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Review

‘Copper and Bronze in Art’ and the Search for Rare Corrosion Products †

by
Gerhard Eggert
Institute of Conservation Sciences, Stuttgart State Academy of Art and Design, 70191 Stuttgart, Germany
†
Dedicated to Professor David A. Scott on the occasion of his 75th birthday.
Heritage 2023, 6(2), 1768-1784; https://doi.org/10.3390/heritage6020094
Submission received: 9 January 2023 / Revised: 29 January 2023 / Accepted: 2 February 2023 / Published: 6 February 2023
(This article belongs to the Section Materials and Heritage)
Browse Figures
Versions Notes

Abstract

:
The information taken from David A. Scott’s book “Copper and Bronze in Art” was crucial for this research on copper corrosion products in Stuttgart. The examples discussed are: (1) the nature and variability of ‘Black Spots’ (or ‘Brown Fuzzies’), mainly copper sulfides, resulting from the action of sulfurous gases on copper containing materials at different relative humidities; (2) cupric hydroxide occurring during maritime corrosion or by cleaning, patination, and pigment synthesis (Bremen blue) when alkaline solutions are used; (3) the wondrous phenomenon of curly malachite, which does not imply any human involvement; (4) chalconatronite formed by the contact of metal to alkaline surface films on soda glass, providing sodium and carbonate ions; (5) the formation of copper formates in contact with glass by the action of electrolytes from the glass hydrolysis and the pollutant formaldehyde reacting to the formate; (6) syntheses and crystal structure determination by powder X-ray diffraction (no single crystals needed) of various basic copper acetates (‘verdigris’) with the aim to better differentiate between historic production methods.

1. Introduction

“The book is a mine of information”, commented the British Museum’s Keeper of Conservation, William Andrew Oddy [1], in his review of David A. Scott’s highly prized and praised work ‘Copper and Bronze in Art: Corrosion, Colorants, Conservation’ [2] (in the following, referred to as SCOTT). One can rarely find a book which so deeply fulfills both the need of students new to the field for a comprehensible introduction and of researchers to identify the frontiers of knowledge. There is also an extra hidden in the text and the appendix: hitherto unpublished data of original research.
This review of two decades of copper corrosion research at the Stuttgart State Academy of Art and Design (ABK) reports how this book worked as a fruitful starting point for the author and his co-workers’ own investigations and their outcome.

2. Black Spots, Brown Fuzzies

“A spectre is haunting Europe”. This was the impression when conservators first heard from Helge Brinch Madsen at the ATM conference in Schleswig in 1976 [3] and the SSCR conference in Edinburgh in 1979 [4] talk about ‘Black Spots on Bronzes’ identified in European museums (Figure 1). As the spots looked like black mould (while containing copper sulfide) and a copper tolerant cladosporium fungus was detectable, Brinch Madsen discussed a ‘microbiological attack’. “The impression was clearly given that there is an epidemic of ‘black spot disease’ sweeping through European museums” [5]. Colleagues went home, took a closer look at their collections—and found them themselves! Some conservators were even fearing that they might spread the ‘infection’ from case to case. This was a kind of scientific déjà vu experience: a cladosporium species had already been thought to be the cause of bronze ‘disease’ as early as 1893 (SCOTT 126), when the recommended ‘therapy’ was sterilisation at 120 °C!
It needed Oddy and Meeks [5] to exorcise the revenant spectre by showing that the formation of copper sulfides can be explained with chemistry alone.
At about the same time, the New World developed another term (SCOTT 232): ‘Brown Fuzzies’, then unknown in Europe. Such ‘fuzzies’ were (first?) mentioned in writing by Tom Chase as a ‘brown-to-black mossy-looking substance which forms on some bronzes in the Walters Collection (the renowned Walters Brown Fuzzies)’ ([6]: 228,229). They occurred together with silver sulfide whiskers, and rubber mats were seen as the source of the volatile sulfur. Rarely, the necessary reduced sulfur sources emitting hydrogen or carbonyl sulfide for the occurrence of ‘Black Spots’ were identified at all, with rubber, wool, and objects from anaerobic sites being the main culprits [7]. Elemental sulfur can also cause ‘Black Spots’, as Eggert and Sobottka-Braun [8] demonstrated when discussing a bronze nail from the Mahdia shipwreck. They speculated on the presence of elemental sulfur in sea-logged wood; shortly after that, more than a tonne was discovered in the Vasa timbers [9]. Another (unexpected) material sometimes containing elemental sulfur as an ingredient and causing ‘Black Spots’ is plasticine [10] (see also SCOTT 396, note 2).
SCOTT (232/3) describes problems with X-ray diffraction because of the poor crystallinity of samples. Some spots are indeed amorphous [7,11] and, therefore, hard to characterise. SCOTT (233) analysed samples from brass mounts, clocks, and other items in the Wallace Collection in London. He could not match them with any known copper sulfide, although several copper sulfides of stoichiometries ranging between covellite (CuS) and chalcocite (Cu2S) have been identified on other objects. However, his compound contained zinc and was identified instead as the sulfate namuwite, (Zn,Cu)4SO4(OH)6·4H2O, a zinc substituted brochantite (SCOTT 445). Following this lead, Eggert et al. [7] also reported the sulfates chalcanthite, CuSO4·5H2O, brochantite, Cu4(SO4)(OH)6, and schulenbergite, (Cu,Zn)7(SO4)2(OH)10·3H2O, on brass in connection with ‘Black Spots’. As sulfides (S2-) are known to oxidise in humid air to their thermodynamic stable state (sulfate, SO42-), the occurrence of sulfates on heritage objects may not always be related to air pollution by sulfur dioxide (SO2), the usual explanation for sulfates (e.g., brochantite), which had been adopted by Seeley also for namuwite (SCOTT 234).
Observations on samples from Weichert et al. [12] ten years later and new results from laboratory experiments by Charlotte Kuhn-Wawrzinek [13] show that ‘Black Spots’ on copper objects are indeed not stable, particularly when exposed to a high relative humidity (RH). She was able to synthesise different types of corrosion (‘Trees, bunches, cauliflower’ [12], see Figure 2) ranging between typical ‘Blacks Spots’ and ’Brown Fuzzies’ in reproducible experiments for the first time, in which RH was the decisive factor [13]. For instance, copper coupons stored for 30 weeks over elemental sulfur at 40 °C developed black spionkopite (Cu39S28) plates at 9% RH, and brown djurleite (Cu31S16) needles at 95 % RH [13]. Copper sulfides can undergo further reactions and cause damage to the metal surface of the object when covellite is involved. Kuhn-Wawrzinek’s [13] experiments could not prove a corrosive effect of copper sulfides to copper without direct contact. However, there are hints from mineralogical collections [14] in which the degradation of disulfide minerals resulted in increased levels of sulfurous gases and caused the corrosion of copper (and other metals). ‘Black Spots’ should therefore be removed from objects for long term conservation. Until then, the storage at low RH can decelerate ongoing reactions.
However, in most cases, only small amounts of the ‘Black Spot’ corrosion products can be observed on objects, and the immediate challenge is to find and remove the original source of sulfur when ‘Black Spots’ are discovered in a collection.

