Chemical Composition and Micro Morphology of Golden Laminae in the Wall Painting “La Maestà” by Simone Martini: a Study By

applied sciences

Article Chemical Composition and Micro Morphology of Golden Laminae in the Wall Painting “La Maestà” by Simone Martini: A Study by Optical Microscopy, XRD, FESEM-EDS and ToF-SIMS

Andrea Atrei 1,* , Andrea Scala 2, Marco Giamello 2, Marianna Uva 3, Riccardo Maria Pulselli 4 and Nadia Marchettini 5

1 Dipartimento di Biotecnologie, Chimica e Farmacia, Università di Siena, 53100 Siena, Italy 2 Dipartimento di Scienze Fisiche, della Terra e dell’Ambiente, Università di Siena, 53100 Siena, Italy 3 CREA (Centro Ricerca Energia e Ambiente), Colle di Val d’Elsa, 53034 Siena, Italy 4 Santa Chiara Lab, Università di Siena, 53100 Siena, Italy 5 Dipartimento di Scienze Fisiche, della Terra e dell’Ambiente, Università di Siena, 53100 Siena, Italy * Correspondence: [email protected]

Received: 16 July 2019; Accepted: 13 August 2019; Published: 21 August 2019

Abstract: The chemical characterization of gilding decorations in works of art is fundamental in order to elucidate the techniques and materials used by the artists. In the present work we investigated by a combination of bulk and surface sensitive methods the composition and micro stratigraphy of the gilding laminae in the wall painting of the 14th century “La Maestà”, the masterpiece of Simone Martini. The aim of this study was to determine the composition of the gilding leaves and of the adhesive organic materials used to glue the metallic decorations to the wall painting. Due to the altered state of the samples we could not univocally identify the nature of the adhesive materials. Time of ﬂight secondary ion mass spectrometry measurements showed that the gilding layers consisted of a gold leaf which was laid either directly on a preparation layer or on a tin lamina. The high sensitivity of ToF-SIMS and its spatial resolution allowed us to ﬁnd traces of silver in the gold leaves and in the tin laminae which were not revealed by energy dispersive X-ray analysis.

Keywords: cultural heritage; surface analysis; wall paintings; gilding; FESEM-EDX; ToF-SIMS

1. Introduction For gilding, that is the decoration of ancient works of art (wooden panels, wall paintings, metallic objects), pure gold, gold alloys (with copper or silver) as well as composite laminae of gold and silver (the so called “oro di metà”) were used [1–6]. In some cases, the gold leaves were laid on a thicker tin lamina [1,2]. Gold leaves were prepared by hammering gold coins until extremely thin foils (typical thickness: 1 to 5 microns) were obtained [7]. The most commonly used methods for applying gold decorations on wall paintings were the “a guazzo” (water gilding) or the “a missione” (mordant gilding) techniques. In the “a guazzo” technique, the gold or composite metallic leaves were stuck to the surface by means of a protein-based binder mixed with “bole”, a mixture of clays, iron oxides and hydroxides. In the “a missione” technique, siccative oils (typically linseed oil), mixed with lead compounds as drying agents, were used to glue the metal foils [1,4]. The chemical characterization of the gilding layers (pure gold, alloys or composite laminae) and of the adhesives used to glue the metallic leaves are essential in order to get information about the materials and techniques employed by the artist. The chemical investigation of the gilding decorations, in walls as well as in wooden panel paintings, is challenging since both the inorganic and the organic components must be analyzed.

