In the medieval and early modern periods, European painters had few choices when it came to blue pigments. Ultramarine pigment, produced from the rare mineral lazurite (a component of lapis lazuli), was difficult to source and prohibitively expensive for all but the finest work, although it produced the finest blue colour [1, 2]. Another blue mineral, azurite, produced a good pigment, but was still extremely valuable [3, 4]. The expense of azurite (and ultramarine) drove painters and scientists to seek out alternative methods of obtaining blue pigments, including by synthetic routes.

Blue and green verditers are synthetic copper-containing pigments. Blue verditer is a copper hydroxycarbonate chemically analogous to azurite (Cu₃(CO₃)₂(OH)₂), while green verditer has long been understood to be chemically analogous to malachite, a copper hydroxycarbonate with alternative stoichiometry (Cu₂CO₃(OH)₂). This work, however, identifies for the first time a copper hydroxynitrate, rouaite (Cu₂NO₃(OH)₃), as the main product of laboratory-based replications of the historical verditer production process. Although this work does not directly address the identification of rouaite in historical samples, this is the subject of an additional forthcoming publication. One previous study (of an early seventeenth century miniature painting) has also identified rouaite as a pigment, although its origin is not investigated in detail [5].

Verditers are widely documented in written sources and historical works of art. Due to the expense of alternative blue pigments, the blue colour was much more valuable than the green [6, 7]. The earliest mention of ‘verditer’ occurs in 1506–1507. As the origin of the name likely derives from ‘verd de terre’, it is assumed that successful production of the blue pigment followed that of the green; the earliest possible mention of blue verditer, ‘verditer bis’,1 occurs in 1537–1538 [8, 9].

Verditer or verditer-like pigments have been identified throughout England (and continental Europe) [10–15]. Visual identification in English wall paintings and easel paintings from the sixteenth and seventeenth centuries is common, consistent with the earliest references to blue verditer in surviving records in the 1530s [6, 8, 16, 17].

Blue verditer, identical to azurite when studied by PXRD and Raman spectroscopy, is distinguished from naturally produced azurite by its specific morphology and lack of diverse associate minerals common to natural pigments. Since it is synthesized by precipitation from solution, it exhibits a distinct round shape and consistent, small particle size [6, 8, 11, 18]. It appears highly birefringent and lobed under polarized light [6, 18]. By-products of synthesis such as unreacted calcium carbonate, a synthetic reagent, may be present [19–21]. Natural calcium carbonate is also a common associate mineral of natural azurite and its presence should not be used to definitively draw conclusions about pigment origin [20, 21].

Several common associate minerals of naturally occurring azurite such as earth pigments, cerussite, and kaolinite were also used as pigments or extenders; their presence should not be used to conclusively decide a natural origin. Occurrence of a blue copper carbonate pigment absent of any of these species, however, suggests that the pigment possesses a synthetic origin. Characterisation of impurities and by-products resulting from verditer synthesis may also be useful to conservators in identifying extenders and degraded pigments originally present [11, 22].

The interpretation of the original colour of areas painted with verditers is challenging for conservators. Synthesis produced a range of colours from green to blue. Additionally, verditers were often mixed with organic yellow pigments that are difficult to detect and prone to fading. Yellowing of the medium over time may also make a blue pigment appear greener [11, 22]. Further scientific work on synthetic procedures and degradation reactions is therefore of great value.

Early modern synthesis of blue and green verditer was widely understood at the time to be unreliable, with small variations in synthetic procedure resulting in products spanning from green to blue, as shown in Fig. 1 [6, 10]. Nonetheless, this affordable pigment was evidently widely used, particularly in England [16, 23–25]; later treatises note that it was useful for producing vibrant greens when mixed with organic yellow pigments [11, 26]. Its ease of acquisition is reflected in the multiple accounts detailing the procedures for synthesis.Fig. 1

Verditer synthesis under varying experimental conditions produces a range of colours due to varying proportion of two mineral species, verditer and rouaite (Cu₂(NO₃)(OH)₃); a Rx_Air_50_int. b Rx_Air_4_S1. c Rx_N2_20_cont. d Rx_Air_20_S3. e Rx_Air_4_int, three washes. F Rx_Air_20_int. g Rx_Air_20_int. h Rx_CO2_20_cont. Note that all products are named according to synthesis conditions as outlined in Table 2