3. Cupric Hydroxide: Corrosion, Colourant, Conservation

Copper(II) hydroxide has rarely been reported as a mineral (spertiniite) in conservation science, as the very short reviews in the book (just six sentences, SCOTT 98) and the preceding article [15] prove. Therefore, we were surprised when we detected it by X-ray powder diffraction (XRPD) in a blue patina on a classical French centrepiece (c. 1800 AD) made of brass with a polished surface. The patina was hidden in gaps, only recovered during demounting (Figure 3, [16]).
However, can this be intentional patination, inside only on an invisible spot? The formation of Cu(OH)2 needs an alkaline medium, and there is a volatile base traditionally used for the household cleaning of brass: ammonia. Ammonia is corrosive to copper alloys; a long duration of exposure dissolves a considerable amount of Cu2+ as the well-known dark blue Cu[NH3]42+ complex. However, when ammonia evaporates upon drying, this complex decomposes again and leaves a residue of Cu(OH)2. As evaporation proceeds quickly, there is not enough time to form visible amounts of copper salts on the outside of the cleaned surface by atmospheric corrosion during cleaning. However, ammonia trapped in gaps has enough time to corrode the alloy. The result is (amongst some Zn compounds) Cu(OH)2, as yielded by a demonstration experiment with a drop of ammonia between two brass sheets (Figure 4).
SCOTT emphasises the double role of many compounds as both a pigment and corrosion product. However, even he missed the role of the duck-egg blue copper hydroxide in patination when objects are intentionally exposed to fumes of ammonia (for recipes, see [17]). In addition, there is also a rare pigment which many (but not all) colour chemists since the 19th century have identified as copper(II) hydroxide: Bremen blue ([16], 65f.]. It was not directly precipitated from copper(II) solutions (which yields an unstable compound), but rather, was formed indirectly by ‘blueing’ solid green basic copper(II) salts (sulfate, chloride, nitrate) with caustic solutions. Nevertheless, the first measurements came as a surprise: the powder sample of Bremen blue in the historic collection of the Stuttgart Institute of Painting Technology consisted mainly of permanent white (BaSO4). Raman microscopy gave evidence of an organic dye, possibly copper phtalocyanine (SCOTT 315). BaSO4 was already reported from a copper-free Bremen blue sample (no. 136) from Vienna ([18], Figures 3–5) Apparently, the term Bremen blue was not only used as a label for copper(II) hydroxide, but also for imitations with blue organic dyes of a similar colour. Further analyses of samples and source texts of the 19th century are needed.