Appl. Sci. 2019, 9, 3452; doi:10.3390/app9173452 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, x 3452 FOR PEER REVIEW 22 of of 89 components must be analyzed. For this purpose, a combination of several microscopy and Forspectroscopy this purpose, techniques a combination is needed. of several In the microscopy present work, and spectroscopy we investigated techniques by optical is needed. microscopy In the present(OM), field work, emission we investigated scanning by electron optical microscopymicroscopy (OM), (FESEM) field coupled emission with scanning energy electron dispersive microscopy X-ray (FESEM)analysis (EDX), coupled micro with X-ray energy diffraction dispersive (XRD) X-ray and analysis time of (EDX), flight micro secondary X-ray ion diff massraction spectrometry (XRD) and time(ToF-SIMS) of flight microsamples secondary ion of mass the gilding spectrometry decorations (ToF-SIMS) from the microsamples wall painting of theof the gilding beginning decorations of the from14th century, the wall painting“La Maestà” of the by beginning Simone Martini of the 14th (Figure century, 1). ToF-SIMS “La Maest isà” a by particularly Simone Martini suitable (Figure tool 1to). ToF-SIMSinvestigate is samples a particularly from works suitable of toolart [8–11]. to investigate This mass samples spectrometry, from works surf oface art sensitive [8–11]. Thistechnique mass spectrometry,allows one to surface analyze sensitive simultaneously, technique allowson the one same to analyze sample, simultaneously, both the inorganic on the sameand sample,organic bothcomponents. the inorganic Moreover, and organic images components. of the distribution Moreover, of imagessecondary of the ions distribution over the ofanalyzed secondary surface ions over(chemical the analyzed maps) can surface be obtained. (chemical The maps) aim canof this be obtained. study was The to aimdetermine of this studythe composition was to determine of the thegilding composition leaves and of theof the gilding materials leaves used and ofto the glue materials the metallic used tolaminae glue the to metallic the wall laminae painting to theand wall the paintingpreparation and layer the preparation over which layer the overgilding which leaves the gildingwere laid. leaves The were combined laid. The use combined of these usetechniques of these techniquespermitted permittedus to determine us to determine the stratigraphy the stratigraphy and composition and composition of the of thevarious various layers layers of of the microsamples. In particular, the high sensitivity and lateral resolution of ToF-SIMS were exploited to detect metalmetal impurities impurities in thein the gilding gilding decorations. decorations The.detection The detection of trace of elements trace elements offers the possibilityoffers the topossibility identify theto identify provenance the provenance of materials of and materials the techniques and the techniques used to make used a work to make of art. a work of art.

Figure 1. Wall painting “La MaestMaestà”à” by SimoneSimone Martini.Martini. The sampling points are marked with yellow and red red circles. circles. The The results results of ofthe the analysis analysis presented presented here hereare for are sample for sample A (halo A of (halo St. Taddeus, of St. Taddeus, middle middleright of the right painting) of the painting) and sample and B sample(decoration B (decoration of the canopy, of the top canopy, right of top the rightpainting). of the painting).

2. Materials and Methods 2. Materials and Methods 2.1. OM Observations 2.1. OM Observations Thin sections of microfragments cut perpendicularly to the surface were observed with a polarized Thin sections of microfragments cut perpendicularly to the surface were observed with a light microscope (LEICA DMRXP) equipped with HP PL Fluotar objective lenses, a digital reﬂex polarized light microscope (LEICA DMRXP) equipped with HP PL Fluotar objective lenses, a digital camera and relative software for image acquisition, transmitted and reﬂected light sources. reflex camera and relative software for image acquisition, transmitted and reflected light sources. Appl. Sci. 2019, 9, 3452 3 of 8

2.2. FESEM-EDX Measurements A ﬁeld emission scanning electron microscope (FESEM) SIGMA VP Zeiss, Germany equipped with a system for EDX spectroscopy (Oxford Instruments, INCA x-sight 7378, Abington, UK) was used. Images were recorded by collecting backscattered electrons. EDX spectra were measured with an energy of the primary electron beam of 20 keV. Samples were coated with graphite (Balt-tec SCD 050 sputter-coater, Scotia, NY, USA) in order to prevent sample charging.

2.3. Micro XRD Measurements A powder X-ray diﬀractometer with Bragg–Brentano geometry (Philips X’Pert PRO PW 3040/60 with X’Celerator PW 3015 detector, Almelo, The Netherlands) equipped with a multiple purpose sample stage (MPSS) for massive specimens and mono-capillary collimator tube was used. This instrument allows investigating very small samples and/or elliptical areas on thin sections with minor axis length of 100 µm. Diﬀractograms were collected with the Cu Kα radiation, in a 3–70◦ 2θ range and a step of 0.017◦.

