te begrijpen.

255

257 Table A.1, Part 1/5: Summary of the observed discolourations on traditional inorganic mediaeval pigments. Pigments sensitivity to RH (relative humidity) and pollutants. If the pigment is reported in literature to be stable to a specific parameter, the symbol “–ˮ is used. If the corresponding cell is left blank, no information could be retrieved from published works.

Pigment High RH Fluctuating RH RH + chlorides (Cl-)

Swelling of paint Calcium carbonates (white) layers, Swelling of paint layers CaCO3 mechanical stress Blackening on wall paintings, watercolours Lead white and manuscripts. 2PbCO ∙Pb(OH) 3 2 Synergistic effects with the other parameters. Dissolution. Formation and transport of As5+ Orpiment (yellow) ions. Whitening As2S3 (formation of As2O3) in presence of light and of oxidizing conditions Initial darkening, then Massicot (yellow) degradation to PbO cerussite/hydrocerussite

Lead tin yellow type I: Pb2SnO4; - type II: PbSn1-xSixO3

Yellow, red, brown ochres Hydration of oxides (red Fe O , FeOOH, Fe O ; 2 3 3 4 to yellow) MnxOy

Realgar/pararealgar Formation and transport (orange/red, yellow) of As5+ ions. Whitening As4S4 (formation of As2O3)

Litharge (orange) PbO

Unstable. Synergy with Involved in Vermillion (red) photodegradation and discolouration and HgS oxidation. sulphation

Red lead Unstable, blackening Involved in Pb3O4 upon light exposure discolouration

Green earths: Discolouration of oil glauconite and celadonite layers

Malachite (green) Reaction without Recrystallization Recrystallization CuCO3∙Cu(OH)2 colour change

Verdigris (bluish green) Hydrolysis of the organic xCu(CH3COO2)∙yCu(OH)2∙zH2O moieties

258 Pigment High RH Fluctuating RH RH + chlorides (Cl-)

Green copper resinate copper salts of abietic acid

Cu chlorides (green) Polymorphism Polymorphism Polymorphism Cu2Cl(OH)3

Cu sulphates (green) Polymorphism Polymorphism CuSO4∙yCu(OH)2∙zH2O

Ultramarine blue Na8[Al6Si6O24]Sn

Vivianite (blue) Fe3(PO4)2∙8H2O

Glass alteration, ions Smalt (blue) leaching and CoO∙nSiO 2 discolouration

Azurite Discolouration:black, Discolouration - 2CuCO3∙Cu(OH)2 green

Egyptian blue Green discolouration Green discolouration CaCuSi4O10

Carbon black C

259 Table A.1, Part 2/5: Summary of the observed discolourations on traditional inorganic mediaeval pigments. Pigments sensitivity to RH (relative humidity), pollutants and oxidizing agents. If the pigment is reported in literature to be stable to a specific parameter, the symbol “–ˮ is used. If the corresponding cell is left blank, no information could be retrieved from published works. RH + pollutants Oxidizing Pigment RH + salts (SO2, SO3, NOx) agents Formation of gypsum, mechanical stress Calcium carbonates (white) (increased volume), CaCO 3 lixiviation of soluble material Formation of various Lead white salts, sometimes not Blackening Blackening 2PbCO3∙Pb(OH)2 white. Cerussite is the most stable. Whitening Orpiment (yellow) (formation of As2S3 As2O3)

Initial darkening, then Massicot (yellow) degradation to PbO cerussite/hydrocerussite

Lead tin yellow Browning/ type I: Pb SnO ; 2 4 darkening type II: PbSn1-xSixO3

Yellow, red, brown ochres Hydration of oxides, Fe O , FeOOH, Fe O ; formation of hydrated 2 3 3 4 MnxOy sulphates Realgar/pararealgar (orange/red, yellow)

As4S4 Degradation to Degradation to Litharge (orange) cerussite/hydrocerussite, cerussite/hydrocerussite, PbO sulphate and sulphate and phosphates phosphates

Vermillion (red) HgS

Discolouration, as lead Red lead Browning/ Whitening salts are most of the Pb O darkening 3 4 time white/whitish

Green earths: glauconite and celadonite

Malachite (green) Reaction without colour Reaction without colour CuCO3∙Cu(OH)2 change change

Browning Verdigris (bluish green) Formation of Cu salts (peroxide xCu(CH COO )∙yCu(OH) ∙zH O 3 2 2 2 species)

260 RH + pollutants Oxidizing Pigment RH + salts (SO2, SO3, NOx) agents

Green copper resinate copper salts of abietic acid

Cu chlorides (green) Formation of Cu salts Cu2Cl(OH)3

Cu sulphates (green) Polymorphism Formation of Cu salts CuSO4∙yCu(OH)2∙zH2O

Ultramarine blue Discolouration (grey) Na8[Al6Si6O24]Sn

Discolouration Vivianite (blue) to green, and Fe3(PO4)2∙8H2O finally to yellow In tempera: Smalt (blue) discolouration CoO∙nSiO2 (dosimetry). No synergy of SO2 and NOx

Azurite - discolouration (green) 2CuCO3∙Cu(OH)2

Egyptian blue CaCuSi4O10

Alteration, Carbon black especially if C impurities are present

261 Table A.1, Part 3/5: Summary of the observed discolourations on traditional inorganic mediaeval pigments. Pigments sensitivity to alkali and acids. If the pigment is reported in literature to be stable to a specific parameter, the symbol “–ˮ is used. If the corresponding cell is left blank, no information could be retrieved from published works.

Pigment Alkali Acids Hydrogen sulphide (H2S)

Calcium carbonates (white) Decomposition CaCO3 Soluble. Blackening (especially Formation of manuscripts and oxalates and watercolours). Formed PbS Lead white Blackening acetates upon can be convered to PbSO , 2PbCO ∙Pb(OH) 4 3 2 exposure to oxalic and finally and acetic acid cerussite/hydrocerussite respectively. again. Soluble. Not Orpiment (yellow) recommended Soluble As S 2 3 for wall paintings

Massicot (yellow) Formation of black PbS PbO

Lead tin yellow Formation of type I: Pb2SnO4; - acetates and Formation of PbS type II: PbSn1-xSixO3 carboxylates

-, formation of Ca Yellow, red, brown ochres oxalates from the Fe O , FeOOH, Fe O ; - 2 3 3 4 other minerals in Mn O x y the ochre Realgar/pararealgar (orange/red, yellow) As4S4 Litharge (orange) PbO

Unstable, but Vermillion (red) still used in wall HgS paintings Unstable. Formation of Soluble. Red lead black PbO and Sensitive: formation of PbS 2 Formation of Pb O PbSO in a or PbSO 3 4 4 PbO , PbCO 4 sulphation 2 3 process Green earths: Soluble Soluble glauconite and celadonite Bluish discolouration on Decomposition, pure pigment; selective Malachite (green) Discolouration further reaction of attack on other pigments in CuCO3∙Cu(OH)2 (black) 2+ Cu ions the mixture if H2S is of biological origin

262 Pigment Alkali Acids Hydrogen sulphide (H2S)

Formation of Verdigris (bluish green) Formation of Bluish discolouration on blue copper xCu(CH COO )∙yCu(OH) ∙zH O oxalates pure pigment 3 2 2 2 hydroxides

Green copper resinate copper Darkening salts of abietic acid

Formation of Cu chlorides (green) (unstable) blue Oxalates Cu2Cl(OH)3 Cu(OH)2

Cu sulphates (green) Oxalates CuSO4∙yCu(OH)2∙zH2O

Stable. Stable. Ultramarine blue Discolouration Discolouration Na [Al Si O ]S 8 6 6 24 n (grey) (grey)