4. A Wonder: Curly Malachite

In the search for traces of organic remains, the microscopic inspection of metal finds has become routine in the last few decades. The more you look, the more you see. In 2001, bundles of curved green fibres (‘curls’) on an Alamannic bronze fibula were detected at the ABK [19]. Until then, this phenomenon has only been mentioned before on Roman bronzes by Scott ([20], Figure 8; SCOTT 106, Plate 21) as a form of malachite, not to be mistaken as textile. However, what do they tell us?
We could confirm the identification as malachite by qualitative spot tests, XRPD, and Fourier Transform Infrared Spectroscopy (FTIR). The curls seem to contain a little zinc (mostly close to the detection limit). They occurred on brass, bronze, and copper-containing silver alloy objects and there are reports of Bronze Age finds. Whilst most of the curls formed only part of a circle (radius some tenth of a mm), some looked similar to a corkscrew (helix), and others similar to a clock spring, under the microscope (Figure 5). In the electron microscope, no structure of any organic material, to be expected for negative or positive casts of them in corrosion products (‘pseudomorphs’), could be detected. A Field Emission-Scanning Electron Microscope (FE-SEM) image (Figure 6) of a curl cross section shows that the individual fibres of a sub-µm diameter are polygonal, as to be expected with minerals. An intensive search in the mineralogical literature brought some curl reports from mines to light, excluding any human involvement in their growth [19]. Giving no clue to the age or to alloys, even to human involvement at all, the malachite curls are somewhat disappointing as an historic information source.
However, they are a beautiful wonder of nature. A recent Dutch publication [21], while reporting further examples and proposing a scheme for their metric characterisation, could not solve the enigma of their growth either. Interestingly, visually quite similar curls formed by fibre bundles of barium sulfate have been grown in the laboratory. Here, nanoparticles diffuse in a viscous solution without sedimentation and form aggregates through the fusion of high-energetic crystal planes [22]. This might imply a growth mechanism that is independent of the specific material. Indeed, needle or curly crystals can be grown from solutions in porous terracotta sherds from quite a number of salts. Curled growth seems to be preferred by fast evaporation [23]. For similar experiments with malachite, one has to find a solvent which readily dissolves it and leaves no traces of evaporation, and there is one: aqueous ammonia. As we have seen in the section on cupric hydroxide, the Cu[NH3]42+ complex decomposes upon evaporation and all of the NH3 is set free. Therefore, solutions of malachite in ammonia in varying concentrations were filled inside porous terracotta pots, which were then covered with an airtight lid. Within days, green efflorescences occurred on the outside of the pot; however, unfortunately, no conditions could hitherto be found to produce curls until now. As long as there is no way to produce malachite curls in the lab, their occurrence on objects proves a long period of natural corrosion, i.e., their authenticity. So, at last, this provides information of cultural significance, which can be drawn from the occurrence of curls.
Once again, the curls proved the common wisdom: you only recognise what you know. After Stuttgart students were told about the phenomenon, three out of eight detected them in block-lifted objects from Alamannic graves. Scott [20] was right to assume that they are ‘probably seen more often by conservators than the literature would suggest’. Therefore, they do not necessarily prove that objects without context belong together.