2.4. ToF-SIMS Measurements A TRIFT III instrument (Physical Electronics, Chanhassen, MN, USA) with a gold liquid metal ion gun as the primary ion source was used for the ToF-SIMS measurements. The primary ion beam energy was 22 keV and a typical ion current of 600 pA, respectively. ToF-SIMS spectra were collected selecting Au+ primary ions. The spectra were measured in bounced mode with a typical mass resolution, m/∆m, around 2000. The spectra were recorded within static conditions keeping the dose of primary ions below 1012 ions/cm2. An electron gun was used to neutralize the positive charging of the samples. + + + + The m/z scale was calibrated using the mass peaks of CH3 , Na ,C2H3 and C3H5 .

2.5. Sampling Points and Sample Preparation “La Maestà” by Simone Martini is a wall painting, covering an area of ca. 110 m2, on the northern east wall of the “Sala del Mappamondo” in the city hall in Siena. This artwork, completed in 1315, was subsequently modified by Simone Martini in 1321. The “Maestà”, the masterpiece of Simone Martini, has been studied by art historians ever since for the style, chronology, commissioning and political meanings [12,13]. Samples were collected in the points marked by circles in Figure1 during the cleaning campaign of the wall painting in 2017, in agreement with the superintendency and the restorer. The results shown here are for the samples of type A and B, taken from the regions marked with red circles in Figure1. The samples are representative of two types of gilding: gold leaf on the preparation layer (sample A) and gold leaf laid on a thicker tin lamina (sample B). The gold leaf directly applied on the preparation layer was used for the finer details, like the halos of saints, whereas decorations with larger thickness, like the canopy on the top right part of the painting, were made by using gold leaf on a tin lamina. The samples embedded in epoxy resin (Epofix) were cut perpendicularly to the sequence of layers to obtain two specular parts of each sample. One part was glued on a glass slide and polished until a thickness of 30 µm was reached for the OM observation as thin section. The OM of thin sections allows to get information about the thickness and colours of the layers as well as the mineralogical characterization of the phases presents [14]. The other part of the sample was polished and cut by means of a microtome (Ultratome, LKB, Bromma, Sweden) to obtain as much as possible flat and free of contamination surfaces for the ToF-SIMS measurements [15].

3. Results The OM and FESEM images of the sample A are shown in Figure2a. The OM image of the thin section shows a preparation layer with an average thickness of ca. 150 µm. The grains in this layer were identiﬁed as minium (Pb3O4) and yellow ochres on the basis of their appearance with transmitted Appl. Sci. 2019, 9, 3452 4 of 8

Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 9 polarized light. The results of the OM observations were conﬁrmed by the EDX results. On top of this layer,lamina which (with works an average as the thickness “bole” layer below in the5 µm) typical can been “a guazzo” seen. This technique, lamina theis visible gold lamina as a bright (with strip an averageat one edge thickness of the below sample 5 µ m)in canthe beenFESEM seen. image This lamina(Figure is2b). visible The asthickness a bright stripof the at metallic one edge lamina of the sampleranges from in the 2 FESEMto 5 µm. image For other (Figure samples2b). The of this thickness type (i.e., of the a gold metallic leaf on lamina a preparation ranges from layer), 2 to FESEM 5 µm. Forimages other show samples that ofonly this some type residues (i.e., a gold of the leaf gold on ale preparationaf are present. layer), The FESEMsignals of images other showmetals, that such only as somecopper residues or silver, of the were gold not leaf detectable are present. above The the signals noise of level other in metals, the EDX such spectra as copper measured or silver, on werethe gold not detectableleaf (Figure above 2b). theHence, noise the level EDX in theanalysis EDXspectra rules out measured that the on gilding the gold leaf leaf is (Figure a copper-gold2b). Hence, or thecopper-silver EDX analysis alloy rules or a out composite that the gildinglamina leafof gold is a copper-goldand silver foils or copper-silveras recently found alloy in or other a composite gilding laminadecorations of gold [6]. and The silver composition foils as recently of the preparation found in other layer gilding as determined decorations by EDX [6]. The (Table composition 1) is similar of theto that preparation of the preparation layer as determined layer for the by metal EDX decorati (Table1)ons is similar in the wall to that paintings of the preparationby Giotto in layerthe Scrovegni for the metalChapel decorations [2]. in the wall paintings by Giotto in the Scrovegni Chapel [2].