Vivianite (blue) Fe3(PO4)2∙8H2O

Smalt (blue) Glass alteration Glass alteration CoO∙nSiO2 Bluish discolouration on pure pigment; selective Azurite Discolouration: Decomposition, attack on other pigments in 2CuCO3∙Cu(OH)2 green further reaction the mixture if H2S is of biological origin Egyptian blue - - CaCuSi4O10

Alteration, Carbon black especially if C impurities are present

263 Table A.1, Part 4/5: Summary of the observed discolourations on traditional inorganic mediaeval pigments. Pigments sensitivity to biological attack, and effect of material selection. If the pigment is reported in literature to be stable to a specific parameter, the symbol “–ˮ is used. If the corresponding cell is left blank, no information could be retrieved from published works. Biological Other pigments/ Pigment Binders attack additives Reacts with verdigris. Cu Formation of oxalates, Calcium carbonates (white) Formation of ions catalyse degradation proteinates, CaCO oxalates of binders and formation 3 carboxylates of oxalates Drying properties. Blackens when mixed Lead white Formation of soaps with red lead. Raction Oxalates 2PbCO3∙Pb(OH)2 which increase layer with S containing transparency pigments not clear Reacts with verdigris and Different stability in 5+ lead white. As deposit Orpiment (yellow) oil/water based around Fe/Mn particles. As S mediums. Unstable in 2 3 Formation of As O and oil 2 3 H2S Massicot (yellow) PbO

Lead tin yellow Drying properties. type I: Pb2SnO4; Formation of lead type II: PbSn1-xSixO3 soaps Fe promotes photo- Ca oxalates Yellow, red, brown ochres oxidation, Mn curing of (from the Fe O , FeOOH, Fe O ; the oil. Reaction - 2 3 3 4 other Mn O products witf x y minerals) proteinaceous binder Reacts with verdigris and Realgar/pararealgar Different stability in lead white. As5+ deposit (orange/red, yellow) oil/water based around Fe/Mn particles. As4S4 mediums Formation of As2O3 and H2S Very reactive towards Litharge (orange) organic binders, PbO commonly used as dryer. Soaps formation Protect vermillion from Mixtures with lead white, light and from external minium, and other Vermillion (red) chloride sources. pigments show increased - HgS Watercolour medium stability of vermillion. offers little to no Reactivity with some protection. No oxalates materials Sensitive to S containing Protect the pigment pigment (arsenic from light and humidity. PbO can sulphides, vermillion and 2 Lead soaps, lead Red lead result from ultramarine): formation of hydroxide, lead Pb O biological dark PbS and white 3 4 acetates and finally activity PbSO , and Pb arsenate lead carbonates are 4 species. It promotes lead formed. white blackening

264 Biological Other pigments/ Pigment Binders attack additives

Green earths: glauconite and celadonite

Cu acts as a Organometallic Malachite (green) biocide. compounds (oxalates, CuCO3∙Cu(OH)2 Acidic carboxylates, resinates, conditions acetates, etc.) The acidic conditions, presence of Cu, light and Promotes drying; Verdigris (bluish green) pollution are involved in formation of soaps and xCu(CH COO )∙yCu(OH) ∙zH O cellulose degradation (20). 3 2 2 2 metalloproteins Darkening when mixed with orpiment or lead white. Formation of carboxylates, Green copper resinate copper metalloproteins. salts of abietic acid Photoxidative processes occur. Cu chlorides (green) Cu2Cl(OH)3

Cu sulphates (green) Formation of CuSO4∙yCu(OH)2∙zH2O Cu salts

Catalytic effect of the Ultramarine blue Pb neutralizes the acidity pigment on binders Na [Al Si O ]S of the binder 8 6 6 24 n hydrolysis

Vivianite (blue) Protect the pigment Fe3(PO4)2∙8H2O from degradation Leaching of ions, Ca neutralizes the acidity changes in Co Smalt (blue) of the binder, Pb more coordination and CoO∙nSiO reactive than K to form 2 formation of soaps soaps (grey, blanching) Discolouration of small particles; formation of bluish verdigris (with Azurite Cu acts as a humidity); formation of 2CuCO ∙Cu(OH) biocide 3 2 Cu proteinates in tempera; yellowing of binders Egyptian blue Darkening in gum CaCuSi4O10

Carbon black Darkening upon + Pb dryers: white C infrared laser exposure discolouration

265 Table A.1, Part 5/5: Summary of the observed discolourations on traditional inorganic mediaeval pigments. Pigments sensitivity to light, chemical analyses and high temperature. If the pigment is reported in literature to be stable to a specific parameter, the symbol “–ˮ is used. If the corresponding cell is left blank, no information could be retrieved from published works. Laser / ion Pigment Light High temperature beams

Calcium carbonates (white) Decomposition to CaO - -/discolouration CaCO3 and CO2 Influence of the binding medium. Discolouration Lead white Formation of (formation of 2PbCO3∙Pb(OH)2 massicot-like massicot, litharge, red degradation / lead) discolouration

Sensitive, especially Darkening, and then Orpiment (yellow) to green. Synergy Green lasers whitening (formation As2S3 with relative humidity. of As2O3 and SO2)

Irreversible Massicot (yellow) Discolouration darkening, or PbO (formation of red lead) whitening

Lead tin yellow Decomposition at 900 type I: Pb SnO ; - 2 4 °C type II: PbSn1-xSixO3 Dehydration of hydroxides/ Dehydration of Yellow, red, brown ochres oxihydroxides hydroxides/ -, If impurities are Fe O , FeOOH, Fe O ; (yellow to red); oxihydroxides; 2 3 3 4 present, unstable MnxOy modification of modification of the Mn the Mn phases phases (darkening) (darkening) Realgar/pararealgar Temporary Sensitive, especially (orange/red, yellow) Green lasers. darkening/Whitening to green. As4S4 (formation of As2O3)

Stable, but Discolouration Litharge (orange) massicot can be (formation of PbO formed. massicot, red lead)

Blackening, Vermillion (red) Blackening, related to related to halogen HgS halogen impurities impurities

Grey to brown Affects lean layers of discolouration, water soluble paint. related to Both blackening pigment’s Red lead (PbS) and lightening composition. Formation of litharge Pb O (PbSO , PbCO ) 3 4 4 3 Formation of PbO occur. Related to (which can be re- pigment’s oxidised to composition. minium)

266 Laser / ion Pigment Light High temperature beams

Browning, Green earths: discolouration of oil glauconite and celadonite layers

Malachite (green) Discolouration - Discolouration (black) CuCO3∙Cu(OH)2 (black)

Verdigris (bluish green) Browning, darkening + -/ discolouration PIXE xCu(CH3COO2)∙yCu(OH)2∙zH2O (release of Cu )

-/-/darkening Green copper resinate copper 2+ Darkening (reduction of Cu X-rays salts of abietic acid + to Cu )

Cu chlorides (green) Cu2Cl(OH)3

Cu sulphates (green) CuSO4∙yCu(OH)2∙zH2O

Ultramarine blue Discolouration (grey)/ - - Na8[Al6Si6O24]Sn stable

Vivianite (blue) Decomposition with Fe3(PO4)2∙8H2O discolouration

Smalt (blue) Further discolouration CoO∙nSiO2 of degraded particles

Azurite Discolouration - Discolouration (black) 2CuCO3∙Cu(OH)2 (black)

