Multi-Analytical Study of Copper-Based Historic Pigments and their Alteration Products

Abstract

Copper-containing materials such as verdigris are commonly found in historic and artistic works of art, often at advanced states of decay. Applied on paper as inks and watercolors, many of which needed a binder such as gum arabic, the intrinsic instability of this pigment was known since the medieval period. The decay of verdigris (a mixture of copper acetates) as a pigment, as watercolor, and as a dye, was studied using a combination of vibrational (Fourier transform infrared) and X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) instrumental techniques. Changes in the copper oxidative states and the formation of copper oxide were monitored during accelerated degradation as powdered solids and applied on mockup samples (with and without binder). Accelerated aging of both commercially available and synthesized verdigris pigments showed the presence of an intermediate species, Cu(CH₃COO)₂•3Cu(OH)₂•2H₂O, which points to the beginning of the decay processes, that culminates in the formation of Cu(II) oxide. However, the presence of gum arabic seems to delay deterioration, by temporarily reducing Cu(II) to Cu(I), even when the final product includes Cu(II). This novel application of XPS and supporting techniques has significant implications in art conservation, as the identified behavior helps explain the better preservation state of some works of art.

Keywords

Pigment degradation X-ray photoelectron spectroscopy XPS X-ray diffraction XRD Fourier transform infrared FT-IR

Introduction

There is a growing fascination with the research of historic and artistic materials, as well as their decay. Areas of interest include the artists' creation processes and techniques (technical art history),^1,2 the development and evaluation of art conservation methodologies,^3–5 and research into constituent materials' decay.^4,6–10 Degradation of pigment is particularly interesting, given that new findings can challenge conservation professionals' course of action.¹¹ Verdigris is one of the pigments falling into this category. Commonly present in works of art dating back as far as antiquity, this copper-based pigment has been identified in various states of degradation in leather and parchment,^12,13 fabric,¹⁴ waxy paint,¹⁵ (oil and canvas) paintings,^7,9,16–18 and paper.^8,19–22

The term verdigris is now applied to a blue-green pigment composed of (a mixture of) copper(II) acetates. However, throughout history, the term could also refer to copper carbonates (malachite, azurite), chlorides (atacamite, brochantite),² and sulfates (posnjakite, langite).²³ Being a synthetic pigment, verdigris' chemical composition is the direct result of its synthesis; its various degradation mechanisms are strictly associated with the media in which it was applied. Its application can be as a pure pigment or as a mixture. To all these considerations, conservation professionals must add the conditions in which the object was and will be treated, stored, and/or displayed to evaluate it.

For the purpose of this paper, we will be referring to verdigris as the blue-green pigment obtained by reacting copper and acetic acid. Studies have shown that obtaining a single phase of this pigment is challenging.^24,25 Therefore, finding more than one form of verdigris in both works of art and in experiments is expected. Chemically, verdigris can predominantly be divided in neutral and basic, with the most common compounds being (Cu(CH₃COO)₂• H₂O) and (Cu(CH₃COO)₂•Cu(OH)₂•5H₂O), respectively.²⁶ The pigment has been identified in numerous oil paintings and its degradation has been the subject of detailed studies. For instance, it was reported that ligand exchange reactions take place between fatty acids and verdigris.²⁷ Degradation products include copper oxalates, carboxylates, and even formates, which were previously thought to be intermediate products from fatty acids carboxylates to oxalates.⁷ Similarly, research into the discoloration observed when verdigris was mixed with linseed oil, suggested a light-induced mechanism that favors decarboxylation and reduction from Cu(II) to Cu(I).⁹

Every artifact containing verdigris can be affected by its established reactivity; its deleterious effects are particularly concerning with more fragile materials, such as those on paper, e.g., illuminated manuscripts and maps.^{12,19,21–23,28–31} Although many seem in good condition, it has been demonstrated that soluble copper ions, present in inks and watercolors with pigments such as verdigris, are particularly active towards the so-called “corrosion” of paper.³² That is, copper ions induce hydrolysis, oxidation, and radical-driven reactions that contribute to the subsequent degradation of cellulose.^5,33–35 Much of the research on this pigment and the effects copper ions have on paper has focused on treatment and it has been commonly in par with iron gall ink research.^34–37 Detailed studies on the development and use of antioxidants, complexing agents, and deacidification processes has shown various degrees of success in slowing down copper ion-induced paper decay.^3–5,8 However, similar studies to those made on verdigris with oil media remain scant. Herein, we report on the results that build onto preliminary work³⁸ and discuss detailed accelerated degradation studies of verdigris using gum arabic on cellulose, a common binding agent and media in writing and illumination. The present article reports on the application of a multi-analytical approach, including X-ray photoelectron spectroscopy (XPS), for tracking oxidation state changes that lead to the decay of verdigris in cultural heritage applications; ongoing investigations focus on substrates other than cellulose.

Experimental

Preparation of Pigments

Verdigris pigments were prepared by adapting historical recipes reported by Pliny et al.,^39,40 Scott,⁴¹ and Thompson.⁴² Two verdigris crystals (neutral and basic) were formed and collected; see Wiggins et al. for more details regarding their preparation and characterization.³⁸

Preparation of Samples

We studied the degradation of verdigris pigments dependent and independent of an organic substrate. First, both the independent powder pigments and mixtures of pigment powders with cellulose powder were aged. The degradation related to the presence of cellulose was studied by mixing each pigment and microcrystalline cellulose (ACROS Organics, particle size 50 μm) in a 1 : 1 (w/w) ratio. Mixtures of pigment and powdered cellulose complement our study of paper colored with the pigments, as we could evaluate an organic substrate (cellulose) with pigment, without the influence of any binding media. This sample set was prepared by placing approximately 0.3 g of each pigment or pigment–cellulose mixture in 2 mL gas chromatography vials with caps (Sun SRi). Septa from these caps were replaced with porous, non-woven, polyester fabric (Reemay) to allow equilibrium with the environmental conditions, while minimizing mass loss due to air circulation within the chamber.