5. Rare Copper Carbonates

There is more to copper(II) carbonates in heritage science than malachite or azurite. SCOTT (117-120) devotes a whole section to chalconatronite, Na2[Cu(CO3)2]∙3H2O, which occurs as a corrosion product on copper alloys that are in contact with sodium carbonate solutions. It was first identified by Gettens and Frondel in 1955 on Egyptian bronzes from soda rich soil [24]. The formation as a secondary product on museum objects is often caused by residues of a sodium sesquicarbonate buffer solution, used for the treatment of bronze disease [25,26]. Indeed, synthetic chalconatronite can be precipitated by dropping a copper(II) solution into such a carbonate buffer [27].
SCOTT (119) also reports the rare occurrence as a pigment on a 16th century manuscript and in Mayan wall paintings. His comment: “This finding is intriguing: further research into the etiology of these salts is needed to determine if they are original pigments or alteration products.” Indeed, sodium carbonate occurs in migrating salt solutions in wall plasters. This is possibly the reason that Bellendorf [28] detected chalconatronite on 15th to 18th century burial plates cast from quaternary copper alloys (Cu/Zn/Sn/Pb) mounted on the wall of Franconian chapels. Keeping an open eye for historic recipes for pigment production, SCOTT (118) yielded a mixture containing chalconatronite in his replication experiment of a historic Chinese pigment recipe by adding a sodium salt mixture (including carbonate) to a copper(II) solution prepared from cuprite and alum.
In Stuttgart, we found another sodium and carbonate source for the formation of chalconatronite: corroding soda glass in contact with copper alloys [29]. The alkaline surface films formed on the glass during hydrolysis also contained these ions, as they absorb carbon dioxide from the air. In our research on Glass Induced Metal-corrosion on Museum Exhibits (GIMME) [30], in ca 5% of all of the cases investigated, chalconatronite could be detected by Raman microscopy. The objects included baroque reliquaries with set glass gems, enamel on metal (16th century and a modern replica of intentionally unstable composition [31], Christmas tree glass baubles with wires, glass buttons with metal shanks, a glass figure with a wire support, and miners’ lamps (Figure 7; [29]).
Despite being a resourceful collector of compounds, SCOTT missed (and from hindsight, rightfully) an arcane basic sodium copper(II) carbonate, Na3[Cu2(CO3)3(OH)]·4H2O (ICDD 00-28-1084), which was listed—without any further information—as a corrosion product identified through powder diffraction ([32],: 167) on a cover glass of a daguerréotype (photosensitised silver layer on copper). Through synthesis experiments and the evaluation of XRPD peak lists and FTIR spectra, we could prove [33] that the postulated compound [34] is nothing other than chalconatronite; consequently, ICDD have now deleted this chart. Therefore, the listing by Barger and White [32] can now be taken as further proof of chalconatronite on another group of combined glass/metal objects.
One reason for the misidentification is the limited quality of the XRPD reference for chalconatronite (ICDD 00-22-1458), which was taken from a Guinier film in 1969 and is still in use for modern studies of heritage corrosion products (e.g., SCOTT 424, 446). In contrast to modern diffractograms, the angle measurements were not precise and stopped at 2Θ = 51°, and the intensities were only estimated visually. To analyse complex mixtures, it should now be replaced by high quality data derived from the crystal structure, e.g., ICDD 01-71-1490 ([33], Table I), covering a larger range with truly quantitative values for intensities. Such a re-evaluation could possibly solve the riddle of chalconatronite, which has been reported as a minor component together with quartz, atacamite, and cuprite in the outer concretion of the Riace Warrior A, recovered from the sea off Calabria (SCOTT 120, 328; [35]). As a result of its high solubility, chalconatronite cannot form in seawater. This might, therefore, indicate some kind of treatment of the bronze after the salvage and before it entered the conservation labs in Florence in 1975, as SCOTT (328) suggests. However, given the limited resolution in the angle and intensities of earlier powder diffraction instruments, and the quality of the JCPDS reference card at the time of measurement, one would recommend a re-analysis of the old sample with modern instrumentation to confirm its presence or absence in a complex mixture.
Another copper carbonate alteration product, a sodium copper(II) acetate carbonate, NaCu(CH3COO)CO3, was detected by SCOTT (302) in samples from the Burrell collection. Thickett and Odlyha [36] first described and detected it on 184 Egyptian artefacts in the British museum stored in oak cupboards that emitted significant amounts of acetic acid. Through desiccator experiments, Paterakis [37] showed that it can result from chalconatronite being exposed to acetic acid vapours. The storage of chalconatronite (as corrosion product on combined glass/metal objects) in oak cupboards might, therefore, be the explanation for our few encounters of this compound in the GIMME survey [30,38].
There is another compound that has a nearly identical Raman spectrum (so contains both acetate and carbonate). Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDX) showed that potassium and copper are the only heavier elements present (no sodium). It was detected by us on the silver mounting of a gold ruby glass (known to be produced from potash glass) in the Dresden Green Vault (Figure 8; [39]).
The original diffraction data published by SCOTT (447) proved its identity with his sample from a brass chandelier (Kelvingrove A 1960.28 bf, not A6082 BF as published) containing K, Cu, C, and O (Environmental SEM). SCOTT (301) was also able to isolate single crystals, but unfortunately, their measurements were lost during the accidental flooding of the crystallographer, Kenneth Hardcastle’s, lab (email to the author by Hardcastle). Therefore, the crystal structure remains unknown. With our additional information from the Raman spectrum (KCu(CH3COO)CO3?), the synthesis of this compound and the determination of the crystal structure might be possible. Single crystals are no longer a mandatory prerequisite for this (see below).
SCOTT (302) mentioned another pale blue compound, detected by Thickett and Odlyha [36], whose diffraction pattern matched with ICDD 31-453 given, as “copper chloride acetate”, with C2H2ClCuO2 as its sum formula. SCOTT (302, 316 note 28) noticed that copper(II) acetate chloride should have one more hydrogen: Cu(CH3COO)Cl. Consulting the ICDD reference literature [40] now showed that the error is not with the formula, but with the name; the correct name is copper(I) chloroacetate, the covalently bound chlorine replaces a hydrogen atom in the acetate anion. The ICDD has now corrected its error (email to the author, 2 November 2016). Although the match of the diffraction data looks quite good ([36], Table I), and no alternative match could be found in the ICDD in November 2016, there is no reasonable explanation how such a compound could form on a bronze. There is absolutely no evidence and no rationale for the use of chloroacetic acid, a laboratory chemical that does not occur in nature, by metal conservators. However, if it is not Cu(I)(CH2ClCOO), there must be another, hitherto unknown, compound with a very similar diffraction pattern. The search for rare copper corrosion products in heritage science is not finished yet.