Egyptian blue - CaCuSi4O10

Photodegradation of Carbon black Stable to 248 nm the aromatic structure Burning C laser catalysed by lead

267 268 Table B.1, Part 1/3: Analysis of Kremer pigment particles sampled with a cotton swab, processed by Qcalc and SinerX. Results of pigment analysis (white and yellow/orange). The given values are calculated based on a given concentration of Fe = 1 ppm and are expressed in percentage of detected elements. The standard deviation sj is calculated on three measurements per sample. The main element(s) for each pigment are marked in bold. Lead tin Lead tin Naples Yellow Lead white Realgar Orpiment yellow I yellow II yellow ochre Pigment 2PbCO3·Pb PbSn1- Fe2O3·H2O Pb2SnO4 Pb2Sb2O7 As4S4 As2S3 (OH)2 xSixO3 +clay+SiO2

% sj % sj % sj % sj % sj % sj % sj Qcalc 0.0 0.0 6.9 0.6

K SinerX 5.9 0.4

Qcalc 0.2 0.0 0.6 0.1 0.5 0.1 0.2 0.0 0.4 0.2

Ca SinerX 0.0 0.0 2.2 0.1 0.4 0.0 0.1 0.1 0.9 0.0 Qcalc 0.0 0.0 2.0 0.0

V SinerX 3.5 0.0

Qcalc 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Cr SinerX 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Qcalc 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Mn SinerX 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Qcalc 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.1 0.0 0.0 0.0 90.0 0.5

Fe SinerX 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 88.0 0.4

Qcalc

Co SinerX

Qcalc 0.1 0.0 0.0 0.0 0.1 0.0 0.1 0.1 0.2 0.0 0.2 0.0 0.1 0.0

Ni SinerX 0.1 0.0 0.1 0.0 0.2 0.0 0.2 0.0 0.1 0.0 0.1 0.0 0.0 0.0

Qcalc 0.1 0.0 0.0 0.0 0.1 0.0 0.1 0.1 0.1 0.0 0.1 0.0 0.2 0.0

Cu SinerX 0.1 0.0 0.1 0.0 0.2 0.0 0.2 0.0 0.1 0.0 0.1 0.0 0.9 0.0

Qcalc 0.1 0.0

Zn SinerX 0.0 0.0

Qcalc 99.6 0.0 99.5 0.0

As SinerX 99.7 0.0 99.7 0.1

Qcalc 0.0 0.0 0.0 0.0 0.0 0.0

Sr SinerX 0.2 0.0 0.3 0.0 0.3 0.0 0.0 0.0

Qcalc 7.1 0.8 0.4 0.1 6.4 0.3 6.1 3.9

Cd SinerX 3.6 0.3 3.5 0.3 1.9 0.4 1.9 0.8

Qcalc 0.2 0.0 0.0 0.0

Hg SinerX 0.3 0.0 0.4 0.1

Qcalc 92.1 0.7 99.0 0.1 92.8 0.3 93.6 4.0 0.2 0.0

Pb SinerX 88.3 0.6 93.5 0.3 97.0 0.4 97.5 0.8 0.9 0.0

Qcalc 0.2 0.0

Bi SinerX 7.2 0.9

269 Table B.1, Part 2/3: Analysis of Kremer pigment particles sampled with a cotton swab, processed by Qcalc and SinerX. Results of pigment analysis (red and blue). The given values are calculated based on a given concentration of Fe = 1 ppm and are expressed in percentage of detected elements. The standard deviation sj is calculated on three measurements per sample. The main element(s) for each pigment are marked in bold. Fe oxide Cinnabar Vermillion Co blue Lapis lazuli Ultramarine Azurite red Pigment 2CuCO3· Fe2O3 HgS CoO·Al2O3 Na8[Al6Si6O24]Sn Cu(OH)2

% sj % sj % sj % sj % sj % sj % sj Qcalc 1.2 0.4 0.2 0.4 3.2 0.1 0.7 0.1

K SinerX 0.4 0.0 2.1 0.1 0.2 0.0

Qcalc 0.1 0.0 4.4 3.3 96.8 0.7 92.7 0.1 0.7 0.1

Ca SinerX 0.0 0.0 4.2 0.2 95.3 0.5 94.0 0.1 0.4 0.0 Qcalc 0.2 0.1 0.4 0.2 0.7 0.0 0.4 0.0

V SinerX 0.2 0.0 1.0 0.2 1.0 0.0 0.3 0.1

Qcalc 0.3 0.0 0.0 0.0 0.0 0.0 3.7 2.7 0.0 0.0

Cr SinerX 1.0 0.0 0.0 0.0 0.0 0.0 5.5 0.0 0.1 0.0

Qcalc 0.2 0.0 0.0 0.0 0.2 0.0 0.0 0.1 0.1 0.0 0.1 0.0

Mn SinerX 0.6 0.5 0.0 0.0 0.4 0.2 0.0 0.0 0.1 0.0 0.1 0.0

Qcalc 99.1 0.1 1.1 0.0 0.1 0.0 2.1 1.8 1.0 0.1 3.2 0.1 2.6 0.0

Fe SinerX 97.4 0.4 0.6 0.1 0.1 0.0 0.1 0.0 0.8 0.1 2.3 0.1 1.2 0.0

Qcalc 84.4 11.7

Co SinerX 80.0 0.4

Qcalc 0.0 0.0 0.1 0.0 0.1 0.0 0.3 0.2 0.3 0.1 0.0 0.0 0.1 0.0

Ni SinerX 0.1 0.0 0.1 0.0 0.7 0.3 0.5 0.2 0.0 0.0 0.1 0.0

Qcalc 0.2 0.0 0.1 0.0 0.2 0.0 0.4 0.3 0.9 0.1 0.1 0.0 95.8 0.1

Cu SinerX 1.0 0.0 0.1 0.0 0.1 0.0 0.7 0.2 1.5 0.1 0.2 0.0 96.7 0.2

Qcalc 0.0 0.0 0.1 0.1 4.6 3.4 0.1 0.0 0.0 0.0 0.3 0.0

Zn SinerX 0.0 0.0 0.3 0.1 7.8 0.2 0.2 0.0 0.0 0.0 1.0 0.1

Qcalc 0.0 0.0 0.1 0.1 0.2 0.0 0.0 0.0

As SinerX 0.0 0.0 0.1 0.0 0.3 0.3 0.2 0.1

Qcalc 0.0 0.1 0.0 0.0

Sr SinerX 0.2 0.0 0.1 0.0

Qcalc

Cd SinerX

Qcalc 98.1 0.2 99.4 0.0

Hg SinerX 98.1 0.1 99.4 0.3

Qcalc 0.1 0.0 0.0 0.0 0.0 0.0

Pb SinerX 0.4 0.2 0.1 0.0 0.0 0.0

Qcalc

Bi SinerX

270 Table B.1, Part 3/3: Analysis of Kremer pigment particles sampled with a cotton swab, processed by Qcalc and SinerX. Results of pigment analysis (green and black). The given values are calculated based on a given concentration of Fe = 1 ppm and are expressed in percentage of detected elements. The standard deviation sj is calculated on three measurements per sample. The main element(s) for each pigment are marked in bold.