To study the effects of an organic substrate on the pigments, verdigris was applied to cellulose paper (Whatman filter paper no. 1), both as a dye and as a watercolor. Commercially available (Kremer) and our neutral and basic verdigris pigments were dissolved in water at 5% (w/w). Various concentrations of gum arabic were added to each solution. X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) showed the Kremer verdigris were neutral verdigris crystals. The basic verdigris pigment was not readily soluble in water, so a small aliquot of 0.75 mL of 5% acetic acid was added to 15 mL of solution to aid in dissolution by slightly neutralizing the pigment, while still maintaining its basic character. Gum arabic mixtures of each pigment (commercially acquired, neutral, and basic) were prepared at 1%, 5%, and 10% (w/w) against the total weight of the verdigris solution.

Solutions of the verdigris pigments with no gum arabic were drop-cast onto five strips of Whatman filter paper no. 1 (13 × 3 cm) in 20 μL aliquots and repeated five times, which were allowed to dry between applications to concentrate the pigment. This process was repeated with the 1%, 5%, and 10% (w/w) gum arabic concentrations for a total of 20 paper strips (4 × 5 replicates). Within the environmental chamber, these strips were hung from the grates by commercially purchased, metal binder clips attached to the edges of the verdigris-colored papers. Mylar strips were used as support and buffers along the edges of the papers to avoid contamination from the clips. One sample strip of each type was withheld from aging to act as controls, and the subsequent replicates were removed after one, three, five, and seven days (see Fig. 1).

Aging Studies

Pigments were studied both as a pure powder pigment and as part of organic matrices (mixed in cellulose powdered and applied to cellulose paper) using an ESPEC BTL 433 test chamber. The accelerated degradation cycle ramped the chamber parameters to 60 ± 2 ℃ and 85 ± 2% relative humidity (RH) (15 min), then held at these conditions for up to 21 days. Samples of the paper were taken at one, three, five, and seven days (Fig. 1). The pigment–cellulose mixtures were sampled at three and seven days. Independent pigment powder samples were sampled at three, seven, and twenty-one days.

Instrumentation

Fourier transform infrared was used to monitor changes to the pigment and cellulose aging through functional groups. The spectra were collected using a Bruker Optic Vertex 70 FT-IR spectrometer and Hyperion 2000 Microscope with a single-point attenuated total reflectance (ATR) attachment. Spectra were collected using OPUS 6.0 (v.6.0.72) software, averaging 128 scans in the 4000–600 cm⁻¹ region with a resolution of 4 cm⁻¹.

X-ray diffraction was used to identify the crystalline materials and their changes over the aging process. It was performed using a Rigaku D/max Rapid II diffractometer with a copper anode X-ray tube and 0.3 mm collimator. Samples adhered to a glass loop by Parabar 10312 (Hampton Research) were secured to the sample stage in spin mode (0–360 ° rotation), 10 °/s, 40 kV, 30 mA, and a total collection time in the range of 15–30 min. Pigment paper fibers used 45 kV, 40mA, and a total collection time in the range of 60–90 min. Rigaku RAPID/XRD software (v.2.4.2) was used to operate the instrument and collect the data, while Rigaku 2DP software (v.2.0.1.1) was used to select and process the diffraction pattern. Rigaku PDXL 2 software (v.2.3.1.0) was used to interpret and compare to a reference database, the Powder Diffraction File from the International Center for Diffraction Data (ICDD).

X-ray photoelectron spectroscopy (XPS) was used to determine the composition and oxidation states of elements, specifically studying copper changes. It was performed on cuttings (1 × 1 mm) from the artificially aged filter papers adhered to carbon tape (Nisshin EM Co., Ltd.) for analysis with a Thermo Scientific K-Alpha + XPS instrument. This instrument was equipped with a monochromatic Al K_α source (hυ = 1486.6 eV) and it operated at a base pressure of 8 × 10⁻⁹ mbar. For each sample, measurements were taken in three different spots with a 400 μm X-ray spot size and the counts were summed. An electron flood gun was used to reduce charging effects. Copper 2p high-resolution spectra were collected with 25 scans across the 965–925 eV range. The pass energy was 20 eV at 0.1 eV/step and a 50 s dwell time. Data were collected with Thermo Avantage (v5.962) and analyzed with CasaXPS (version 2.3.18PR1.01), where the spectra were calibrated using the C1s peak at 284.6 eV.

Results and Discussion

Experiments with Pure Powder Pigments and Pigment–Cellulose Mixtures

X-ray diffraction and FT-IR analysis of verdigris pigments, prepared following a simple historical recipe that consists of letting vinegar fumes slowly react with copper,^39–42 showed two products: neutral (Cu(CH₃COO)₂•H₂O) and basic (Cu(CH₃COO)₂•Cu(OH)₂•5H₂O) verdigris.²⁶ As expected, neutral verdigris is a green-turquoise product, while basic verdigris is light blue color. These two verdigris species represent two possible starting materials used by artists for their different coloring.^{13,28,43–45}

While studying verdigris' impact on cellulose is extremely important, we focused our current efforts solely in understanding the pigment in the presence of cellulose, with and without gum arabic. Accelerated degradation tests were performed first on powdered pigments with no organic substrate/media; second, on a mixture of the pigment and powdered cellulose; and third, on paper applied as watercolors (with gum arabic) and as dyes (dissolved in water). Studying the pigments' degradation on paper was critical to contribute to the understanding of a near real-case scenario degradation. Studying mixtures of powdered verdigris and cellulose, on the other hand, allowed for experiments in the absence of any binder, with a comparatively higher ratio of pigment to organics, and with comparatively greater surface areas than on a paper sheet.