6. The Formation of Copper(II) Formates

Routine business: a green corrosion product (spot test for Cu positive) was detected during conservation on a 17th century limewood box of board games between turquoise enamel and silver (ca 5.5% Cu), but its diffraction data (Debye-Scherrer camera; [41], Table I] could not be matched with anything in the Powder Diffraction File (PDF). A search in SCOTT’s Appendix D could not find a match, as judged by the three strongest peaks in the tables, either. A year later, another object was found with the same corrosion product: a wire spiral on a Chinese theatre hat showed corrosion where it was in contact with an apparently unstable glass bead, showing iridescence and crizzling. Another search in SCOTT’s tables: when neglecting Scott’s third strongest peak at d = 6.76 Å, there was a very good match with an unidentified sample from the Fitzwilliam Museum, discussed in the organic salts chapter (SCOTT 301, 446). This led us to the Trentelman et al. (one of the co-authors being Scott) paper on “a new pale blue corrosion product” [42] detected mainly on archaeological bronze finds. According to their XPS and Raman data, it is a mixed sodium copper (1:1) formate acetate; therefore, I coined the abbreviation ‘socoformacite’ [41]. As the formate:acetate ratio was estimated by the authors to be between 1:2 and 2:1, the general formula might be given as NaCu(HCOO)1+X(CH3COO)2−X, with 0 ≤ x ≤ 1. With sodium from the glass and copper from the alloy, this was the first time that we understood that there are joint glass/metal corrosion products [30,38], and our research on glass-induced metal corrosion started. Other than expected, carbonates are rare as copper corrosion products and formates dominate [43]. This corresponds with the analyses of the salts present in films on glass [44] as the ubiquitous formaldehyde can react directly in alkaline medium to formate via the Cannizzarro reaction, which is trapped in the liquid.
In 50% of the more than 250 GIMME cases, ‘socoformacite’ was detected ([43], for an example, see Figure 9), but we were unable to synthesise the compound from the solutions with varying Cu2+:Na+ and acetate:formate ratios [41]. In our Raman spectra, the peak at ca 940 cm−1 for the highly Raman active symmetric C-C stretch in the acetate anion was surprisingly small, variable, and even sometimes missing. Therefore, we started to doubt the necessary presence of acetate. As the formation of chalconatronite is proven for bronze finds and GIMME (see above), a synthetic sample was stored in a desiccator with a formaldehyde/formic acid-rich atmosphere (ca 200 ppm each, no acetic acid or acetaldehyde, 75% relative humidity) for a period of ~six months. The idea derived from SCOTT (446): his unmatched peak at ca 6.76 Å could very well be explained as a residue of some unchanged chalconatronite (strongest peak at 6.9 Å). Indeed, we obtained ‘socoformacite’, together with sodium formate. There were two lessons to be learned: firstly, the product can be produced acetate free; secondly, do not coin a compound name before you are absolutely sure of its composition.
All of our samples (>100) were finely grained, and no single crystal was observed under the stereomicroscope. No chance to determine the crystal structure and with it the overall formula of the compound? Robert E. Dinnebier from the Max Planck Institute of Solid State Research Stuttgart took up the challenge, and succeeded in determining the structure from high precision XRPD measurements alone by applying sophisticated methods of structure evaluation, yielding a basic sodium copper(II) formate oxide with Cu4Na4O(HCOO)8(OH)2·4H2O as its formula [45]. Indeed, no acetate appears in the structure. The powder diffraction data derived from the structure fit extremely well with Trentelman et al.’s measurement ([42], Table I), proving the identity of the compounds. In addition, the formula does not exclude the possibility that real samples might be contaminated with acetate (e.g., absorbed on the surface of crystallites), as we could also detect acetate through ion exclusion chromatography [43].
This was not the only unknown copper formate identified because of SCOTT’s diffraction tables. At the border between enamel and copper on a small copper fitting (Ø 6.3 cm) in quatrefoil form, with émail champlevé depicting a kneeling angel (MAK Frankfurt inv. no. 6341, early 14th century, Limoges), light blue and green corrosion products were found [46], the blue one being the sodium copper formate (Figure 10). The diffraction data of the green one perfectly matched with SCOTT’s film 770 (433), a synthetic basic copper(II) formate without a PDF reference card. Just by chance, we obtained the single crystals of this compound when treating copper coupons in patination experiments with ammonia and formic acid. This allowed us to solve the structure and determine the formula as copper(II) formate trihydroxide, Cu(OH)3HCOO [47], correcting the formula by Scott et al. ([48], Table I) taken from the synthetic literature. The compound contains no sodium, but as it can form only at a pH > 8, the influence of glass hydrolysis providing the alkaline environment is clear. The normal condensed humidity on the surfaces is slightly acidic (pH 5.6) because of the equilibrium with the carbon dioxide in the air.
The compound was also detected on a glass flute made in 1807 CE with silver mountings, a silver box with gold ruby glass inlay, a glass framed daguerreotype, and a Limousin émail peint copper plaque, c. 1500 [46]. In the GIMME project {30,38], Cu(OH)3HCOO was detected in one third of all cases, often together with the sodium copper(II) formate. This indicates that the (unknown exact) conditions of formation for both might be similar, and it, again, emphasises the dominance of copper formates in glass-induced metal corrosion, while they quite rarely occur without glass contact [49].
The latest example of a hitherto unknown copper formate corrosion product on heritage objects characterised structurally by XRPD was a mixed basic copper(II) zinc formate, occurring on brass in contact with glass, and, in simulation experiments: Zn(10-6x)Cu(3+6x)(HCOO)8(OH)18∙6(H2O) [50,51]. On the reflector of a miner’s lamp (Figure 7), this compound occurred together with chalconatronite. A current overview of all glass-induced metal corrosion compounds can be found in [38].