Malachite Verdigris Cu resinate Green earth Viridian Magnetite Pigment CuCO3· Cux(CH3COO)y Cu salts of K[(Al,Fe)(Fe,Mg)] Cr2O3·2H2O Fe3O4 Cu(OH)2 (OH)z·nH2O abietic acid (AlSi3,Si4)O10(OH)2 % sj % sj % sj % sj % sj % sj

Qcalc 2.7 0.1

K SinerX 1.7 0.1

Qcalc 0.0 0.0 82.2 0.1 0.5 0.8 0.67 0.10

Ca SinerX 0.0 0.0 84.1 1.0 0.1 0.1 0.00 0.00 Qcalc 0.9 0.1

V SinerX 0.8 0.1

Qcalc 0.0 0.0 0.0 0.0 99.0 0.0 0.12 0.01

Cr SinerX 0.0 0.0 0.0 0.0 99.3 0.1 0.00 0.00

Qcalc 0.5 0.0 0.43 0.00

Mn SinerX 0.9 0.1 0.99 0.00

Qcalc 0.1 0.0 0.1 0.0 0.4 0.1 13.0 0.3 0.9 0.0 99.34 0.02

Fe SinerX 0.0 0.0 0.0 0.0 0.2 0.0 10.7 0.7 0.4 0.0 99.01 0.00

Qcalc 0.1 0.0 0.0 0.0

Co SinerX 0.0 0.0 0.0 0.0

Qcalc 0.2 0.0 0.1 0.0 0.4 0.0 0.0 0.0 0.0 0.0 0.01 0.00

Ni SinerX 0.2 0.0 0.2 0.0 0.3 0.1 0.1 0.0 0.0 0.0 0.00 0.00

Qcalc 99.5 0.1 99.6 0.1 99.0 0.1 0.3 0.0 0.1 0.0 0.06 0.01

Cu SinerX 99.4 0.1 99.2 0.1 98.6 0.2 0.5 0.0 0.1 0.0 0.00 0.00

Qcalc 0.2 0.0 0.1 0.0 0.3 0.1 0.0 0.0 0.0 0.0 0.04 0.00

Zn SinerX 0.3 0.1 0.6 0.1 0.6 0.1 0.0 0.1 0.0 0.0

Qcalc

As SinerX

Qcalc 0.3 0.0

Sr SinerX 1.1 0.1

Qcalc

Cd SinerX

Qcalc

Hg SinerX

Qcalc 0.3 0.1 0.0 0.0

Pb SinerX 0.3 0.1 0.1 0.0

Qcalc

Bi SinerX

271 272 Table C.1, Part 1/3: Overview of the studied Cu-containing samples listed according to their commercial identification. Provenance and measurement conditions are given (number of points * seconds, accumulations), for both used lasers. Finally, the identified materials on the basis of the recorded vibrational signature are listed. Description, New identification based on Raman Denomination 785 nm 532 nm provenance spectra 3x30s, - Malachite Cu (CO )(OH) 30acc 2 3 2 10310 Kremer - 3x30s, - Malachite Cu (CO )(OH) natural, extra fine 30acc 2 3 2 1035 Kremer - 3x40s, Chrysocolla (Cu,Al) H Si O (OH) ·zH O, - 2 2 2 5 4 2 natural 30acc malachite Cu2(CO3)(OH)2 573 Reppia (NW 300s, 3x40s, Chrysocolla Allophane (Al O (SiO ) ·2.5-3.0H O) Italy) 70acc 40acc 2 3 2 1.3-2.0 2 145 Libiola (NW 3x300s, 3x200s, Brochantite Cu (SO )(OH) Italy) 30acc 30acc 4 4 6 1160 Libethen 120s, 3x60s, Libethenite Libethenite Cu (PO )(OH) (Slovakia) 70acc 80acc 2 4 3x150s, 15acc, Brochantite Cu (SO )(OH) , Brochantite 555 (Morocco) - 4 4 6 3x60s, antlerite Cu3(SO4)(OH)4 80acc 3x30s, 6x300s, Brochantite, Baccu Locci, 10acc, Brochantite Cu (SO )(OH) , 10acc 4 4 6 pyromorophite (Sardinia, Italy) 3x60s, pyromorphite (Pb (PO ) Cl) - 5 4 3 40acc 3x30s,

Brochantite, Corchia (NW - 10acc, Brochantite Cu4(SO4)(OH)6,

serpierite Italy) - 3x60s, serpierite(Ca(Cu,Zn)4(OH)6(SO4)2·3H2O) 10acc 2x120s, 3x30s, 456 (Voltaggio, 10acc 10acc Langite Cu (SO )(OH) , Langite 4 4 6 NW Italy) 3x300s, 3x120s, brochantite Cu4(SO4)(OH)6 30acc 40acc 3x30s, 103900 Kremer - Malachite Cu (CO )(OH) 20acc 2 3 2 Atacamite 1320 Monte 3x30s, Fucinaia (central - Atacamite Cu Cl(OH) 15acc 2 3 Italy) 1192 Piombino 3x30s, Paratacamite - Clinoatacamite Cu Cl(OH) (central Italy) 20acc 2 3

273 Table C.1, Part 2/3: Overview of the studied Fe-containing samples listed according to their commercial identification. Provenance and measurement conditions are given (number of points * seconds, accumulations), for both used lasers. Finally, the identified materials on the basis of the recorded vibrational signature are listed. New identification based on Denomination Description, provenance 785 nm 532 nm Raman spectra 17410 Kremer (Cyprus) - blue 3x100s, 3x40s, Celadonite green 60acc 20acc 17400 Kremer (Cyprus) – 3x60s, 3x60s, Celadonite, anhydrite green 30acc 80acc 40830 Kremer (France) – 3x30s, 3x60s, PG 7, bismutoferrite, gypsum, natural 30acc 30acc magnesite, calcite, quartz 40821 Kremer (Verona, NE 3x20s, 3x20s, Glauconite/ PG 7, green earth, calcite Italy) - natural 20acc 20acc Celadonite Calcite, kaolinite, carbon 3x30s, 3x30s, 40800 Kremer - natural (amorphous and diamondlike), 40acc 20acc celadonite, rhodonite or fowlerite 3x200s, Green earth (unspecified), calcite, 11110 Kremer (Russia) – 3x60s, 50acc quartz, kaolinite, dolomite, natural 100acc anhydrite 41700 Kremer – ground and 3x40s, 3x30s, PG 7, green earth, norbergite, purged green earth, 30acc 30acc calcite, magnesite, gypsum enhanced with viridian 11100 Kremer (Bavaria) - 3x30s, 3x30s, Glauconite, calcite natural 30acc 20acc 40810 Kremer (Bohemia) – 3x120s, 3x60s, Glauconite natural 20acc 60acc 11000 Kremer (Verona, NE Celadonite 3x30s, 3x20s, Italy) – worse quality of the Anatase, calcite, goethite 30acc 30acc historical pigment 110005 Kremer (Verona, NE 3x60s, 3x60s, Anatase, calcite, celadonite, Italy) – worse quality of the 10acc 80acc altered green earth historical pigment

274 Table C.1, Part 3/3: Overview of the studied synthetic samples listed according to their commercial identification. Provenance and measurement conditions are given (number of points * seconds, accumulations), for both used lasers. Finally, the identified materials on the basis of the recorded vibrational signature are listed. Description, New identification based on Denomination 785 nm 532 nm provenance Raman spectra 3x200s, 3x60s, Copper acetates Verdigris 44450 Kremer 100acc 100acc Cu (CH COO) (OH) ·nH O (0.4mW) x 3 y z 2 3x100s, 1220 Kremer - Copper resinates 60acc CR-KIK1 (1981: Cu salts of abietic 3x60s, verdigris + Abies - Copper resinates acid 60acc pectinata resin) CR-KIK2 (1973: 3x150s, - Fluorescence resinate + oil) 60acc Cu-containing 3x60s, 10 3x60s, wollastonite 10064 Kremer Tridymite (Ca,Cu) Si O acc 15 acc 3 3 9