The pure powdered pigments and the mixture of powdered pigments with powdered cellulose were aged for seven days at 50 ℃ and 60% RH in open vials (more details in the Experimental section). Figure 2 shows XRD patterns of pigments' aging: control (with no organics present) of basic (left) and neutral (right) verdigris. After seven days of aging, new peaks at 9.41 ° and 18.73 ° suggest the formation of the less common basic verdigris species Cu(CH₃COO)₂• 3Cu(OH)₂•2H₂O as an intermediate before showing any visual changes pointing to degradation.^41,46,47

At seven days of aging, the pure basic verdigris, without organics, is almost entirely converted to the above-mentioned intermediate structure. The conversion reaction for neutral verdigris seems to be slower, as Cu(CH₃COO)₂•H₂O is still present, along with the intermediate, after seven days. The pigments mixed with cellulose showed similar trends. X-ray diffraction of both basic and neutral verdigris mixtures showed a partial conversion to the intermediate species Cu(CH₃COO)₂•3Cu(OH)₂•2H₂O, albeit slightly more pronounced for basic verdigris.

The FT-IR spectra of both pigments, displayed in Fig. 3, further supports the alteration suggested by XRD, which, to the naked eye, is associated with a shift to paler hues of both basic and neutral pigments. Unreacted basic verdigris showed carbonyl bands at 1564, 1537, and 1420 cm⁻¹, which shifted to 1527 and 1415 cm⁻¹ when aged with cellulose powder. Unreacted neutral verdigris, on the other hand, exhibited the carbonyl bands at 1595, 1442, and 1421 cm⁻¹, and shifted to 1547 and 1412 cm⁻¹ when aged with cellulose powder.^7,41,46,47

Altogether, FT-IR and XRD evidence suggest both basic, Cu(CH₃COO)₂•Cu(OH)₂•5H₂O, and neutral, Cu(CH₃COO)₂•H₂O, verdigris degrades into the same intermediate form: Cu(CH₃COO)₂•3Cu(OH)₂•2H₂O. This intermediate, identified as such because its presence becomes noticeable in the samples during the aging process and it is not detected once the final product, copper oxide, is predominant. The intermediate was detected in the absence as well as in the presence of cellulose. However, its presence seems to promote and increase the production of the alteration product. The deleterious effects of copper-containing species with paper have been the focus of attention for paper conservators and heritage conservation scientists alike; however, in contrast to our research, the research has been heavily focused on treatment development and evaluation.^4,8,44,48,49 While there are several established chemical structures for Cu(II) acetates or verdigris, the present study documented that two starting species (basic and neutral) will transform into a different verdigris species, the intermediate Cu(CH₃COO)₂• 3Cu(OH)₂•2H₂O during their degradation, as pigment, and in the presence of cellulose. This species has been documented previously, but the impact of other common constituent documents (gum arabic) has not been well-documented.^41,47,50

Experiments on Paper

To help establish an overall degradation pathway for verdigris on cellulose, the preliminary work published in 2018 was continued.⁴¹ Previous results suggested both basic and neutral verdigris pigments degrade into an intermediate state of verdigris (Cu(CH₃COO)₂•3Cu(OH)₂•2H₂O) that continues to degrade into copper oxide.

An independent set of aging experiments (50 ℃, 60% RH, seven days) was performed on unisized paper (Whatman filter paper no. 1), where commercially available (Kremer) and lab prepared verdigris (basic and neutral) were applied as dye (mixed in water). X-ray diffraction and FT-IR results of freshly applied pigments on paper showed the presence of neutral (Cu(CH₃COO)₂•H₂O) and basic (Cu(CH₃COO)₂• Cu(OH)₂•5H₂O) verdigris species, respectively.^7,41,47 Figures S1 and S2 (Supplemental Material) show FT-IR spectra for pigments (lab-synthesized and commercially available) and Whatman filter paper with and without basic verdigris before degradation, respectively. In addition, samples of the basic verdigris already contained the intermediate species (Cu(CH₃COO)₂•3Cu(OH)₂•2H₂O) before aging (Fig. 4). Figure 4 shows the intermediate species (9.41 ° and 18.73 °) at 0 days of aging, which seems to be connected to the rapid transformation of the basic pigment into to Cu(II) oxide (35.5 ° and 38.6 °).^51,52 Since copper oxide is a common degradation product in cultural heritage objects where copper salts have reacted with organic materials (e.g., parchment, binders, etc.), its presence was expected.^36,53,54 The XRD pattern following seven days of aging showed a broad feature of the baseline (see first half of Fig. 4). This is a result of both amorphous cellulose fibers included in the analysis sample and possibly amorphous alteration products, as previously suggested in degradation studies by Bette et al.⁵⁵

This new set of experiments puts verdigris' decay on paper into a greater conservation context. It has been well established that both basic and neutral verdigris pigments degrade into copper oxides. Additionally, the evidence presented here allowed identification of the intermediate (Cu(CH₃COO)₂•3Cu(OH)₂•2H₂O) as part of the oxidative alterations of the pigment. More importantly its presence seems to be the indicator of a subsequent rapid degradation process, as decay was more pronounced when applied to paper as a dye (aqueous solution) and not as the cellulose–pigment (in the previous section) mixture. Using XRD, the crystalline changes to these pigments were determined during the degradation process, in the presence and absence of cellulose.