(a) (b)

FigureFigure 2.2. (a) Optical Optical microscopy microscopy (OM) (OM) image image of of the the thin thin section section of ofsample sample A, A,reflected reflected light, light, dark dark field. field. (b) (Fieldb) Field emission emission scanning scanning electron electron microscopy microscopy (FESEM (FESEM)) image image of sample of sample A and A energy and energy dispersive dispersive X-ray X-rayanalysis analysis (EDX) (EDX)spectra spectra measured measured on the gold on the leaf gold (spot leaf analysis) (spot analysis) and on the and preparation on the preparation layer (analyzed layer (analyzedarea 50 × 50 area µm 502). The50 µmetallicm2). The leaf metallic is visible leaf as is bright visible regi ason bright on the region preparation on the preparation layer. In the layer. preparation In the × preparationlayer grains layerof various grains materials of various appear materials with different appear withlevels di offf grey.erent levels of grey.

Table 1. Elemental analysis of the preparation layer of sample A as determined by EDX. The analysis Table 1. Elemental analysis of the preparation layer of sample A as determined by EDX. The analysis was performed on an area large enough to represent an average composition. was performed on an area large enough to represent an average composition. Element Atomic % Element Atomic % FeFe 28 28 Si 26 Si 26 Pb 16 AlPb 16 13 CaAl 13 9 CuCa 9 6 KCu 6 2 K 2 In the preparation layer relatively high concentrations of iron and lead were found. These results are consistentIn the preparation with the micro layer XRD relatively measurements high concentrations which show of the iron presence and lead of minium were found. (Pb3O 4These) and whiteresults lead are (2 consistent PbCO3 Pb(OH) with 2the) (Figure micro3). XRD The peaksmeasurements of yellow ochreswhich areshow too the weak presence to be detectable of minium in the(Pb di3Oﬀ4)ractogram. and white Copper,lead (2 PbCO which3 Pb(OH) was detected2) (Figure by EDX, 3). The is probablypeaks of presentyellow ochres as copper are resin,too weak a drying to be agentdetectable used in the “adiffractogram. missione” technique Copper, [which1]. was detected by EDX, is probably present as copper resin, a drying agent used in the “a missione” technique [1]. Appl. Sci. 2019, 9, 3452 5 of 8 Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 9

Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 9

Figure 3. DiffractogramDiﬀractogram ofof thethe preparationpreparation layerlayer ofof samplesample A.A. TheThe peakspeaksof ofminium minium (Pb (Pb33OO44)) and whitewhite