PG 50 3x150s, 30 3x120s, 44100 Kremer Cobalt titanate Co TiO acc 30 acc 2 4 3x30s, 3x30s, 44200 Chromium oxide Cr2O3 Cr oxide 30acc 30acc 3x10s, 3x30s, 44204 Chromium oxide Cr O 100acc 30acc 2 3 Cr oxide hydrated 3x40s, Chromium oxide and chromium oxide 4425 - 60acc hydrated Cr2O3 and Cr2O3·2H2O 3x300s, 3x120s, PG 7 C H Cl CuN to PG 7 30acc 32 3 13 8 10acc C HCl CuN (0.4mW) 32 15 8 3x120s, 3x60s, PG 36 PG 36 C Br Cl CuN 10acc 100acc 32 6 10 8 3x300s, 3x120s, PG 8 30acc PG 50 C H FeN O Na 40acc 30 18 3 6 (0.4mW)

275 Table C.2, Part 1/3: Overview of the Raman band positions of the identified Cu-containing green materials of interest for cultural heritage, with both the used laser wavelengths. The relative intensity values are given according to (Vandenabeele and Moens 2012). Cu-containing materials -1 Identified Band positions in cm materials 785 nm 532 nm 3382w, 3380vw, 1638vw, 1493m, 1462w (shoulder), Malachite 1368vw-w, 1097-1059w, 820vw, 750w, 720w, 599w, 535w- - (Figure 4.5, a) m, 435m, 351w, 267m, 216m, 180vs, 169s, 154vs 144s, 118m Chrysocolla 3630vs, 2750m-s, 1460vw, 1300w-m, 1073m, 988s-vs, (Figure 4.5, b - 973m, 792w, 678m, 464w-m, 400m, 340m, 209vw and c) 3470w, 1125vw, 1069w, 1051w, 1020s, 1010 (shoulder), Libethenite 975vs, 862vw, 815vw, 650vw, 628w, 585vw, 558w, 466w, 975s, 300vs (Figure 4.6, a) 450 (shoulder), 300m, 280vw, 250vw, 228m, 195m, 154m, 75m 3588vw-w, 3567vw-w, 3403vw-w, 3371vw-w, 3262vw, 973vs, 725w, 620w, 1129vw, 1105vw, 1097vw, 1076vw, 973vs, 910vw-w, 421w, 390w, 240vw-w, Brochantite 871vw, 776vw, 728vw, 620w, 609w, 595w, 507w, 483w, 195w-m, 168vw, 156w-m, (Figure 4.6, b) 448w, 424w, 387w-m, 366vw-w, 319w, 262vw, 243vw-w, 137w-m, 123w, 117w, 227vw, 198w, 188vw, 177vw, 170vw, 157vw, 150vw, 139w, 92w 132vw, 125vw, 119vw, 105vw, 90w-m 3587w, 3578w, 3399vw, 3370vw, 1156vw, 1127vw, Langite (Figure 1094vw, 970vs, 605w, 1093vw, 970vs, 611vw-w, 510vw, 487vw, 450 (shoulder), 4.6, c and d) 428m, 239m, 130w 432m, 242w 3581w, 3489vw-w, 1171vw-w, 1122vw, 1076w, 988vs, Antlerite (Figure 784vw, 748vw, 501w, 484w, 470 (shoulder), 442w, 416m, - 4.6, e) 341vw, 334vw, 296vw, 267w, 248vw-w, 230vw, 216vw, 173vw, 149vw, 143vw, 124w, 108w, 97w, 82vw, 70vw 3616vw, 3570vw, 1168vw, 1132w, 1115(shoulder), 1085vw, Serpierite - 991vs, 651vw, 605vw, 474w (broad, shoulder), 445w, 426w, (Figure 4.7, a) 415w, 338vw, 244vw-w, 218vw 3252vw, 2662vw, 2534vw, 2422vw, 2041w, 1501vw, 1529vw, 1150vw, 1013vw, 971vw, 944m, 918vs, 819m, Pyromorphite - 576vw, 553vw, 411w, 391w, 344vw, 324vw, 178w-m, 157w- m, 106vs, 89s-vs Connellite 3550vw-w, 984m-s, 585w, 524w, 446w, 404vs, 350w, - (Figure 4.7, b) 262vw, 236vw, 192w, 184w, 132w-m 981vs, 854vs 3600-3400w, 979s, 829vs (broad, asymmetric), 613vw, Chalcophyllite (asymmetric); 464, 208, 495m, 454m, 393w-m, 370 (shoulder), 332vw, 305vw, (Figure 4.7, c) 128 (quartz) 272vw, 219vw, 177vw, 118w-m Spangolite 968vs, 615w, 520m, 968vs, 615w, 520s, 410m, 168w-m (Figure 4.7, d) 410w, 168w Atacamite 3436 m to vs, 3346 m to vs, 3328 w to m 977w, 911m, (Figure 4.8, a, b - 846vw, 822w, 591vw, 511m, 452vw, 362-357vw-w, 263w, and c) 237vw, 216vw, 149m, 138m, 119vs, 106w, 65w 3445vs, 3355vs, 3311m, 967m, 929m, 897m, 867vw, 803w, Clinoatacamite - 575vw, 514s, 444w, 424w, 364m, 266vw, 166vw, 142s, (Figure 4.8, d) 119vs 3420vs (broad), 2942vw, 1638vw, 1357vw, 1103w, 982vw, Allophane 859vs 858m, 720vw, 502w-m, 396w, 364w

276 Table C.2, Part 2/3: Overview of the Raman band positions of the identified Fe-containing green materials of interest for cultural heritage, with both the used laser wavelengths. As explained in the text, no intensity values are given for the green earths.

Fe-containing materials

-1 Identified Band positions in cm materials 785 nm 532 nm Bavarian green earth (11100 Calcite (1087, 711) Glauconite (3561, 597, 264) Kremer; Figure 4.9, a)

Bohemian green earth (40810 Glauconite (450) Kremer)

Cyprian blue green earth Celadonite (3560) (17410 Kremer; Figure 4.9, b)

Celadonite (<300, 966, 3560) Cyprian green earth (17400 Kremer) Anhydrite (1017), unidentified compounds (1210, 1290)

PG7 Green earth from France, light Gypsum (1136, 1008, 493, 414), (40830 Kremer; magnesite (1764, 1094, 741, 328, Figure 4.9, c) Bismutoferrite (1536, 1290, 1219, 695, 440, 212), calcite (1086, 712, 283, 156), 347, 332, 145) (Frost et al. 2010) quartz (463), water molecules (3403, 3498), unassigned (2874, 2828, 2220, 2190, 1614, 670)

PG 7 (Scherrer et al. 2009) Green earth from Verona (40821 Kremer) Unspecified green earth (694, 349, 332, 294, Calcite (1086, 712, 283, 156) 147)

Calcite (1087), kaolinite (395, 426, 469, 509)

Green earth light Amorphous carbon and diamond-like (40800 Kremer) structures (1600, 1400, 1270), unspecified Celadonite (609, 550, 467, 341, 253, green earth (673, 604, 547 and 465), kaolinite 164) hydroxyl bending (913), rhodonite or fowlerite (508, 568, 714) (Buzatu and Buzgar 2010) Calcite (1084, 288). Unspecified Green earth from green earth, probably glauconite Russia (11110 Unspecified green earth (698, 398, 356, 266 and 147). Quartz Kremer) (464), Si-O-Si stretch of kaolinite (633). Calcite and dolomite (1084

277 Fe-containing materials

-1 Identified Band positions in cm materials 785 nm 532 nm and 1096). Anhydrite (1126 and 1113)

Anatase (636, 510, 398, 148), calcite (1088) Green earth from Verona (11000 Some bands can be attributed to khandite Kremer) minerals: 797 (hydroxyl vibration of surface Al- Goethite (515, 480, 395, 298, 215) OH) and 280. Unidentified bands, broad and overlapping: (1232, 1293, 1419, 1523, 1690) Anatase (147, 395, 516, 637). Calcite (1087, 280, 355, 713) Green earths (celadonite) signature is also visible, even if the spectrum of TiO2 is Green earth from overwhelming (587, 703) Verona (bulk) (110005 Kremer) Celadonite (959). Altered green earth (589).