Experiments on Paper with Gum Arabic

Gum arabic was a common binder in works of art on paper, including illuminated manuscripts, where verdigris was used.^44,56–58 To evaluate the role of this substance in the degradation pathway, aging experiments (50 ℃, 60% RH, seven days) were performed on unisized paper (Whatman filter paper no. 1). Commercially available (Kremer, neutral) and lab-synthesized verdigris (basic and neutral) were applied both as dye (mixed in water) and as watercolor (mixed in an aqueous solution of gum arabic) in a range of gum arabic concentrations (0%, 1%, 5%, and 10%). See the Experimental section for sample details.

Surface analysis of the samples was performed using XPS, which allowed for accurate tracking of the changes in oxidation states of copper-containing species. Because this technique provides information about the chemical environments, but not of the exact species, the XPS discussion will be limited to oxidation states only. Given its high spatial resolution, attempting to sample the same site to run complementary analysis like XRD was impossible. However, other areas of the same mockup sample were analyzed using XRD and FT-IR. Monitoring of the cellulosic changes with FT-IR did not show any conclusive trends. Alterations to these cellulose substrates could not be readily distinguished with IR spectroscopy and complementary studies are ongoing.

X-ray photoelectron spectroscopy analysis of commercially available (neutral) and lab-synthesized (basic and neutral), both as unaged pigments and applied to paper as aqueous solutions, showed mixtures of Cu(I) and Cu(II) states. This is consistent with previous results in Wiggins et al.³⁸ Although this result could be interpreted as a mixed-valence compound being present, it is possible the ultrahigh vacuum required to run XPS dehydrated the samples, resulting in a partial reduction.

The presence of gum arabic seems to be associated with the reduction of copper to Cu(I). Even at 0 days of aging, basic verdigris is predominantly in a Cu(II) state without gum arabic. Neutral verdigris (Fig. S1, Supplemental Material), on the other hand, is approximately an even mixture of Cu(I) and Cu(II). The XPS high-resolution scan of the copper 2p region (Fig. 5) suggests a correlation between higher concentrations of gum arabic and more Cu(I). Higher concentrations of gum yield an increase in peaks at 953.3 eV (2p_1/2) and 933.1 eV (2p_3/2), meaning a reduction to Cu(I). In addition, there are more Cu(I) species in the sample.⁵¹

Higher concentrations of gum arabic imply higher concentrations of polysaccharides and reducing sugars. The results showed gum arabic mixed with all forms of verdigris reduce some verdigris species from Cu(II) to Cu(I), with higher concentrations of gum arabic leading to more Cu(I). Cu(I) species produce more hydroxyl radicals than Fe(II), and Cu(II) species produce more hydroxyl radicals than other common transition metal ions on cellulose (in Fenton-like reactions frequently associated with paper decay).^34,59 Reducing sugars with Cu(II) acetate in water (Barfoed's reaction)⁶⁰ suggests objects such as illuminated manuscripts are inherently prone to degrade, as they contain all starting reagents that lead to Cu(I) species as byproducts. For instance, chelating treatments used to stop the catalytic effects of Fe(II), present in historical inks, are non-specific and can bond to transition metals^59,61–63 Therefore, the role of copper ions and their oxidation state need to be considered when selecting conservation procedures, as copper acetates (verdigris) can be found as part of these inks as well.

Interestingly, pigments seem to react with gum arabic to (temporarily) oxidize copper during the accelerated degradation process, as Cu(I) oxidizes into Cu(II) species. Figure 6 shows lab-synthesized neutral verdigris in 10% gum arabic, which showed the highest Cu(I) species at the start. Over the course of aging (three and seven days), the peaks at 955.1 eV (2p_1/2), 944.7–941.1 eV (satellite), and 934.9 eV (2p_3/2) slightly increased, possibly indicating some conversion to a Cu(II) compound, such as Cu(II) oxide.⁵¹

Commercially available neutral verdigris showed an oxidative change as well. Samples that started with higher amounts of Cu(II) (lower amounts of gum arabic) remained in that state throughout the experiment, possibly reaching stoichiometric equilibrium or perhaps evidencing a different pathway to copper oxide (ongoing research). This shows XPS evidence of the oxidation of the copper species during the artificial aging process, which in turn informs the oxidative decay occurring for the pigments on cellulose.

The presence of Cu(I) species, significantly present when the pigment is mixed with gum arabic, seems to be associated with a slower pigment degradation. X-ray photoelectron spectroscopy results showed Cu(I) and Cu(II) were present during the entire aging time in the presence of gum. In contrast, Cu(II)-containing species are overwhelmingly present in its absence. In addition, the XRD data on the same mockups suggest a slower reaction in the presence of gum arabic, by showing the same starting material (Cu(CH₃COO)₂•3Cu(OH)₂•2H₂O) during the seven days of aging. Figure 7 shows the XRD patterns (after seven days of aging) for basic verdigris with 10% gum arabic and a control sample, with no gum arabic. While the sample with no gum arabic only shows peaks for CuO (35.5° and 38.6°), the sample with 10% gum arabic shows the presence of the initial intermediate verdigris (9.45° and 18.96°) as well as the final product of CuO.^41,46,47 Furthermore, visual assessment of these samples showed that those with more gum arabic are less discolored and brown, compared to the controls, with no gum arabic (Fig. 1). This suggests that without gum arabic, under these conditions, the reaction goes to completion, while in the presence of gum arabic, the reaction progresses slowly. In both XRD patterns in Fig. 7, intense broad baselines in the range of 10°–25° indicate amorphous materials, either cellulose fibers from the substrate or amorphous alteration products, which could not be identified.⁵⁵