Verditer was produced from copper–water, a byproduct of the refining industries wherein nitric acid (aqua fortis) and copper metal reacted to form copper nitrate. Refiners’ operations were often small scale and verditer production tended to be a secondary source of income [7]. Christopher Merrit’s The Art of Refining (1677–78), describes the process:

‘They put into a Tub a Hundred Pound weight of Whiting, and thereon pour the Copper- Water, and stir them together every Day, for some Hours ... when the Water grows pale, they take it out, and set [it] by for farther Use, and pour on more of the Green Water; and so continue till the Verditer be made...’ [27]

Other sources include John Harris (early 1700s) and Robert Dossie (1758), providing similar procedures [26, 28]. Robert Boyle (1669) tells of successful synthesis by heating the solution before reacting it with whiting [29]. Boyle may have been intentionally misled by a refiner looking to protect trade secrets [6]. The problem of unreliability was paramount for verditer producers, as Merrit notes in his appendix to a translation of an Italian glass-making treatise published in 1662: ‘they can assign no reason, nor can they hit on a certain rule to make constantly their Verditer of a fair Blew, to their great disprofit, the Blew being of manifold greater value than the Green.’ [30]

Further complicating the discussion of verditer synthesis is evidence of many other syntheses for blue pigments, likely cuprammonium salts, dating back as far as the eighth century [31–35]. These syntheses are outside the scope of this work and products have not been detected in many surviving samples, though the possibility of their use should not be ruled out.

MacTaggart & MacTaggart studied the products of historical verditer syntheses in 1980, which provided a basis for this work. The green and blue synthetic products were characterized by polarised light microscopy (PLM). They found that the product colour was dependent on the synthesis temperature; the blue product was not formed above 12°C, a result that this work brings into question [6].

This led to the hypothesis that the success of the English verditer industry was due to cold English winters, while syntheses were not successful during warmer months. Historians have cited this work to explain why the English verditer industry succeeded where those in warmer continental Europe failed [8]. The mechanism for successful blue verditer synthesis, however, has not yet been elucidated, and has important implications for the historical understanding of pigment production and availability in the early modern period.

Although previous scientific research has investigated the products of the ‘refiners’ verditers’ synthesis method, the products of this synthetic procedure have not been previously characterised by chemically specific analytic methods. This work combines visual analysis, electron microscopy (SEM), Raman spectroscopy, and Powder X-Ray Diffraction (PXRD) to study the products of verditer synthesis.

The objectives of this work, therefore, is to address these open questions:

What products are formed in the “refiners’ verditers” synthesis?

The products formed in these reactions are also relevant as copper corrosion products in both industrial and archaeological settings [36–41], and as synthetic catalyst precursors for mediation of various chemical contaminants [42–47]. Their formation and reactivity under a range of reaction conditions, subject to significant debate in the literature, is therefore relevant beyond the area of pigment studies [39, 48, 49].

The use of ‘verditer’ throughout the text refers to blue verditer, the synthetic analogue of azurite. Azurite refers to the naturally occurring mineral. ‘Green verditer’ indicates the green synthetic pigment often previously identified as synthetic malachite, instead found here to be rouaite, a synthetic copper nitrate. It is important to note that blue verditer and azurite (and similarly, synthetic malachite and malachite) share the same Raman spectra and powder X-Ray diffraction patterns, and therefore, reference data from the American Mineralogist Crystal Structure Database (AMSCD) for azurite and malachite are referenced throughout the text under the mineral names. The terminology has been chosen to reflect the historical texts and previous scientific work on these materials and to minimise confusion. Table 1 summarises the nomenclature used in this work.Table 1

Summary of nomenclature

All reaction procedures studied are summarised in Table 2, and products are named and referred to throughout the text according to the relevant reaction conditions used.Table 2

Summary of synthetic routes studied

The syntheses studied are summarised in Table 2, which details the reaction conditions as well as resulting products for each route. Reactions are named systematically and referenced throughout the text as such.

Previous work reports the formation of a variety of products ranging in colour from green to blue [6]. This range was observed in repetitions of syntheses in this work as well, with most results under standard reaction conditions producing an intermediate blue-green product.