PG 7 (Scherrer et al. 2009) Verona green earth (41700 Unspecified green earth (694 and 349). Kremer) Norbergite MgFeCa(SiO4)F(OH) (979 and Calcite (1086, 711, 282) and gypsum 955) (Frost et al. 2007). (1134, 1008, 415) Carbonates: calcite (1086), magnesite (1091 and 738), gypsum (1006).

278 Table C.2, Part 3/3: Overview of the Raman band positions of all the identified synthetic green materials of interest for cultural heritage, with both the used laser wavelengths. The relative intensity values are given according to (Vandenabeele and Moens 2012).

Synthetic origin -1 Identified Band positions in cm materials 785 nm 532 nm 2941vw, 2700vw, 1670vw, 1531vw, 1441vw-w, 1421w, 1365vw, 1095vw, Verdigris 1057vw, 951s, 937w, 878vw, 700w, 686w 948m, 320m, 180vs (Figure 4.10, a) (shoulder), 633vw, 551vw, 321vs, 301 (shoulder) 267vw, 253vw, 232w, 225w, 215w, 187w, 179vw, 144vw, 125vw, 107m Cu resinate 2797 (broad), 1610vs, 1445w, 1300w-m, (Figure 4.10, b) 1201w-m, 708m, 435w, 229w, 124w-m Cu resinate 514nm: 2933vs, 2873m (shoulder), 1619m, 1461w, 1201w, 707vw-w (Figure 4.13, c) 2933vs (broad), 2873m (shoulder), 1610m (broad, asymmetric), 1446w-m, 1407w, Cu resinate 1301vw-w, 1200w, 1072vw, 1051vw, - (Figure 4.13, d) 978vw, 933vw, 887vw, 834vw, 774vw, 708w, 614vw (broad), 441vw-w, 367vw, 312vw, 221vw, 119m, 75vw Trydimite 1075 w-m (broad), 789w, 433m, 355vs, 1075w-m (broad), 789w, 550vw, 434m, (Figure 4.10, c 302s-vs, 210m 355vs, m-s, 210m, 141w, 112w and d) 710 vs, 645w, 618w, 520w-m, 460w-m, Co titanate 709vs, 520m, 460m, 330vw, 236vw, 380vw, 330w, 236vw, 175w, 150w-m, (Figure 4.10, e) 175s-vs, 150w (shoulder), 119w, 80vw 115vw Cr oxide (Figure 613m, 555vs, 351m, 299w 597w-m, 542vs, 391vw, 340w, 304w 4.11, a) Cr oxide hydrated - 624vw, 583w, 552vw-w, 487vs, 266w (Figure 4.11, b, c and d) 3059vw, 2810vw, 2632vw, 2580vw, 1551m 1535vs, 1508vw (shoulder), 1387vw, (shoulder), 1532m, 1498m, 1475w, 1434vw, 1334m, 1280m, 1207m, 1082vw-w, PG 7 (Figure 1379w, 1330w, 1277w-m, 1192w, 1079w, 977vw, 951vw, 814vw, 770m, 740s, 4.9, d) 972vw, 814w, 769vw, 686vs, 642vw, 514vw, 689vs, 640vw, 345w, 331vw, 290vw-w, 506vw, 462vw, 368vw, 346vw, 332vw, 223vw, 197vw, 165vw, 147vw, 99m 233vw-w, 163vw, 146vw 1523m-s, 1427vw, 1368vw, 1315m, 2597vw, 1530m, 1489m, 1426vw, 1376m, PG 36 (Figure 1265m, 1186w, 1053vw, 957vw-w, 748vs, 1317w (shoulder), 1272m, 1186w, 1166w, 4.12, a, b and 662m, 571vw, 534vw, 514vw, 479vw, 1057w, 966vw, 914vw, 771w, 661vs, c) 360vw (shoulder), 330w, 276vw, 258vw, 568vw, 533vw, 452vw, 326vw, 222vw, 223w, 184vw, 155vw, 92vw 167vw-w, 115vw-w 1590w, 1550vw-w, 1511w-m, 1469w, 1589w, 1511w, 1468w, 1446w, 1417w, 1444w, 1416vw, 1355-1345m, 1324w, 1357m, 1322w, 1208w, 1085-1077m, 1302vw, 1254w-m, 1147w, 1076w-m, PG 8 (Figure 1056m, 1021m, 879vs, 752vs, 734m 1056m, 1021m, 878m, 752vs, 668s, 4.12, d) (shoulder), 688w, 671w-m, 636s, 617s, 633s, 614m, 533s, 494m, 470s, 447s, 536m, 495w, 465m, 441m, 405w, 381w-m, 380m, 351m, 313m, 223w, 196w, 151w, 354w, 314m, 285w, 222w, 151m, 110vs 119w

279 280 Table D.1: Raman bands of carbonaceous materials.

OTHER -1 BAND ASSIGNMENT POSITION (CM ) SYMBOLS

G G1 E2g2, E2g From 1550 to 1620

A1g; edge effects as oxides or C=C bonds; in plane defects and heteroatoms; sp3-sp2 From 1301-1317 (for a NIR/IR D1 D carbon bonds; volatile compounds, excitation to 1390 polyenes, ions 2 E’2g or E2g or oxidised sp carbons; non sandwiched graphene layers; defects as D2 D’, G2 From 1599 to 1635 imperfect graphite or disordered E2g;

splitting of degenerated E2g; E1u Tetrahedral carbons (defects outside the carbon plane); organic molecules, fragments and functional groups of soot; D3 D”, A oxygen on the surface of graphite; sp2 From 1440 to 1550 carbons; interstitial defects; amorphous

carbons; methylene group vibrations; ν3 of trans polyenic molecules Defects; soot features, like mixed sp2-sp3 bonds or polyenic structures; char and coal tar; diamondlike carbon in anthracite; sp3 From 1050 with deep UV excitation to D4 I carbons; nanocrystalline diamond or trans 1127 to 1275

polyene ν1; defects related to oxygen rich precursors as wood FULLERENES Phototransformation/oxidation 1459-1469 and 930-970

RHOMBOHEDRAL B2 1400 GRAPHITE 2g

281 Table.2: Carbon containing black Kremer pigments studied in Paragraph 4.3.2. Literature is not always consistent about materials’ composition: here are briefly reported the descriptions found on Kremer’s website, references [K] and (Winter 1983) [W]. USED PIGMENT PRODUCER’S DESCRIPTION LITERATURE FORM ORIGIN LASER

Black earth (see [K]: black chalk. [W]: ± ordered graphite, quartz, 785 nm/ Figure 4.14 c; 11280 From Andalusia. Crystalline. Mineral iron oxides, other minerals. 532 nm Figure 4.16)

[K]: coke of ivory, 60% is inorganic: (Ca, Mg) Genuine. Ivory wastes are heated to 800 Ivory black/ phosphate and carbonate, dhallite, the rest is 12000/ °C in closed iron vessels: pure animal 785 nm/ Ivory black pieces collagen and lipids. [W]: coke from collagen, Coke. Animal 120005 charcoal containing phosphates and 532 nm (see Figure 4.18) carbonate-hydroxyapatite constitute the possibly soluble salts. inorganic fraction.