The results discussed here seem to suggest the degradation from the intermediate verdigris (Cu(CH₃COO)₂• 3Cu(OH)₂•2H₂O) to copper(II) oxide (CuO) occurs by more than one mechanism. Figure 7 shows that after seven days of aging and in the absence of gum arabic, only the degradation product CuO was detected. In contrast, in the presence of gum arabic, both the intermediate and CuO are detected. Because we are considering CuO to be the product of a complete degradation process, we think the gum arabic slows down the degradation, leading to an incomplete degradative oxidation of the pigment. Feasible reasons include: (1) a stable equilibrium between the various saccharides present in gum arabic and verdigris variants; (2) pigment simultaneously reacting with cellulose and gum arabic; or (3) a different, specific, mechanism from that occurring in the absence of gum, yet to be studied. Our findings have significant implications for conservation, as this could help explain dissimilar degradation in artifacts and could potentially lead to novel treatments.

The presence of numerous transition metal ions in historic inks is a well-documented cause of cellulose's degradation.^{34,35,59,64–66} In particular, the presence of Fe(II) has been the subject of numerous studies pertaining to iron gall inks and their treatment.^{5,32,35,37,61,63,65,67–69} Of all ions under study, copper has been deemed the most efficient to promote auto-oxidation and depolymerization of cellulose either as Cu(I) in the pseudo-Fenton's reaction to form hydroxyl radicals,^35,64,70,71 or as Cu(II) to act as an oxidation agent with cellulose carboxylates.^{33,54,72–75} Our findings show once more that copper is indeed a clear cause of decay, but in the presence of gum arabic, Cu(I)-promoted degradation is slower than that of Cu(II) without gum.

Conclusion

This study expands on previous work regarding the characterization and alteration of verdigris pigment, with a focus on identification of its crystalline species. This paper focused on determining the verdigris pigments' oxidative changes when applied to organic materials during accelerated degradation. Both lab-synthesized neutral and basic verdigris were found to alter into the same intermediate verdigris species, Cu(CH₃COO)₂•3Cu(OH)₂•2H₂O, with and without cellulose being present. This alteration was put into the context of the pathway of verdigris to copper oxide on paper, as identified by XRD. Expanding on this work, the role of gum arabic, a common binder for paper documentation, was evaluated as well. Gum arabic formed reduced copper species in the verdigris watercolors on paper. Those copper species appear to slow down the ultimate reaction to the degradation product, copper(II) oxide. These findings were documented by XPS and XRD with supporting FT-IR analysis.

This research documents the alteration in various forms of verdigris during the aging process on cellulose and the oxidation states the pigment undergoes with gum arabic binders on cellulose. While this stage focused on changes to only the pigment as it ages on paper, ongoing investigations focus on alterations to organic substrates as well as their changes during the oxidative process. This information benefits conservators, and conservation scientists aiming to preserve and treat historical documents, where verdigris plays a key role in their degradation. Continued work in characterizing the species that were not identified in this report and in better assessing the role of a reduction agent such as gum arabic on the overall degradation process is ongoing.

Supplemental Material

Supplemental material for Multi-Analytical Study of Copper-Based Historic Pigments and their Alteration Products

Supplemental Material for Multi-Analytical Study of Copper-Based Historic Pigments and their Alteration Products by Marcie B. Wiggins, Emma Heath, Karl S. Booksh and Jocelyn Alcántara-García in Applied Spectroscopy

Footnotes

Acknowledgments

The authors thank: the Winterthur Museum, Garden, and Library’s Scientific Research and Analysis Laboratory (SRAL) for the use of the XRD and accelerated aging systems; Brain Baade at the University of Delaware for materials and assistance in preparing many of the pigments; Jim Schneck (Winterthur Museum) for photography; the National Science Foundation (CHM 1506853) and the University of Delaware’s Office of Graduate and Professional Education for support; the University of Delaware’s Surface Analysis Facility for XPS support, as well as the NSF (CHE 1428149) and the NIH NIGMS COBRE program (P30-GM110758).

Conflict of Interest

The authors report there are no conflicts of interest.

Funding

The Andrew W. Mellon Foundation provided funding for the accelerated aging unit.

Supplemental Material

All supplemental material mentioned in the text is available in the online version of the journal.

References

De la Roja

J.M.

San Andrés

Cubino

N.S.

, et al. “Variations in the Colorimetric Characteristics of Verdigris Pictorial Films Depending on the Process used to Produce the Pigment and the Type of Binding Agent Used in Applying It”. Color Res. Appl. 2007. 32(5): 414–423.

Valadas

Freire

R.V.

Cardoso

Mirao

, et al. “On the Use of the Unusual Green Pigment Brochantite (Cu₄(SO₄)(OH)₆) in the 16th-Century Portuguese-Flemish Paintings Attributed to the Master Frei Carlos Workshop”. Microsc. Microanal. 2015. 21(2): 518–525.

Malešič

Kolar

Anders

“Evaluation of Treatments for Stabilization of Verdigris and Malachite Containing Paper Documents”. Restaurator. 2015. 36(4): 283–305.

Ahn

Hartl

Hofmann

, et al. “Investigation of the Stabilization of Verdigris-Containing Rag Paper by Wet Chemical Treatments”. Heritage Sci. 2014. 2(1): 12.