Characterisation of the product by Raman spectroscopy confirmed the presence of blue verditer (Cu₃(CO₃)₂(OH)₂) with characteristic bands at 400, 831, and 1090 cm⁻¹. Representative Raman spectra for samples synthesized under cold conditions with intermittent stirring are presented in Fig. 3. Unreacted calcium carbonate was also observed. In addition to blue verditer, a green basic copper nitrate was identified, rather than the expected malachite (Cu₂CO₃(OH)₂).Fig. 3

a Representative Raman spectra of synthetic samples (Rx_Air_4_int), following one (black), two (teal), or three (orange) washes. Reference spectra for rouaite (green) and azurite/verditer (blue) are provided for comparison [52]. Significant bands of rouaite (*) and azurite/verditer (+) are marked. The convoluted band at 400/404 cm⁻¹ as well as significant rouaite (454, 1046) and azurite/verditer bands (831, 1090) are labelled. b The shift of the convoluted 400/404 cm⁻¹ band following subsequent washes is shown, indicating a more significant contribution from verditer after subsequent washes

Careful analysis of Raman spectra as well as PXRD (Fig. 4) revealed that this compound was rouaite (Cu₂NO₃(OH)₃), the metastable monoclinic polymorph of gerhardtite that is most commonly observed in synthetic samples [51]. PXRD analysis additionally showed malachite present as a minor component, as well as unreacted calcium carbonate. Rouaite exhibits characteristic Raman bands at 404, 454, and 1046 cm⁻¹ (marked ∗ in Fig. 3a), and can therefore be readily differentiated from verditer and synthetic malachite. Verditer, on the other hand, shows characteristic Raman bands at 400, 831, and 1090 cm⁻¹ (marked + in Fig. 3a).Fig. 4

PXRD diffraction patterns of synthetic samples Rx_Air_4_int (black); Rx_Air_20TC_int (teal) and Rx_Air_50TC_int (orange) conditions are shown. Reference patterns for rouaite (green), azurite/verditer (blue), malachite (red), and calcite (grey) are provided for comparison [52]. Significant bands of rouaite (R), azurite/verditer (A), malachite (M), and calcite (C) are marked

Microscopic analysis of samples, on the other hand, showed similarity between samples composed of primarily rouaite those composed of primarily verditer (Fig. 5). Examination of synthesized verditer/rouaite mixtures by polarised light microscopy, used in previous studies of these experiments to characterise blue and green verditer, showed structures in agreement with earlier studies of verditers as well as commercial verditer: small, lobed crystals that are highly birefracting [6].Fig. 5

PLM images in light field and through crossed polars of a, d Commercial blue verditer (L. Cornellisen); b, e Rx_Air_4_int, three washes, blue verditer; c, f Rx_Air_4_S2, synthetic rouaite/ blue verditer/malachite. Note lobed “cauliflower” structure and birefringence under crossed polars consistent with earlier literature on verditer and verditer-like pigments [6, 18]

Rouaite, with chemical formula Cu₂(NO₃)(OH)₃, has not been identified in reference synthetic green verditer samples studied previously (Additional file 1: Fig. S1), and has not been identified in historical wall painting samples to date, although in a later publication Mactaggart and Mactaggart suggested that the green synthetic product may be gerhardtite (rather than malachite) [53]. Rouaite has also been noted as a common impurity in the synthesis of other mixed-metal hydroxycarbonates, suggesting that its presence in this synthesis should not be unexpected [42].

Product morphologies were studied further by scanning electron microscopy of coated particles. Rouaite shows a distinct tabular morphology, and is readily differentiated from verditer, which appears as smaller intersecting plates forming roughly spherical aggregates (Fig. 6). The appearance of rouaite observed here is consistent with samples synthesised by precipitation published elsewhere [49]. The appearance of verditer is consistent with that of various reference samples that have been studied previously as part of ongoing work on its characterisation as well as results published elsewhere [6, 18, 19, 54, 55].Fig. 6

a, b: Rx_Air_4_int; c, d Rx_Air_20_int. Verditer morphology circled in blue. Rouaite morphology circled in green

Products were more blue following subsequent reactions (second, third wash) of the same product in which additional unreacted carbonate and copper nitrate solution were added to product and reacted. Figure 7 shows the products resulting from one, two, and three washes with corresponding CIEL*a*b* colour values. After subsequent washes, there is a shift to more negative b* (more blue) and less negative a* (less green). While this multistep procedure was initially studied due to its presence in verditer recipes, these results led to a crucial hypothesis about the synthetic mechanism.Fig. 7

