New Insights Into Synthetic Copper Greens: the Search for Specific

heritage Article New Insights into Synthetic Copper Greens: The Search for Specific Signatures by Raman and Infrared Spectroscopy for Their Characterization in Medieval Artworks Juliana Buse 1,2, Vanessa Otero 1 and Maria J. Melo 1,* 1 Department of Conservation and Restoration and LAQV-REQUIMTE, NOVA School of Sciences and Technology of NOVA University Lisbon, 2829-516 Monte da Caparica, Portugal; [email protected] (J.B.); [email protected] (V.O.) 2 Department of Information Sciences, Federal University of Ceará, Fortalez CEP 60020-181, Brazil * Correspondence: [email protected] Received: 16 April 2019; Accepted: 23 May 2019; Published: 4 June 2019 Abstract: A systematic investigation of medieval copper green pigments was carried out based on written sources: 21 manuscripts, dating from 50–70 to 1755 AD, were sourced and 77 recipes were selected, translating into 44 experiments. Reconstructions from medieval recipes were prepared and characterized through a multianalytical approach to disclose the original pigment formulation that is often described as verdigris. Based on the results obtained, we propose three main groups of copper green pigments, group 1, in which only Cu(CH COO) H O is formed; group 2, where this acetate 3 2· 2 is found together with copper oxalates; group 3, in which atacamite is present as the major green component or as a signature compound. The products formed are in perfect agreement with that predicted by the state-of-the-art research on the mechanisms of atmospheric corrosion of copper. This knowledge, together with our experience on craft recipes to prepare medieval paint materials, allowed us to recover a lost medieval recipe to produce a copper green pigment based mainly on atacamite, a basic copper chloride, which has been recently detected, by Raman and infrared spectroscopy, in artworks ranging from Catalonia and the Crown of Aragon panel painting to Islamic manuscripts. Keywords: copper green pigments; medieval pigments; conservation; Raman microscopy; infrared spectroscopy 1. Introduction 1.1. Medieval Copper Greens Copper greens were used as pigments since antiquity until the discovery and dissemination of viridian green, a chromium oxide, in the 19th century [1–5]. Except for malachite, a basic copper carbonate available as a mineral, all the other copper green pigments may have been synthesized during the medieval period as will be discussed in this paper. The medieval artefacts, in which synthetic copper greens have been detected, range from artistic to technical, such as manuscript illuminations and maps. Amid the synthetic copper greens, “copper resinates” and “verdigris” are still the best known medieval copper greens [1,4–7]; as we will show in this work, the main pigment obtained reproducing the medieval process for making verdigris is a neutral acetate of copper (Figure1). More recently, using an analytical approach similar to ours, other copper pigments have been characterized in medieval artworks, such as basic copper sulfates in medieval illuminated manuscripts dated from the 15th century by Eremin et al. [8,9]; basic copper acetates admixed with chlorides as well as oxalates, Heritage 2019, 2, 1614–1629; doi:10.3390/heritage2020099 www.mdpi.com/journal/heritage Heritage 2019, 2 1615 Heritage 2019, 2 FOR PEER REVIEW 2 43 alsochlorides from theas well 15th century,as oxalates, in Catalonia also from and the the 15th Crown century, of Aragon in Catalonia paintings and (oil andthe temperaCrown of paintings) Aragon 44 bypaintings Salvad ó(oilet and al. [ 10tempera,11]. In paintings) this latter work,by Salvadó the authors et al. [10,11]. are uncertain In this onlatter the work, origin the of theseauthors green are 45 compounds,uncertain on inthe particular, origin of thethese copper green oxalates: compound Weres, in they particular, applied the as syntheticcopper oxalates: pigments Were or werethey 46 theyapplied formed as synthetic due to paintpigments degradation? or were they Eremin formed et al. due [12 ],to in paint a systematic degradation? study Eremin on the et materials al. [12], andin a 47 techniquessystematic ofstudy Islamic on manuscripts,the materials described and techniques a variety of of Islamic mixtures manuscripts, of copper greens described to produce a variety subtle of 48 variationsmixtures of in copper color; greens they detected to produce all thesubtle above variations described in color; compounds they detected and atacamite all the above alone. described These 49 authorscompounds also and discuss atacamite the provenance alone. These of these authors greens, also eitherdiscuss produced the provenance artificially of asthese copper greens, corrosion either 50 productsproduced or artificially as “natural as mixtures copper resultingcorrosion