Peach black Declared soot/ 12010 Genuine peach-stone soot. [K], [W]: chars. Vegetable 785 nm (see Figure 4.20 a) expected chars.

[K]: synonym of vine black. Chars of woody Declared Grape black 12015 Genuine grape-seeds soot. materials from vine. [W]: chars of woody soot/expected Vegetable 785 nm (see Figure 4.20 c) materials from vine. chars.

Declared Cherry black 12020 Genuine cherry-stone soot. [W]: chars. soot/expected Vegetable 785 nm (see Figure 4.20 b) chars.

[K]: soot produced from resin, brushwood, pine Produced from the tannic acid of oak bark Declared humic Atramentum chips and vine dregs [Vitruvius]. Other author Organic 12030 in reaction with iron salts. Different from salts/expected 532 nm (see Figure 4.22 d) cite the term for soot blacks, writing inks. Black matter carbon-blacks and iron oxides. soot. dye for leather. Can refer also to ivory black.

Shungite [K]: amorphous/nanocrystalline carbon from 12040 <80µm Crystalline. Mineral 785 nm (see Figure 4.17 b) Russia (Shunga), used for icons.

282 USED PIGMENT PRODUCER’S DESCRIPTION LITERATURE FORM ORIGIN LASER [K]: soot of beechwood, collected close to the flame and washed before use. Contains as well Wood (beechwood, Fagus sp.) soot of Bistre flame carbon, chars, cokes and tarry materials. brownish shade, or coal soot, that was Vegetable, (see Figure 4.21 a 12100 Same as bituminous coal, atramentum. [W]: Flame carbon. 532 nm washed and treated. Only used in smoke and b) flame carbon prepared from beechwood soot. It watercolor paint. also contains chars, possibly some coke and uncarbonized tarry materials. Ink of cuttlefish, obtained by drying the whole ink sac of Sepiidae or just its liquid content. The hue is brownish. 78% melanin, 10% CaCO , 7% MgCO , 2% 3 3 Animal – 785 nm/ Sepia alkaline sulphates and chlorides, 0.8% [K]: pigment extracted from Cephalopoda 12400 Proteins. organic 532 nm (see Figure 4.23) other compounds. Melanin structure is still containing eumelanin (indolic units). matter unknown. It was used in watercolours. The genuine Sepia ink is not lightfast and nowadays “Sepia” indicates synthetic materials.

Black chalk [K], [W]: ± ordered graphite, quartz, iron oxides 12451 Shale rich in carbon, from France. Crystalline. Mineral 785 nm (see Figure 4.17 c) (same as black earth).

Kasslerbraun in the German website. [K]: the term can refer to calcined brown ochres Lignit coal from a mine near Cassel, containing iron oxides and organic matter in the Organic Van Dyck brown Crystalline/humic 41000 Germany. It’s composition may vary, form of humus or bituminous matter. This is a rock/organic 532 nm (see Figure 4.22 a) salts. since it is a natural product, but consists general term referring to humic earths and matter mainly of humic acids and iron oxides. associated pigments.

[K]: humic earth associated with Van Dyck Cassel earth Saftbraun in the German website. Sodium Organic 41050 brown, containing bitumen. If calcined gives a Humic salts. 532 nm (see Figure 4.22 b) salts of humic acids. matter darker color.

283 USED PIGMENT PRODUCER’S DESCRIPTION LITERATURE FORM ORIGIN LASER

Product of bones carbonization above [K], [W]: coke from collagen, hydroxyapatite 400 °C but below 800 °C. Carbon Ca (OH)(PO ) . The original structure is lost Bone black 47100 5 4 3 Coke. Animal 785 nm particles are incorporated in the inorganic during carbonization: conversion to graphite is bone matrix. possible.

Nearly pure amorphous carbon: [K]: flame carbon. Industrially produced since Furnace black condensed smoke of a luminous flame the 1920s. [W]: carbon-black belonging to soots 47250 Flame carbon. Smoke 532 nm (see Figure 4.21 c) (oil, tar, pitch, resin) and still contains and smokes. unburnt oil. It has a bluish shade.

[K]: by-product from coal-tar and lamp black. Asphaltum is natural bitumen (solid or viscous) Absent from Asphaltum or artificial. 1 (4): mixture of organic and 47600 From USA. Winter’s 532 nm (see Figure 4.22 c) inorganic materials. The organic fraction description. (bitumen) acts as the medium, while the inorganic components are extenders.

Graphite Graphite powder, < 75 μm. Minimum 80% [K]: pure carbon that may contain silica, iron (see Figure 4.15 47710 of carbon, the rest is ashes and 0.8% of oxides and clays. [W]: ordered carbon of Crystalline. Mineral 785 nm and 4.17 a) sulfur. mineral origin.

[K]: chars, non graphitizable carbon. After water evaporation, at 280 °C the wood breaks down producing charcoal plus vapour, acetic Charcoal Carbon pyrolize of wood carbonization. 47800 acid, methanol, tars and non-condensable gas. Char. Vegetable 785 nm (see Figure 4.19) 78-82% of carbon. The reaction is exothermic and reaches 400 °C. If heat is provided, tar decomposes. [W]: chars of vegetable origin.

284 *Coccato, A.; Moens, L.; Vandenabeele, P., 2017. On the stability of mediaeval inorganic pigments: a literature review of the effect of climate, material selection, biological activity, analysis and conservation treatments. Heritage Science 5.1: 12.

Coccato, A.; Costa, M.; Rousaki, A.; Clist, B.; Karklins, K.; Bostoen, K.; Manhita, A.; Cardoso, A.; Barrocas Dias, C.; Candeias, A.; Moens, L.; Mirão, J.; Vandenabeele, P., 2017. Micro-Raman spectroscopy and complementary techniques (hXRF, VP-SEM-EDS, μ-FTIR, Py-GC/MS) applied to the study of beads from the Kongo Kingdom (Democratic Republic of the Congo). Journal of Raman Spectroscopy. doi: 10.1002/jrs.5106.

*Coccato, A., Vekemans, B., Vincze, L., Moens, L. and Vandenabeele, P., 2016. Pigment particles analysis with a total reflection X-ray fluorescence spectrometer: study of influence of instrumental parameters. Applied Physics A, 122(12), p.1051.

Barone, G., Bersani, D., Coccato, A., Lauwers, D., Mazzoleni, P., Raneri, S., Vandenabeele, P., Manzini, D., Agostino, G. and Neri, N.F., 2016. Nondestructive Raman investigation on wall paintings at Sala Vaccarini in Catania (Sicily). Applied Physics A, 122(9), p.838.

*Coccato, A., Bersani, D., Coudray, A., Sanyova, J., Moens, L. and Vandenabeele, P., 2016. Raman spectroscopy of green minerals and reaction products with an application in Cultural Heritage research. Journal of Raman Spectroscopy, 47, p. 1429.