leadlead (2PbCO(2PbCO33 Pb(OH)2)) are are marked marked by by empty empty and and fi ﬁlledlled circles, respectively. Figure 3. Diffractogram of the preparation layer of sample A. The peaks of minium (Pb3O4) and white 2 The wholelead (2PbCO ToF-SIMS3 Pb(OH) spectrum2) are marked of by sample empty and A fi (notlled circles, shown), respectively. measured on a 500 500 µm 2 area, The whole ToF-SIMS spectrum of sample A (not shown), measured on a 500 ×× 500 µm area, including both the gold leaf and the preparation layer, exhibits peaks of hydrocarbon fragments having including both the gold leaf and the preparation layer, exhibits peaks of hydrocarbon 2fragments The whole ToF-SIMS spectrum of sample A (not shown), measured on +a 500 × 500 µm area,+ mhaving/z values m/z close values to thoseclose ofto thethose elements of the elements detected bydetected EDX. Forby EDX. instance, For Cinstance,2H3 (m /Cz227.02)H3+ (m and/z 27.02) Al (andm/z including+ both the gold leaf +and the preparation +layer, exhibits peaks of+ hydrocarbon fragments 26.98),+ C H (m/z 39.02)+ and K (m/z 38.96),+ C H (m/z 40.03)+ and Ca (m/z 39.96).+ These peaks of Al (mhaving/z 326.98), 3m/z values C3H3 close (m/z to 39.02) those ofand the K elements (m/z 338.96), detected4 C 3byH 4EDX. (m/ Forz 40.03) instance, and C 2CaH3+ ((mm//zz 27.02) 39.96). and These hydrocarbonpeaksAl of+ (hydrocarbonm/z ion26.98), fragments C3H ion3+ ( mfragments are/z 39.02) mainly and are due K mainly+ to(m surface/z 38.96), due contaminationto C 3surfaceH4+ (m/z contamination 40.03) of theand sample Ca+ (ofm / duringthez 39.96). sample cutting These during and handling.cuttingpeaks and ToF-SIMSof handling. hydrocarbon is ToF-SIMS a surface ion fragments sensitive is a surface are technique, mainly sensitive due thus to technique, surface even relativelycontamination thus even small relatively of amountsthe sample small of during impurities amounts + inof theimpuritiescutting outermost and in handling. regionthe outermost of ToF-SIMS a sample region is are a surface detectable.of a sample sensitive The are technique, peaks detectable. of the thus isotopesThe even peaks relatively of Cuof the small(m /isotopesz 62.9amounts and of 64.9)Cu+ + + and(m/z of62.9of Pbimpurities and(m 64.9)/z 206.0, in and the 207.0 outermostof Pb and+ (m / 298.0)rez gion206.0, areof 207.0 a observed,sample and are298.0) in detectable. agreement are observed, The with peaks in the agreementof EDXthe isotopes results. with of The theCu peak EDX + + ofresults. Au(m /(Thezm 62.9/z 196.9)peak and 64.9)of is Au also and+ ( detectablem of/ zPb 196.9) (m/z inis206.0, also the 207.0 spectradetectable and (Figure 298.0) in theare4a). spectraobserved, The location (Figure in agreement of 4a). the The gold with location laminathe EDX of over the results. The peak of Au+ (m/z 196.9) is also detectable in the spectra (Figure 4a). The location of the thegold sample lamina cross over sectionthe sample was cross found section by acquiring was found maps by ofacquiring the lateral maps distribution of the lateral of the distribution gold signal. of gold lamina over the sample cross section was found by acquiring maps of the lateral distribution of The peaks of Ag (m/z 106.9 and 108.9) were observed when the analyzed area was reduced and localized the goldthe goldsignal. signal. The The peaks peaks of ofAg Ag (m (m/z/ z106.9 106.9 andand 108.9)108.9) werewere observed observed when when the the analyzed analyzed area areawas was over the gold lamina (Figure4a). Despite the weak signal of silver, the overlay of the Au and Ag maps reducedreduced and andlocalized localized over over the the gold gold lamina lamina (Figure (Figure 4a). DespiteDespite the the weak weak signal signal of silver, of silver, the overlay the overlay indicatesof theof Authe that andAu the andAg silver Agmaps maps is indicates localized indicates that mostly that the the within silversilver theis localizedlocalized zone of mostly themostly gold within within lamina the the (Figurezone zone of 4the b).of goldthe It is gold not possiblelaminalamina (Figure to quantify (Figure 4b). 4b). It the is It not concentrationis not possible possible to to ofquantify quantify silver in thethe the concentration gold leaf. However, of of silver silver in anthein upperthe gold gold leaf. limit leaf. However, of However, ca. 1 at% toan theupperan silver upper limit concentration limit of ca. of ca.1 at% 1 at% is to a tothe reasonable the silver silver concentrat concentrat estimationion since isis aa reasonable thereasonable signal estimation of estimation silver wassince since not thedetectable signalthe signal of byof EDXsilver [silver 16was]. wasnot detectablenot detectable by byEDX EDX [16]. [16].