Photograph of Rx_Air_4_int following one, two, and three washes with CIELAB coordinates for products from one and three washes. Average L*, a*, and b* values and standard deviations from three data collections are presented for each product

The result suggests that the formation of verditer proceeds via a gradual displacement following initial formation of rouaite. The shift in relative verditer proportion was confirmed by PXRD of products following one, two, and three subsequent reactions (Fig. 7). Integrated peak area ratios presented in the inset table in Fig. 8 confirm that, following subsequent washes, azurite increases in abundance relative to rouaite in the product. This is consistent with visual and colorimetric examination of products (Fig. 7 and Additional file 1: Table S2), allowing the use of colour as a proxy for relative phase composition.Fig. 8

PXRD patterns of Rx_Air_4_int after first, second, and third washes. Data are normalized to the intensity of the rouaite peak at 12.8, and peak intensities of verditer bands at 17.2 and 17.4 are observed to increase following the second and third washes. Inset table provides the integrated peak area ratio A(rouaite)/A(azurite) determined by integrating the peak areas (12.0–13.25 2θ for rouaite, 16.5–18.0 2θ for azurite) from the non-normalised patterns for each sample. The integrated peak area ratios similarly show increasing azurite content relative to rouaite following subsequent washes

The conversion of rouaite to verditer is dependent upon the replacement of nitrate anions with carbonate anions. It was therefore proposed that the synthesis may be influenced by the concentration of carbonate anions in solution and therefore the carbonate-carbon dioxide equilibrium as shown in the following reactions:1CO32-+H+→HCO3-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{CO}}_{3}^{{2^{ - } }} + {\text{H}}^{ + } \to {\text{HCO}}_{3}^{ - }$$\end{document}2HCO3-+H+→CO2+H2O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{HCO}}_{3}^{ - } + {\text{H}}^{ + } \to {\text{CO}}_{2} + {\text{H}}_{2} {\text{O}}$$\end{document}

This equilibrium is dependent on factors such as partial pressure of carbon dioxide (pCO₂) and solution pH.

The conversion of rouaite to verditer may be analogous to ‘hydroxy double replacement reactions’ in which anions in a layered salt structure are loosely bound and therefore readily exchanged. Rouaite has been used as a precursor to synthesise organometallic catalysts in which nitrate anions are replaced by organic anions [49, 56, 57].

While nitrate anions are loosely bound in layered hydroxy salts (Fig. 9a), the larger negative charge of carbonate anions means that they are not readily exchanged once bound within the crystal structure. Figure 9b shows the crystal structure and bonding in azurite/verditer, in which carbonate anions are multiply bound to nearby copper ions; it is apparent that this crystal structure is radically different, a possible explanation for the irreversibility observed.Fig. 9

Crystal structures and information for a–c rouaite and d–f azurite. Nitrate ions in rouaite are singly bound to copper ions and located in intercalated layer. Carbonate anions in azurite are multiply bound to copper above and below. Structures visualised using VESTA (Version 3.5.8) [60] and data from the American Mineral Society Crystal Database [59]

Several other copper minerals (including langite, botallackite, and paratacamite) as well as organometallic catalysts form via facile ion exchange with rouaite, in which loosely bound intercalated nitrate anions are exchanged for loosely bound chloride and sulphate anions [49]. The original crystal habit, or shape, is often maintained in the product [48, 49, 58].

It is also possible that conversion occurs via dissolution and recrystallization [49]. The structural similarity of azurite to rouaite, both in terms of lattice parameters and bonding of carbonate anion, is low, as observed in Fig. 9, suggesting that dissolution and recrystallisation is more likely than anion exchange [49, 59]. In this case, the structural and electronic differences between rouaite and azurite still hold as an explanation for the irreversibility for conversion.

Initial work focused on the possible thermal dependence proposed by MacTaggart & MacTaggart and widely accepted as an explanation for the superior quality of English blue verditer over French verditer produced in the early modern period [6, 8, 9, 24]. Reactions at ambient room temperature (approximately 20 °C) produced mixtures of rouaite and verditer in varying proportions (Fig. 10, Additional file 1: Table S2). Intermittent stirring slightly favoured verditer formation compared to continuous stirring, but this is inconsistent as well.Fig. 10

Average CIELAB values a* (red-green) and b* (blue-yellow) for ten samples synthesised at 20 °C (continuous stirring, 0.31M [Cu²⁺], in air). Each sample data point is plotted in the corresponding colour value to aid in interpretation. Full colorimetry results are presented in Additional file 1: Table S2

Although the variability of results suggests that temperature may play a minor role in the successful production of blue verditer, it did not control the synthetic outcome and does not explain why the reaction succeeds or fails. A cooler climate (therefore colder reaction temperatures) does not explain the production of a blue product rather than a green one.