285 Lauwers, D., Candeias, A., Coccato, A., Mirao, J., Moens, L. and Vandenabeele, P., 2016. Evaluation of portable Raman spectroscopy and handheld X-ray fluorescence analysis (hXRF) for the direct analysis of glyptics. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 157, p.146.

Rousaki, A., Coccato, A., Verhaeghe, C., Clist, B.O., Bostoen, K., Vandenabeele, P. and Moens, L., 2016. Combined Spectroscopic Analysis of Beads from the Tombs of Kindoki, Lower Congo Province (Democratic Republic of the Congo). Applied spectroscopy, 70(1), p.76.

*Coccato, A., Jehlicka, J., Moens, L. and Vandenabeele, P., 2015. Raman spectroscopy for the investigation of carbon‐based black pigments. Journal of Raman Spectroscopy, 46(10), p.1003.

Wood Science and Technology II - Microclimates for Panel Paintings, Maastricht, The Netherlands, 2016

Thickett, D., Odlyha, M., Coccato, A., Grontoft, T., Bonaduce, I., Vandenabeele, P., Colombini M.P. Advances in Microclimate Frame Research.

2nd International Conference on Innovation in Art (InArt 2016), Ghent, Belgium, 2016

Coccato, A.; Rousaki, A.; Costa, M.; Clist, B.; Bostoen, K.; Vandenabeele, P. Combined historical, physical anthropology, archaeological, and archaeometrical approaches to understand glass beads from Kongo Central province, Democratic Republic of the Congo (DRC).

8th International Congress on the Application of Raman Spectroscopy in Art and Archaeology (RAA 2015), Wroclaw, Poland, 2015

Coccato, A.; Rousaki, A.; Verhaeghe, C.; Clist, B.; Bostoen, K.; Vandenabeele, P.; Moens, L. Combined spectroscopic analysis of beads from the tombs of Kindoki, Mbanza Nsundi, Lower Congo.

286 XI International GeoRaman Conference (GeoRaman XI 2014), St. Louis, USA, 2014

Vandenabeele, P.; Van Pevenage, J.; Lauwers, D.; Coccato, A.; Rousaki, A.; Lycke, S.; Moens, L. In situ Raman analysis: the ultimate solution for archaeometry?

4th International Congress Chemistry for Cultural Heritage, Brussels, Belgium, 2016.

Coccato, A.; Moens, L. Vandenabeele, P.; On the sensitivity of pigments towards climatic factors and various pollutants: a review.

Rosier, F.; Dubois, H.; Van der Snickt, G.; Sanyova, J.; Coudray, A.; Glaude, C.; Janssens, K.; Lauwers, D.; Coccato, A.; Vandenabeele, P. Identification of restorations on the Cumean Sibyl of the brothers Van Eyck’s Ghent Altarpiece (1432)

Society of Africanist Archaeologists (SAfA) 23rd Biennial meeting, Toulouse, France, 2016.

Costa, M.; Coccato, A.; Rousaki, A.; Verhaeghe, C.; Clist, B.; Bostoen, K.; Moens, L.; Mirão, J.; Vandenabeele, P. Analysis of beads from Congo by means of a non-destructive multi-analytical approach.

XII International GeoRaman Conference (GeoRaman XII), Novosibirsk, Russia, 2016

Coccato, A.; Rousaki, A.; Costa, M.; Clist, B.; Bostoen, K.; Moens, L.; Vandenabeele, P. Raman (532 and 785 nm) study of glass beads from the Kongo Central province, Democratic Republic of the Congo (DRC). Bersani, D., Coccato, A., Lauwers, D., Vandenabeele, P., Barone, G., Raneri, S., Mazzoleni, P., Quartieri, S., Sabatino, G., Manzini, D. Raman analysis of paintings: comparison between different excitation wavelengths in mobile systems.

2nd International Conference on Innovation in Art (InArt 2016), Ghent, Belgium, 2016

Coccato, A.; Vekemans, B.; Vincze, L.; Moens, L.; Vandenabeele, P. Optimization of Total Reflection X-ray Fluorescence analysis of pigments.

287 Gomes, S.; Lorena, M.; Vandenabeele, P.; Lauwers, D.; Coccato, A.; Valadas, S.; Candeias, A. Green colour in the 16th century Portuguese paintings of the Funchal’s Cathedral altarpiece.

Barone, G.; Bersani, D.; Coccato, A.; Lauwers, D.; Mazzoleni, P.; Raneri, S.; Vandenabeele, P.; Manzini, D.; Agostino, G.; Neri, N.F. Non-destructive Raman investigations on wall paintings at Sala Vaccarini in Catania (Sicily).

8th International Congress on the Application of Raman Spectroscopy in Art and Archaeology (RAA 2015), Wroclaw, Poland, 2015

Coccato, A.; Moens, L. Vandenabeele, P. Raman database of green pigments.

Rousaki, A.; Coccato, A.; Bellelli, C.; Carballido Calatayud, M.; Palacios, O.; Custo, G.; Moens, L.; Vandenabeele, P.; Vázquez, C. Micro-Raman analysis of prehistoric rock art in Patagonia (Argentina).

Non-destructive and microanalytical techniques in art and cultural heritage (TECHNART 2015), Catania, Italy, 2015

Coccato, A.; Rousaki, A.; Moens, L. Vandenabeele, P. Optimization of TXRF for the analysis of paint samples.

Lauwers, D.; Coccato, A.; Mirao, J.; Vandenabeele, P.; Candeias, A. Evaluation of portable Raman spectroscopy and handheld X-ray fluorescence spectroscopy for the analysis of glyptics.

XI International GeoRaman Conference (GeoRaman XI 2014), St. Louis, USA, 2014

Coccato, A.; Jehlicka, J.; Moens, L. Vandenabeele, P. Raman spectroscopy for the investigation of carbon-based black pigments.

Chemistry Conference for Young Scientists (ChemCYS 2014), Blankenberge, Belgium, 2014

Coccato, A.; Lauwers, D.; Moens, L.; Vandenabeele, P. The multidisciplinary study of the Ghent altarpiece: analytical chemistry and art.

Lauwers, D.; Coccato, A.; Candeias, A.; Mirão, J.; Moens, L.; Vandenabeele, P. Archaeometrical study of the main altarpiece of the cathedral of Funchal (Madeira).

MEMORI Conference, Madrid, Spain, 2013

288 Coccato, A.; De Laet, N.; Lycke, S.; Van Pevenage, J.; Moens, L.; Vandenabeele, P. Raman study of pigment degradation due to acetic acid vapours.

7th International Congress on the Application of Raman Spectroscopy in Art and Archaeology (RAA 2013), Ljubljana, Slovenia, 2013

Coccato, A.; Moens, L. Vandenabeele, P. Raman spectroscopic investigation of black pigments.

Coccato, A.; De Laet, N.; Lycke, S.; Van Pevenage, J.; Moens, L.; Vandenabeele, P. Raman study of pigment degradation due to acetic acid vapours.

“Analysis and Abatement of water pollution” (Master of Science in Environmental Sanitation, Ghent University, Ghent, Belgium. Academic year 2013-2014.)

“Raman spectroscopy: an ideal tool in the recycle business?” (Bachelor in Chemistry, Ghent University, Ghent, Belgium. Academic years 2013-2014 and 2015•2016.)

“Optimization of sample pretreatment procedures for TXRF analysis for archaeometrical applications: a case study on Yustyd dwelling soil samples.” Thesis of Joyce Vaerewyck (Professional bachelor in Chemistry, HoGent, Ghent, Belgium. Academic year 2013-2014.)

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