(a) (b)

FigureFigure 4. (a )4. Time (a) Time of ﬂight of flight secondary secondary ion massion mass spectrometry spectrometry (ToF-SIMS) (ToF-SIMS) mass mass spectra spectra of theof the m/ zm/z regions (a) (b) of silverregions and goldof silver for sampleand gold A. (forb) Mapssample showing A. (b) Maps total ions,showing Au+ total(red), ions, Ag +Au(green)+ (red), distributions. Ag+ (green) The dashedFiguredistributions. white4. (a) linesTime The markof dashedflight the zonesecondarywhite of lines the mark goldion mass leaf.the zone Onspectrometry of the the left gold of thisleaf.(ToF-SIMS) zoneOn the there left mass of is thethis spectra embeddingzone there of the is resin m/z the embedding resin and on the right the preparation layer. The overlay of the Ag and Au ion andregions onthe of rightsilver the and preparation gold for layer.sample The A. overlay (b) Maps of the showing Ag and Autotal ion ions, distributions Au+ (red), is shownAg+ (green) in the distributions is shown in the last image. The total count rates are 80 for Au and 10 for Ag. lastdistributions. image. The The total dashed count white rates arelines 80 mark for Au the and zone 10 forof the Ag. gold leaf. On the left of this zone there is the embedding resin and on the right the preparation layer. The overlay of the Ag and Au ion distributions is shown in the last image. The total count rates are 80 for Au and 10 for Ag. Appl. Sci. 2019, 9, 3452 6 of 8

Hence,Appl. Sci. ToF-SIMS2019, 9, x FOR measurements PEER REVIEW conﬁrm that the gilding layer consists essentially of6 pure of 9 gold with traces of silver. Golden leaves were typically obtained by hammering coins which were made of pure gold [Hence,1,7] and ToF-SIMS the use measurements of leaves of pure confirm gold that is notthe gi surprisinglding layer takingconsists into essentially account of thepure importance gold with traces of silver. Golden leaves were typically obtained by hammering coins which were made and the wealth commissioning of this wall painting. of pure gold [1,7] and the use of leaves of pure gold is not surprising taking into account the Inimportance the OM and and FESEM the wealth images commissioning of sample of B severalthis wall layers painting. are discernable (Figure5a,b). The FESEM images andIn EDX the OM analysis and FESEM show thatimages in sample of sample B, theB seve goldral leaflayers was are laid discerna on a thickerble (Figure (10–20 5a,b).µ m)The tin foil (FigureFESEM5b). The images EDX and analysis EDX analysis reveals thatshow the that gold in sample leaf consists B, the gold of pure leaf gold,was laid within on a thethicker detection (10–20 limit of theµm) method tin foil (Figure (Figure5 b).5b). TheThe EDX layer analysis below reveals the metallic that the lamina, gold leaf which consists contains of pure gold, mainly within Fe the and Ca, is the preparationdetection limit layer. of the Layers, method which (Figure appear 5b). The dark layer both below in thethe OMmetallic and lamina, FESEM which images, contains are visible betweenmainly the goldFe and leaf Ca, and is the the preparation tin lamina layer. and the Layers, tin lamina which andappear the dark preparation both in the layer. OM Theand FESEM EDX spectra images, are visible between the gold leaf and the tin lamina and the tin lamina and the preparation measured over these regions did not reveal the presence of other elements, besides carbon and oxygen. layer. The EDX spectra measured over these regions did not reveal the presence of other elements, Thus, theybesides probably carbon and correspond oxygen. Thus, to the they glue probably used forcorre thespond adhesion to the glue of the used metallic for the adhesion foils between of the them and tometallic the preparation foils between layer. them and to the preparation layer.