The effect of raising the temperature to 45–55 °C was then evaluated. This produced primarily rouaite with traces of malachite, suggesting that there is a temperature threshold above which verditer production is disfavoured, although this is well above the range of historical ambient air temperatures in England (not exceeding an average of 20 °C in summer and not falling below an average of 3 °C in winter during the sixteenth and seventeenth centuries) [61, 62].

The effects of temperature are still not fully explained. There was significant variability in the hue and relative proportion of verditer/rouaite of products produced at the same temperature and reaction conditions (Fig. 10). It was therefore proposed that temperature may play a role in influencing another factor not previously identified. As solution equilibria may be perturbed by temperature changes, and the rouaite to verditer conversion reaction is dependent on ion concentrations, the influence of carbon dioxide partial pressure (pCO₂) and therefore carbonate ion concentration was investigated.

Variation in the atmospheric pCO₂, an equilibrium sensitive to environmental factors such as temperature, was therefore proposed to influence the carbonate ion availability in solution and therefore verditer production. Initially, extreme conditions were used to study the reaction under very low (nitrogen atmosphere) and very high pCO₂ values (100% CO₂ at 1 atm).

Some verditer formed in very low abundance alongside mainly rouaite under a nitrogen atmosphere (approximately zero pCO₂) and under a synthetic air atmosphere (80% N₂, 20% O₂). The absence of CO₂ in the atmosphere disfavours the presence of carbonate in solution and favours production of carbon dioxide. Raman and PXRD data confirm the presence of primarily rouaite, as presented in Fig. 11. This result, though not historically relevant, is extremely useful in understanding the mechanism of synthesis. It suggests that successful verditer synthesis requires high carbonate concentration in solution, beyond that generated by dissolution of calcium carbonate, while rouaite formation occurs regardless. We note that reaction under synthetic air produces very slightly more verditer (Fig. 11b), which may suggest that stability and reactivity of rouaite are dependent on oxygen levels as well, although further work is necessary to comment on this conclusively.Fig. 11

a PXRD results of Rx_N2_20_cont (black) and Rx_SynAir_20TC_cont (orange) with verditer, rouaite, and malachite patterns provided for comparison [52]. The two conditions produce very similar patterns which correspond closely with reference for rouaite. b A smaller region of the PXRD pattern is shown, with grey regions highlighting very weak peaks in samples 1–3 corresponding to verditer [52]

Extremely pure verditer was observed after reaction under 100% CO₂ at 1 atm. This product was identified by Raman and PXRD analysis (Fig. 12). Rouaite is present after 4 h reaction time (sampled by removal of product suspended in solution from reaction chamber by needle) and absent by 7 h; this suggests that the mechanism of formation of verditer is not altered, but rather, the conversion of rouaite to verditer is greatly facilitated by increased pCO₂. Visually, the product is a deep blue colour (Fig. 2).Fig. 12

a Raman spectra of Rx_CO2_20_cont sampled at 4 h (black) and 7 h (red). Reference Raman spectra of verditer (blue) and rouaite (green) are provided, and the strongest rouaite band at 1046 cm⁻¹ is marked in the 4 h spectrum (*) [52]. b The PXRD pattern of the 7 h product is compared to the reference PXRD pattern for azurite/verditer, confirming the identity of the product as verditer with no detection of rouaite

At lower, but still elevated relative to atmospheric, concentrations of CO₂ (9500, 20,000, 50,000 ppm), however, rouaite was still the major product of the synthesis based on PXRD analysis at room temperature after 7 h as shown in Fig. 13. It is therefore likely that elevated atmospheric CO₂ alongside additional factors influencing carbonate concentration would be necessary to definitively control synthesis.Fig. 13

PXRD patterns of Rx_CO2var_20_cont (black, 100% CO₂ at 1 atm; red, 9500 ppm; blue, 20,000 ppm; yellow, 50,000 ppm) at 20 °C, with continuous stirring. The dominant product for all lower pCO₂ values is rouaite. Inset shows zoomed in region of verditer peaks, emphasizing minimal verditer production at all values studied