7. Some News on an Old Pigment: Verdigris

Verdigris (from vert-de-Grèce, green of Greece) is a collective term for green pigments occurring as transformation products on copper containing materials. It is only used here in its narrower meaning, for different copper acetates formed by the action of vinegar, e.g., on sheet copper. As an artificial corrosion product, this pigment fits into this review of research, and again illustrates the brilliant concept from SCOTT of “combining information on pigments and corrosion products, which are usually treated separately but are often chemically identical” (SCOTT, backcover citing G.E.).
Verdigris is one of the oldest synthetic pigments, used by mankind until the First World War. It was already mentioned by ancient Greek (Dioscorides, Theophrastus) and Roman (Pliny, Vitruvius) authors. “Verdigris is prepared in numerous ways” (nat. hist. 34.26) states Pliny, and he reports various recipes. Chemically, a number of basic copper(II) acetate hydrates of the general formula x[Cu(CH3COO)2]∙y[Cu(OH)2]∙zH2O have been found, in addition to neutral verdigris (pure copper(II) acetate monohydrate, x-y-z = 1-0-1). Gauthier [52] described the compounds x-y-z = 2-1-5, 1-1-5, 1-2-0, and 1-3-2. These syntheses were replicated by Schweizer and Mühlethaler [53], who deposited samples in reference collections such as the National Gallery London and the Doerner Institute Munich. Rahn-Koltermann et al. [54] also reported 1-4-3, but doubted 1-1-5, which was later confirmed again by SCOTT (273). Technical products are normally mixtures of various compounds, depending on their preparation. SCOTT (406-415) replicated a number of syntheses from the chemical literature and historical recipes and provided diffraction peak lists (SCOTT 434-442). He concluded: “The previously published X-ray diffraction data for the basic copper (II) acetates are sometimes contradictory and difficult to understand.” (SCOTT 273). Bette et al. ([55], Table I) tabulated the, sometimes contradictory, literature on the properties of these phases, including 1-3-0, which is easily formed from 1-3-2 by reversible thermal dehydration [56]).
One reason for this is the poor replicability of the syntheses. For example, SCOTT (408-409, 437) yielded two totally different materials for 1-2-0 (compound C and C2), which are not the same as the Schweizer and Mühlethaler sample Cc from the National Gallery measured by SCOTT (437).
Another reason is that only the crystal structures of the 1-0-1 and the 1-3-2 compounds were known and, therefore, reliable data for powder diffraction were only available for these phases. All other compounds need further investigation. In the framework of our DFG funded research project, “In search of structure” (DFG grant EG 137/9-1), Bette et al. [55] were now able to develop a reproducible synthesis for 1-2-0. Powder diffraction measurements at the Diamond Synchrotron allowed them to solve the crystal structure. Sheets in the structure are only bound by small van der Waals forces, and the compound has a tendency for stacking disorder. In general, disorder might change the shape of peaks, and even move their position, again changing the diffractograms. Intensities are often dependent on the measurement geometry: measurements with the sample in a spinning capillary in Debye-Scherrer geometry are to be preferred in contrast to the measurements on flattened powder samples in Bragg-Brentano geometry, which might show stronger texture effects (e.g., horizontal alignment of plates). Unfortunately, the experimental details are not always reported in the literature (including SCOTT).
Identifying another crystal structure is a step forward for a better understanding of the verdigris diffractograms, as illustrated here with a sample of 1-2-0 synthesised by Schweizer and Mühlethaler [53] from the Doerner Institute Munich. Our high precision lab measurement (spinning capillary) is compared to that reported by SCOTT (437) from the sample stored in the National Gallery in Table 1.
It is evident that modern measurements show many more peaks and allow a better quantification of the intensities. The peaks can be compared with those of 1-2-0 and 1-3-2. Indeed, unexpectedly, both phases are present, and one or more unknown, as illustrated in Figure 11.
In a replication experiment for verdigris production according to recipe V in the mediaeval Mappae clavicula by San Andrés et al. [57], copper sheets were exposed to vinegar fumes in a closed container, for six months, at 40 °C. Compounds 1-0-1 and 1-3-2 were identified on the copper using XRPD by their existing PDF cards, the unknown one (sample V-v-2) is clearly 1-2-0, as a comparison with our new data now proves.
The phase 2-1-5 (Compound A, SCOTT 271) has now also been synthesised in Stuttgart and the structure has been solved. It could be detected in old samples from pharmacies [58]. Comparing the diffraction data for this compound with SCOTT’s (437, 442) shows that these samples certainly did not consist of a pure 2-1-5 phase. The crystal structure of neutral anhydrous copper (II) acetate (1-0-0 phase = Compound E, SCOTT 272) could now also be determined by us from the XRPD measurements [59].
All of the crystal structure determinations were supported by the Raman and Fourier Transform Infrared (FTIR) spectra, which contain many structural details, e.g., the number of different acetate sites and the different coordination to copper (see [58] for a detailed discussion).
The final goal is to solve all of the structures of basic copper(II) acetates in order to allow the full quantification of the mixtures via Rietveld analysis. As illustrated by SCOTT’s verdigris tables, the composition varies greatly. Therefore, clues to the production methods are to be expected from such studies, thus contributing to technical art history. Given the significance of colour to man, historic pigments have been investigated since the beginning of analytical chemistry. It is really time now to better understand verdigris, and modern XRPD provides us with a tool to overcome the former limitations.