(a) (b)

FigureFigure 5. (a) 5. OM (a) imageOM image of sample of sample B, B, reflected reflected light, light, dark field. field. (b ()b FESEM) FESEM image image and andEDX EDXspectra spectra of of samplesample B (gold B (gold foil onfoil tin on lamina).tin lamina). The The gold gold leaf leaf isis visible as as a abrighter, brighter, thin thin region region on top on of top a thicker of a thicker region corresponding to tin. (b) The EDX spectra measured on the gold leaf (spot analysis), on the tin region corresponding to tin. (b) The EDX spectra measured on the gold leaf (spot analysis), on the tin lamina and on the preparation layer (analyzed area ca. 20 × 20 µm2). lamina and on the preparation layer (analyzed area ca. 20 20 µm2). × The results of the ToF-SIMS analysis of sample B are similar to those of sample A, besides the The results of the ToF-SIMS analysis of sample B are similar to those of sample A, besides the presence of the peaks of the isotopes of tin (m/z 116.9, 117.9, 118.9 and 119.9) (Figure 6a). The presencechemical of the maps peaks of of tin, the silver isotopes and gold of tin and (m /theirz 116.9, overlay 117.9, are 118.9 shown and in 119.9)Figure (Figure6b. From6a). the The overlay chemical maps ofmap, tin, it silverturns andout that gold the and signal their of overlaysilver is arelocalized shown mainly in Figure on the6b. tin From lamina. the In overlay addition, map, in this it turns out thatcase the silver signal is present of silver in traces is localized since it mainlywas not ondetected the tin by lamina.EDX analysis. In addition, Although in the this analysis case silveris is presentlimited in traces to just since one itsample was not it should detected be noted by EDX that analysis.presence of Although silver impurities the analysis in the tin is limitedlamina could to just one samplebe it used should to derive be noted information that presence about the of provenance silver impurities of the material. in the tin lamina could be used to derive information about the provenance of the material. The analysis of samples A and B reveals residues of the last restoration carried out between 1989 and 1992. Barium, detected by EDX and ToF-SIMS (m/z 138), comes from consolidation treatments of the wall painting to transform calcium sulphate into barium sulphate. The ion fragments of fluorinated + + + hydrocarbons (m/z 50 CF2 , m/z 69 CF3 , m/z 119 C2F5 etc.) observed in the ToF-SIMS spectra probably originate from previous restorations. As far as the organic components are concerned, ToF-SIMS spectra measured for sample A in the region close to the gilding leaf show peaks which could be attributed to the acylium ions of palmitic + + (C16H31O , m/z 239) and stearic (C18H35O m/z 267) acids and to the ion fragments of monoglycerolester + + of palmitic (C19H37O3 m/z 313) and stearic (C21H41O3 m/z 341) acids. The peaks corresponding to Appl. Sci. 2019, 9, 3452 7 of 8 the ion fragments of lead palmitate and stearate were also detected. Peaks ascribable to deprotonated palmitic and stearic acids were observed in the negative ion ToF-SIMS spectra. Palmitic and stearic acids are the oxidation and hydrolysis products of linseed and other unsaturated oils during ageing of the original materials, as shown in many studies on oil painting ageing [17]. On the other hand, linseed and other oils are not the only source of fatty acids, which are contained also in waxes and egg. Moreover, the presence of several contaminants and the altered state of the samples did not allow us to derive anyAppl. firmSci. 2019 conclusion, 9, x FOR PEER about REVIEW the nature of the adhesive materials. 7 of 9

(a) (b)