Kolar

Možir

Balažic

, et al. “New Antioxidants for Treatment of Transition Metal Containing Inks and Pigments”. Restaurator. 2008. 29(3): 184–198.

De Laet

Lycke

Pevenage

J.V.

, et al. “Investigation of Pigment Degradation Due to Acetic Acid Vapours: Raman Spectroscopic Analysis”. Eur. J. Mineral. 2013. 25(5): 855–862.

Prati

Bonacini

Sciutto

, et al. “ATR-FT-IR Microscopy in Mapping Mode for the Study of Verdigris and its Secondary Products”. Appl. Phys. A. 2015. 122(1): 10.

Hofmann

Hartl

Ahn

, et al. “Studies on the Conservation of Verdigris on Paper”. Restaurator. 2015. 36(2): 147–182.

Santoro

Zarkout

Le Hô

A.-S.

, et al. “New Highlights on Degradation Process of Verdigris from Easel Paintings”. Appl. Phys. A. 2014. 114(3): 637–645.

10.

Coccato

Moens

Vandenabeele

“On the Stability of Mediaeval Inorganic Pigments: A Literature Review of the Effect of Climate. Material Selection. Biological Activity. Analysis and Conservation Treatments”. Heritage Sci. 2017. 5: 12.

11.

Vermeulen

Sanyova

Janssens

, et al. “The Darkening of Copper- or Lead-Based Pigments Explained by a Structural Modification of Natural Orpiment: A Spectroscopic and Electrochemical Study”. J. Anal. At. Spectrom. 2017. 32(7): 1331–1341.

12.

Zoleo

Nodari

Rampazzo

, et al. “Characterization of Pigment and Binder in Badly Conserved Illuminations of a 15th-Century Manuscript”. Archaeometry. 2014. 56(3): 496–512.

13.

Chaplin

T.D.

Clark

R.J.H.

Martinón-Torres

“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”. J. Mol. Struct. 2010. 976(1–3): 350–359.

14.

Daniels

Leese

“The Degradation of Silk by Verdigris”. Restaurator. 1995. 16(1): 45–63.

15.

Liang

Scott

D.A.

“Green-Copper Containing Waxy Paint on two Egyptian Polychrome Artifacts: A Technical Study”. Stud. Conserv. 2014. 59(6): 391–403.

16.

Ren

Atlasevich

Baade

, et al. “Influence of Pigments and Protein Aging on Protein Identification in Historically Representative Casein-Based Paints Using Enzyme-Linked Immunosorbent Assay”. Anal. Bioanal. Chem. 2016. 408(1): 203–215.

17.

Villar

S.E.J.

Edwards

H.G.M.

“An Extensive Colour Palette in Roman Villas in Burgos. Northern Spain: a Raman Spectroscopic Analysis”. Anal. Bioanal. Chem. 2005. 382(2): 283–289.

18.

Samanian

“Identification of Green Pigment Used in Persian Wall Paintings (AD 1501–1736) Using PLM, FT–IR, SEM/EDX, and GC–MS Techniques”. Archaeometry. 2015. 57(4): 740–758.

19.

Arias

T.E.

Montes

A.L.

Bueno

A.G.

, et al. “A Study about Colourants in the Arabic Manuscript Collection of the Sacromonte Abbey Granada Spain. A New Methodology for Chemical Analysis”. Restaurator. 2008. 28: 76–106.

20.

Mendes

Lofrumento

Migliori

, et al. “Micro-Raman and Particle-Induced X-ray Emission Spectroscopy for the Study of Pigments and Degradation Products Present in 17th Century Coloured Maps”. J. Raman Spectrosc. 2008. 39(2): 289–294.

21.

Chaplin

T.D.

Clark

R.J.H.

Jacobs

, et al. “The Gutenberg Bibles: Analysis of the Illuminations and Inks Using Raman Spectroscopy”. Anal. Chem. 2005. 77(11): 3611–3622.

22.

Chaplin

T.D.

Clark

R.J.H.

McKay

, et al. “Raman Spectroscopic Analysis of Selected Astronomical and Cartographic Folios from the Early 13th Century Islamic ‘Book of Curiosities of the Sciences and Marvels for the Eyes’”. J. Raman Spectrosc. 2006. 37(8): 865–877.

23.

Gilbert

Denoel

Weber

, et al. “Analysis of Green Copper Pigments in Illuminated Manuscripts by Micro-Raman Spectroscopy”. Analyst. 2003. 128(10): 1213–7.

24.

Bette

Kremer

R.K.

Eggert

, et al. “On Verdigris. Part I: Synthesis. Crystal Structure Solution and Characterisation of the 1-2-0 Phase(Cu₃(CH₃COO)₂(OH)₄)”. Dalton Trans. 2017. 46(43): 14847–14858.

25.

Bette

Kremer

R.K.

Eggert

, et al. “On Verdigris. Part II: Synthesis of the 2-1-5 phase, Cu₃(CH₃COO)₄(OH)₂·₅H₂O by Long-Term Crystallisation from Aqueous Solution at Room Temperature”. Dalton Trans. 2018. 47(25): 8209–8220.

26.

Kühn

“Verdigris and Copper Resinate”. Stud. Conserv. 1970. 15(1): 12–36.

27.

Gunn

Chottard

Rivière

, et al. “Chemical Reactions Between Copper Pigments and Oleoresinous Media”. Stud. Conserv. 2002. 47(1): 12–23.

28.

Scott

D.A.

Khandekar

Schilling

M.R.