8. Concluding Remarks

To deal with decay in conservation, you need to know its causes. Identifying corrosion products is a prerequisite for this. While the common corrosion compounds became increasingly understood, research interest in the 1980s and early 1990s moved to rare products; Fabrizi and Scott’s [60] contribution on “Unusual copper corrosion products and problems of identity” is a paradigmatic example. However, since then, people have moved on to other themes: what could you do with only low resolution Debye-Scherrer films and a comparably small PDF reference database at the time? You were stuck with unidentifiable patterns and would miss any non-crystalline matter. Without easy access to SEM-EDX, you would not be able to visualise your samples with high magnification and obtain an idea about their homogeneity and elemental composition. Raman microscopes indicating characteristic molecular vibrations (e.g., of multiatomic anions) were unknown in conservation labs.
When David commented on my research results on corrosion products (e-mail of 10 September 2015): “…you are the only one left standing who is doing this work!”, I first felt heroic, but later idiotic (the others had good reasons to flee…). Some thoughts later, I see myself not as the last one standing, but as the first one reviving the field in a situation which has totally changed. SEM-EDX and Raman microscopy are now widespread in conservation science labs. High precision XRPD measurements and the now larger number of known structures allow for much better quantifications of mixtures, even the amount of hitherto overlooked amorphous material can be calculated. You do not necessarily need single crystals anymore for the determination of the crystal structure, even complicated structures can be solved from the powder diffraction data, as demonstrated by us [61]. This is not restricted to metal corrosion products and pigments, as has been also shown for a number of acetate efflorescence compounds on calcareous objects [62]. As these methods of data evaluation will become much more widespread in crystallography, this will revolutionise conservation science.
Sulfates in brown fuzzies, the rarity of cupric hydroxide, the wonder of curly malachite, unexpected occurrences of chalconatronite, diffraction data for hitherto unknown copper formates, and the ‘verisimilitude of verdigris’: we owe all of this knowledge to SCOTT’s ‘Copper and Bronze in Art’, which has provided a sound starting point for our research, and this was just copper, not to tell of all the steely ideas I developed from working with David on the sequel, ‘Iron and Steel in Art’ (discussed in [63]).
David’s curiosity, as documented in his many publications, will certainly continue to inspire new generations of researchers. The notion that we can see only so far because we are standing on the shoulder of giants was first expressed in the 12th century, by Bernard of Chartres [64]. It has now become a literary topos of our culture. In the bible, David is not the giant, but in conservation science! Thank you, giant David, for carrying us dwarves around and enlarging our horizon so much.

Funding

There was no funding for the whole research. For funding of individual projects, see Acknowledgements.

Data Availability Statement

All data can be found in the cited references.

Acknowledgments

David A. Scott’s infectious enthusiasm greatly inspired our research on copper corrosion products in Stuttgart, a joint effort with my co-workers and the institutions who funded them (listed in the order of the material presented here):
  • Maja Ossig (neé Weichert),
  • Charlotte Kuhn-Wawrzinek (“Bad Luck with Brimstone” project, funded by a graduate stipend of the Stuttgart Academy),
  • Britta Schmutzler (RaCoPhInO research, partly supported by an IPERION CH Transnational Access ARCHLAB grant, Agreement no. 654028),
  • Andrea Fischer (GIMME I and II projects funded by Friede Springer Stiftung),
  • Sebastian Bette (DFG-project “In search of structure”, grant EG 137/9-1), and
  • Jörg Stelzner (DBU-pilot project on glass-induced metal corrosion, AZ 33255/01).
Other contributions: The recent success in crystal structure determination is due to Robert E. Dinnebier’s state of the art equipment at the Max Planck Institute for Solid State Research and his unique knowledge in measuring and evaluating XRPD data. Christoph Krekel provided access to Raman microscopy and SEM-EDX in his lab at the Stuttgart Academy. Heike Stege (Doerner Institute Munich) provided verdigris samples from their collection. Thank you all!

Conflicts of Interest

The author declares no conflict of interest.