Figure 6.Figure(a) ToF-SIMS 6. (a) ToF-SIMS mass mass spectrum spectrum for thefor them/ zmregions/z regions of of tin, tin, silver silver andand gold. (b (b) )Maps Maps showing showing total Sntotal+ (red), Sn+ Ag(red),+ (green) Ag+ (green) and and Au+ Au(blue)+ (blue) distributions. distributions. The The dasheddashed white line line marks marks the the zone zone of ofthe the Sn lamina.Sn lamina. The overlay The overlay of the ofSn, the AgSn, andAg and Au Au ion ion distributions distributions is is shown shown in th thee last last image. image. The The total total count rates are 1904, 1797 and 115 for Sn, Ag and Au, respectively. count rates are 1904, 1797 and 115 for Sn, Ag and Au, respectively. The analysis of samples A and B reveals residues of the last restoration carried out between For sample B, in the ToF-SIMS spectra we did not observe peaks attributable to linseed oil or 1989 and 1992. Barium, detected by EDX and ToF-SIMS (m/z 138), comes from consolidation to protein-based materials. This is probably due to the alteration (mineralization) of the organic treatments of the wall painting to transform calcium sulphate into barium sulphate. The ion materials which occurs faster when they are directly in contact with air and light as a result of the fragments of fluorinated hydrocarbons (m/z 50 CF2+, m/z 69 CF3+, m/z 119 C2F5+ etc.) observed in the partial detachmentToF-SIMS spectra of the probably gold leaf originate from thefrom oxidized previous tin restorations. foil. As far as the organic components are concerned, ToF-SIMS spectra measured for sample A in 4. Conclusionsthe region close to the gilding leaf show peaks which could be attributed to the acylium ions of Thepalmitic gilding (C decorations16H31O+, m/z in239) the and wall stearic painting (C18H “La35O Maest+ m/z à267)” by acids Simone and Martinito the wereion fragments characterized of + + by a combinationmonoglycerolester of microscopy of palmitic and (C spectroscopy19H37O3 m/z 313) techniques. and stearic Because (C21H41 ofO3 the m/ presencez 341) acids. ofcontaminants The peaks corresponding to the ion fragments of lead palmitate and stearate were also detected. Peaks ascribable to and the altered state of the organic materials, the analysis by ToF-SIMS did not allow a univocal deprotonated palmitic and stearic acids were observed in the negative ion ToF-SIMS spectra. Palmitic identificationand stearic of theacids adhesive are the oxidation materials and used hydrolysis to glue prod theucts gilding of linseed layers. and other The unsaturated results of theoils EDXduring and ToF-SIMSageing analysis of the original indicate materials, that pure as goldshown was in many used studies for gilding. on oil painting ToF-SIMS ageing revealed [17]. On traces the other of silverhand, in the goldlinseed leaf of and the other sample oils inare which not the the only gold source leaf of was fatty directly acids, which attached are contained to the preparation also in waxes layer. and Foregg. the sampleMoreover, in which the a gold presence leaf wasof several applied contaminants on a tin lamina, and the impuritiesaltered state ofof silverthe samples were did detected not allow mostly us to on the tin lamina.derive any firm conclusion about the nature of the adhesive materials. For sample B, in the ToF-SIMS spectra we did not observe peaks attributable to linseed oil or to Author Contributions:protein-based materials.OM: XRD This measurements: is probably M.G. due andto the A.S.; alteration FESEM-EDX (mineralization) measurements: of M.U.the organic and A.A.; ToF-SIMSmaterials measurements: which occurs A.A., R.M.P.faster when and N.M.; they writing,are directly review in contact and editing with ofair the and manuscript: light as a result all authors. of the Funding:partialThis detachment research was of the partially gold leaf funded from the by oxidized MIUR, tin Grant foil. “Dipartimento di Eccellenza 2018–2022” (Dipartimento DBCF) and by the PSR (Piano Sostegno alla Ricerca) 2018 grant (Dipartimento DFSTA). Acknowledgments:4. ConclusionsThe authors are grateful to Alessandro Terrosi (Dipartimento di Scienze Fisiche, della Terra e dell’Ambiente,The gilding University decorations of Siena, in Italy) the wall and painting Claudia “La Faleri Maestà” (Dipartimento by Simone di Martini Scienze were della characterized Vita, University of Siena, Italy) for preparing the samples. The authors thank Marco Consumi (Dipartimento di Biotecnologia, by a combination of microscopy and spectroscopy techniques. Because of the presence of Chimica e Farmacia, University of Siena, Italy) for the help with the measurements. contaminants and the altered state of the organic materials, the analysis by ToF-SIMS did not allow a Conflictsunivocal of Interest: identificationThe authors of declarethe adhesive no conflict materials of interest. used to glue the gilding layers. The results of the EDX and ToF-SIMS analysis indicate that pure gold was used for gilding. ToF-SIMS revealed traces of silver in the gold leaf of the sample in which the gold leaf was directly attached to the preparation layer. For the sample in which a gold leaf was applied on a tin lamina, impurities of silver were detected mostly on the tin lamina. Appl. Sci. 2019, 9, 3452 8 of 8

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