, et al. “Technical Examination of a Fifteenth-Century German Illuminated Manuscript on Paper: A Case Study in the Identification of Materials”. Stud. Conserv. 2001. 46(2): 93–108.

29.

Davies

B.H.

“Did the Medieval Illuminator Know How to Prevent Oxidative Damage?” Dyes Hist. Archaeol. 2003. 19: 24–31.

30.

Buti

Rosi

Brunetti

B.G.

, et al. “In-Situ Identification of Copper-Based Green Pigments on Paintings and Manuscripts by Reflection FT-IR”. Anal. Bioanal. Chem. 2013. 405(8): 2699–2711.

31.

Muralha

V.S.

Burgio

Clark

R.J.

“Raman Spectroscopy Analysis of Pigments on 16-17th c. Persian Manuscripts”. Spectrochim. Acta, Part A. 2012. 92: 21–28.

32.

M. Strlič, J. Kolar. Ageing and Stabilisation of Paper. Ljubljana, Slovenia: National and University Library, 2005. Pp. 211.

33.

Strlič

Kolar

Pihlar

“The Effect of Metal Ion. pH and Temperature on the Yield of Oxidising Species in a Fenton-Like System Determined by Aromatic Hydroxylation”. Acta Chim. Slov. 1999. 46: 555–566.

34.

Strlič

Kolar

Šelih

V.-S.

, et al. “A Comparative Study of Several Transition Metals in Fenton-Like Reaction Systems at Circum-Neutral pH”. Acta Chim. Slov. 2003. 50: 619–632.

35.

Šelih

V.S.

Strlič

Kolar

, et al. “The Role of Transition Metals in Oxidative Degradation of Cellulose”. Polym. Degrad. Stab. 2007. 92(8): 1476–1481.

36.

Kanngiesser

Hahn

Wilke

, et al. “Investigation of Oxidation and Migration Processes of Inorganic Compounds in Ink-Corroded Manuscripts”. Spectrochim. Acta B. 2004. 59(10–11): 1511–1516.

37.

Malešič

Kolar

Strlič

, et al. “The Influence of Halide and Pseudo-Halide Antioxidants in Fenton-Like Reaction Systems”. Acta Chim. Slov. 2006. 53: 450–456.

38.

Wiggins

M.B.

Alcántara-García

Booksh

K.S.

“Characterization of Copper-Based Pigment Preparation and Alteration Products”. MRS Adv. 2017. 2(63): 3973–3981.

39.

Pliny, H. Rackham, W.H.A. Jones, et al. Natural History. Volume IX: Book 34.26. Cambridge, MA: Harvard University Press, 1979.

40.

Pliny, H. Rackham, W.H.S. Jones, et al. Natural History. Volume IX: Book 34.28. Cambridge, MA: Harvard University Press, 1979.

41.

D.A. Scott. Copper and Bronze in Art: Corrosion, Colorants, Conservation. Los Angeles, CA: Getty Conservation Institute, 2002.

42.

D.V. Thompson. The Materials and Techniques of Medieval Painting. New York, NY: Dover Publications, 1956.

43.

Švarcová

Čermáková

Hradilová

, et al. “Non-Destructive Micro-Analytical Differentiation of Copper Pigments in Paint Layers of Works of Art Using Laboratory-Based Techniques”. Spectrochim. Acta A. 2014. 132: 514–525.

44.

Hofmann

Hartl

Ahn

, et al. “Stabilization of Verdigris”. J. Pap. Conserv. 2016. 17(3–4): 88–99.

45.

Miller

A.M.

Hanson

“The Maker and the Monk: Conservation of the Mercator Atlas of Europe”. J. Inst. Conserv. 2010. 33(1): 29–39.

46.

Salvadó

Butí

Cotte

, et al. “Shades of Green in 15th Century Paintings: Combined Microanalysis of the Materials Using Synchrotron Radiation XRD, FT-IR, and XRF”. Appl. Phys. A. 2013. 111(1): 47–57.

47.

San Andrés

de la Roja

J.M.

Baonza

V.G.

, et al. “Verdigris Pigment: a Mixture of Compounds. Input from Raman Spectroscopy”. J. Raman Spectrosc. 2010. 41(11): 1468–1476.

48.

Ahn

Hofmann

Horsky

, et al. “How Copper Corrosion Can be Retarded– New Ways Investigating a Chronic Problem for Cellulose in Paper”. Carbohydr. Polym. 2015. 134: 136–143.

49.

Zaccaron

Potthast

Henniges

, et al. “The Disastrous Copper. Comparing Extraction and Chelation Treatments to Face the Threat of Copper-Containing Inks on Cellulose”. Carbohydr. Polym. 2019. 206: 198–209.

50.

N. Salvadó Pradell

Pantos

, et al. “Identification of Copper-Based Green Pigments in Jaume Huguet's Gothic Altarpieces by Fourier Transform Infrared Microspectroscopy and Synchrotron Radiation X-ray Diffraction”. J. Synchrotron Radiat. 2002. 9(4): 215–222.

51.

Masciocchi

Corradi

Sironi

, et al. “Preparation. Characterization. and ab initio X-Ray Powder Diffraction Study of Cu₂(OH)₃(CH₃COO)·H₂O”. J. Solid State Chem. 1997. 131(2): 252–262.

52.

Pierson

J.F.

Thobor-Keck

Billard

“Cuprite. Paramelaconite and Tenorite Films Deposited by Reactive Magnetron Sputtering”. Appl. Surf. Sci. 2003. 210(3): 359–367.

53.