fromproducts use of or di ffaserent "natural ores.” mixtures The main resulting analytical from techniques use of 51 useddifferent by Salvadores." óThewere main X-ray analytical diffraction techniques (XRD), us X-rayed by fluorescence Salvadó were (XRF) X-ray and diffraction infrared spectroscopy;(XRD), X-ray 52 whereasfluorescence Eremin’s (XRF) analyses and infrared were based spectroscopy; on Raman whereas microscopy Eremin’s and XRF analyses (infrared were spectroscopy based on was Raman also 53 used,microscopy but with and no XRF conclusive (infrared identification). spectroscopy Previously was also published used, but workswith no on theconclusive identification identification). of copper 54 greensPreviously such published as atacamite works in paintings on the wereidentification carried out of bycopper Naumova, greens Pisareva such as and atacamite Nechiporenko in paintings in the 55 1990s;were carried the characterization out by Naumova, was essentially Pisareva basedand Nechiporenko on transmitted in light the microscopy1990s; the characterization [13,14], and only was for 56 aessentially basic copper based sulfate on transmitted found in 16 lightth century microscopy Russian [13,14], frescoes and were only the for XRD a basic data copper presented. sulfate found 57 in 16th century Russian frescoes were the XRD data presented. 58 2+ 59 FigureFigure 1. 1.Cu(CH Cu(CH3COO)COO)2·HH2O represents a bridged carboxylate Cu2+ complex.complex. 3 2· 2 60 In whatwhat concernsconcerns the the conservation conservation of of this this cultural cultur heritage,al heritage, the the use use of verdigris of verdigris on paper on paper raises raises great 61 concerngreat concern since itssince degradation its degradation corrodes corrodes the organic the support, organic similarly support, to similarly iron gall inks.to iron Additionally, gall inks. 62 theAdditionally, changes in the color changes from green in color to dark from brown green greatly to dark affect brown our perception greatly affect of the our artworks perception [15]. of While the 63 thereartworks has been[15]. muchWhile research there has on treatmentsbeen much for research stabilizing on irontreatments gall inks’ for degradation stabilizing [ 16iron], there gall isinks’ still 64 muchdegradation to be learned [16], there on copperis still much acetate to degradation be learned on [15 copper,17,18]. acetate degradation [15,17,18]. 65 With this this issue issue in in mind, mind, for for this this work, work, we sy westematically systematically synthesized synthesized medieval medieval copper copper green 66 greenpigments pigments based on based written on sources written (treatises sources and (treatises other andtechnical other documents) technical documents)(Table S1). Twenty-one (Table S1). 67 Twenty-onepre-, post- and pre-, medieval post- and manuscri medievalpts, manuscripts,dating from 50–70 dating to from1755 AD, 50–70 were to 1755 sourced AD, and were 77 sourced recipes 68 andwere 77 selected recipes to were be reproduced, selected to be that reproduced, translated thatinto translated44 experiments. into 44 The experiments. pigments obtained The pigments were 69 obtainedcharacterized were through characterized a multianalytical through a multianalytical approach using approach Raman using microscopy Raman microscopy (μRaman), (µ Fourier-Raman), 70 Fourier-transformtransform infrared infraredmicrospectroscopy microspectroscopy (μFTIR), ( µX-rayFTIR), fluorescence X-ray fluorescence microspectroscopy microspectroscopy (μXRF) (andµXRF) X- 71 andray diffraction X-ray diff raction(XRD). Raman (XRD). microscopy Raman microscopy and infrar anded spectroscopy infrared spectroscopy were fundamental were fundamental to fingerprint to 72 fingerprintsignature compounds signature compounds for the three for main the three groups main we groups propose we to propose be representative to be representative of medieval of medieval copper 73 coppergreens, greens,as will be as discussed will be discussed in this work in this (Figure work (FigureS1). S1). 74 1.2. The Synthesis of Cu(CH 3COO)COO)2·HH2OO Using Using Copper Copper in in the the Presence Presence of of Acetic Acetic Acid Acid 3 2· 2 75 In the recipes that were selectedselected forfor thisthis study,study, thethe preparationpreparation ofof Cu(CHCu(CH3COO)COO)2·HH2OO (Figure (Figure 11)) 3 2· 2 76 is usually usually made made from from copper copper sheets sheets in in the the presence presence of, of,but but notnot in contact in contact with, with, a source a source of acetic of acetic acid, 77 acid,such as such vinegar as vinegar in a well-sealed in a well-sealed container. container. Thus, the Thus, formation the formation of medieval of medievalverdigris verdigriscan be described can be 78 describedas

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