Scribal Abbreviation

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

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Heritage 06 00094 g001 550
Figure 1. ‘Black Spots’ and ‘Brown Fuzzies’ in the eye of a classical Zeus figure, photo: C. Kuhn-Wawrzinek, ABK.
Figure 1. ‘Black Spots’ and ‘Brown Fuzzies’ in the eye of a classical Zeus figure, photo: C. Kuhn-Wawrzinek, ABK.
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Figure 2. ‘Cauliflower’ type of ‘Black Spots’ as seen in the FE-SEM, photo: D. Kirchner, DBM.
Figure 2. ‘Cauliflower’ type of ‘Black Spots’ as seen in the FE-SEM, photo: D. Kirchner, DBM.
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Figure 3. Blue cupric hydroxide under the base of a brass column, photo: L. Albrecht, ABK.
Figure 3. Blue cupric hydroxide under the base of a brass column, photo: L. Albrecht, ABK.
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Figure 4. Brass test sheets after 46 d with a drop of ammonia between them, photo: B. Schmutzler, ABK.
Figure 4. Brass test sheets after 46 d with a drop of ammonia between them, photo: B. Schmutzler, ABK.
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Figure 5. Malachite curls on a Frankish brass fragment, image width: 6 mm, photo: A. Stäbler, ABK.
Figure 5. Malachite curls on a Frankish brass fragment, image width: 6 mm, photo: A. Stäbler, ABK.
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Figure 6. FE-SEM image of a cross section of a curl illustrating the polygonal shape of individual fibres, photo: R. Bretzler, fem Schwäbisch Gmünd.
Figure 6. FE-SEM image of a cross section of a curl illustrating the polygonal shape of individual fibres, photo: R. Bretzler, fem Schwäbisch Gmünd.
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Figure 7. Corrosion on a nickel silver reflector (portable X-Ray Fluorescence: ca 60% Cu, 24% Zn, 13% Ni) of a miner’s lamp in contact with glass: chalconatronite and a basic zinc copper formate (see below), photo: A. Schorpp, ABK.
Figure 7. Corrosion on a nickel silver reflector (portable X-Ray Fluorescence: ca 60% Cu, 24% Zn, 13% Ni) of a miner’s lamp in contact with glass: chalconatronite and a basic zinc copper formate (see below), photo: A. Schorpp, ABK.
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Figure 8. Unknown Potassium-copper compound at the gilded silver mounting of a gold ruby glass cup (Green Vault IV 227), photo: R. Richter, Green Vault Dresden.
Figure 8. Unknown Potassium-copper compound at the gilded silver mounting of a gold ruby glass cup (Green Vault IV 227), photo: R. Richter, Green Vault Dresden.
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Figure 9. Glass figure with internal copper wire, sodium copper formate efflorescence out of a fracture in the arm (KHM Vienna KK4144), photo: W. Bernhard, ABK.
Figure 9. Glass figure with internal copper wire, sodium copper formate efflorescence out of a fracture in the arm (KHM Vienna KK4144), photo: W. Bernhard, ABK.
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Figure 10. Copper fitting (Ø 6.3 cm) with émail champlevé, light blue and green corrosion in the contact zone, photo: A. Schwarz, MAK Frankfurt.
Figure 10. Copper fitting (Ø 6.3 cm) with émail champlevé, light blue and green corrosion in the contact zone, photo: A. Schwarz, MAK Frankfurt.
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Figure 11. Comparison of the diffraction patterns of the sample denoted as 1-2-0 phase synthesised by Schweizer and Mühlethaler [53] and of a pure 1-2-0 and 1-3-2 phase, synthesised by Bette et al. [55]. ◆ = other unknown phase(s).
Figure 11. Comparison of the diffraction patterns of the sample denoted as 1-2-0 phase synthesised by Schweizer and Mühlethaler [53] and of a pure 1-2-0 and 1-3-2 phase, synthesised by Bette et al. [55]. ◆ = other unknown phase(s).
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Table
Table 1. Sample of Schweizer and Mühlethaler [53] as measured by SCOTT (437, Compound Cc) and us [55]; only peaks Irel ≥ 5 are listed). Right column: tentative assignment to the phases 1-2-0 and 1-3-2 and U (=unknown).
Table 1. Sample of Schweizer and Mühlethaler [53] as measured by SCOTT (437, Compound Cc) and us [55]; only peaks Irel ≥ 5 are listed). Right column: tentative assignment to the phases 1-2-0 and 1-3-2 and U (=unknown).
SCOTTBetteAssignment
Ireld [Å]d [Å]Irel
2015.4215.65U
10010.5210.431001-2-0
9.3101-3-2
156.546.53201-2-0
6.165U
105.395.39131-2-0
5.2381-2-0
5.0961-3-2?
4.6791-3-2
4.157U
4.09221-2-0
24.034.03121-2-0
3.97121-2-0
3.8911U
53.593.59231-2-0
3.55181-2-0
3.51161-2-0
3.49191-2-0
103.443.4351-2-0
53.123.12391-3-2
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Eggert, G. ‘Copper and Bronze in Art’ and the Search for Rare Corrosion Products. Heritage 2023, 6, 1768-1784. https://doi.org/10.3390/heritage6020094

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Eggert G. ‘Copper and Bronze in Art’ and the Search for Rare Corrosion Products. Heritage. 2023; 6(2):1768-1784. https://doi.org/10.3390/heritage6020094

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Eggert, Gerhard. 2023. "‘Copper and Bronze in Art’ and the Search for Rare Corrosion Products" Heritage 6, no. 2: 1768-1784. https://doi.org/10.3390/heritage6020094

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