Špec

Retko

Ropret

, et al. “The Influence of UV–Vis Radiation, and Oscillations of Temperature and Relative Humidity, on Malachite Alteration in the Presence of Different Organic Binders and Varnishes”. J. Raman Spectrosc. 2014. 45(11–12): 1068–1075.

54.

S. Quartieri. “Synchrotron Radiation in Art, Archaeology, and Cultural Heritage”. In: S. Mobilio, F. Boscherini, C. Meneghini, editors. Synchrotron Radiation: Basics, Methods, and Applications. Berlin; Heidelberg: Springer, 2016. Pp. 677–695.

55.

Bette

Kremer

R.K.

Eggert

, et al. “On Verdigris. Part II: Synthesis of the 2-1-5 Phase, Cu₃(CH₃COO)₄(OH)₂·₅H₂O, by Long-Term Crystallisation from Aqueous Solution at Room Temperature”. Dalton Trans. 2018. 47(25): 8209–8220.

56.

Banik

Stachelberger

Mairinger

, et al. “Analytical Investigations of the Problem of Kupferfrass in Illuminated Manuscripts”. Microchim. Acta. 1981. 1(1–2): 49–55.

57.

Ricciardi

Pallipurath

Rose

“‘It's Not Easy Being Green’: A Spectroscopic Study of Green Pigments Used in Illuminated Manuscripts”. Anal. Methods. 2013. 5(16): 3819.

58.

Barkeshli

Ataie

G.H.

“pH Stability of Saffron Used in Verdigris as an Inhibitor in Persian Miniature Paintings”. Restaurator. 2002. 23(3): 154–164.

59.

J. Kolar, M. Strlič. Iron Gall Inks: On Manufacture. Characterization. Degradation and Stabilization. Ljubljana, Slovenia: National and University Library, 2006. Pp. 253.

60.

Barfoed

“Ueber die Nachweisung des Traubenzuckers neben Dextrin und verwandten Körpern”. Z. Anal. Chem. 1873. 12(1): 27–32.

61.

Neevel

J.G.

“Phytate: A Potential Conservation Agent for the Treatment of Ink Corrosion Caused by Irongall Inks”. Restaurator. 1995. 16(3): 143–160.

62.

J.G. Neevel. “Im(possibilities) of the Phytate Treatment”. In: A.J.E. Brown, editor. The Iron Gall Ink Meeting. Newcastle upon Tyne: Northumbria University, 2000. Pp. 125–134.

63.

J.G. Neevel. “(Im)possibilities of the Phytate Treatment of Ink Corrosion”. In: J.A. Mosk, N.H. Tennet, editors. Contributions to Conservation: Research in Conservation at the Netherlands Institute for Cultural Heritage (ICN). London: James and James, 2002. Pp. 76.

64.

Kolar

“Mechanism of Autoxidative Degradation of Cellulose”. Restaurator. 1997. 18(4): 163–176.

65.

Strlič

Kolar

Pihlar

“The Effect of Metal Ion, pH, and Temperature on the Yield of Oxidising Species in a Fenton-Like System Determined by Aromatic Hydroxylation”. Acta Chim. Slov. 1999. 46(4): 555–566.

66.

Kolar

Malešič

Kočar

Strlič

, et al. “Characterisation of Paper Containing Iron Gall Ink Using Size Exclusion Chromatography”. Polym. Degrad. Stab. 2012. 97(11): 2212–2216.

67.

Strlič

Kolar

Pihlar

“Some Preventive Cellulose Antioxidants Studied by an Aromatic Hydroxylation Assay”. Polym. Deg. Stab. 2001. 73: 535–539.

68.

Šala

Kolar

Strlič

, et al. “Synthesis of Myo-Inositol 1,2,3-Tris- and 1,2,3,5-Tetrakis(dihydrogen Phosphate)s as a Tool for the Inhibition of Iron-Gall-Ink Corrosion”. Carbohydr. Res. 2006. 341(7): 897–902.

69.

Ceres

Conte

Mirruzzo

, et al. “Imidazolium-Based Ionic Liquids for the Efficient Treatment of Iron Gall Inked Papers”. ChemSusChem. 2008. 1(11): 921–926.

70.

Valenzuela

Contreras

Oviedo

, et al. “Copper Catechol-Driven Fenton Reactions and Their Potential Role in Wood Degradation”. Int. Biodeterior. Biodegrad. 2008. 61(4): 345–350.

71.

Malešič

Kočar

Balažic Fabjan

“Stabilization of Copper- and Iron-Containing Papers in Mildly Alkaline Environment”. Polym. Degrad. Stab. 2012. 97(1): 118–123.

72.

Beeson

W.T.

V.V.

Span

E.A.

, et al. “Cellulose Degradation by Polysaccharide Monooxygenases”. Annu. Rev. Biochem. 2015. 84(1): 923–946.

73.

Hosny

W.M.

Hadi

A.K.A.

El-Saied

, et al. “Metal Chelates with Some Cellulose Derivatives. Part III. Synthesis and Structural Chemistry of Nickel(II) and Copper(II) Complexes with Carboxymethyl Cellulose”. Polym. Int. 1995. 37(2): 93–96.

74.

Zhongyue

Yuyue

Peng

, et al. “Structure and Properties of Cu(II) Complex Bamboo Pulp Fabrics”. J. Appl. Polym. Sci. 2010. 117(3): 1843–1850.

75.

Faubel

Staub

Simon

, et al. “Non-Destructive Analysis for the Investigation of Decomposition Phenomena of Historical Manuscripts and Prints”. Spectrochim. Acta, Part B. 2007. 62(6): 669–676.

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