N. Sancho, M. San Andrés, S. Santos, J.M. de la Roja, Colorimetric measurements of different variants of verdigris: powder and bound pigments, e-conservation Journal 5, 2017
Available online 15 November 2017
doi: 10.18236/econs5.201706

Colorimetric measurements of different variants of verdigris: 
powder and bound pigments

N. Sancho, M. San Andrés*, S. Santos, J.M. de la Roja


Facultad de Bellas Artes. Depto. Pintura y Restauración
Universidad Complutense de Madrid
Address: C/ Greco nº2. (28040) Madrid
*e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Abstract

There are many variants of the pigment known as verdigris corresponding to different compositions and colours. All of them are copper salts resulting from the corrosion of copper or its alloys and their colours can be green, green-bluish or blue. On the other hand, the nature of the binder used to apply them on pictorial layers have a clear influence on the final colour. In this paper, the results corresponding to the colorimetric measurements of different varieties of verdigris pigment are presented. These are neutral verdigris [Cu(CH₃COO)₂·H₂O], basic verdigris [xCu(CH₃COO)₂·yCu(OH)₂·zH₂O] and the variety known as "salt green" consisting of a mixture of atacamite and paracamite, both idiomorphs of Cu₂Cl(OH)₃. All of them have been obtained in our laboratory using recipes explained in old texts. These pigments have been binded whit two types of binders (arabic gum and linseed oil) and pictorial layers have been applied on a support. The relationship between the pigments, the paintings colours and their compositions have been established. The colourimetrical changes produce over time has also been quantified.

1. Introduction

Verdigris is an historical synthetic pigment that has been widely used throughout history on easel and wall paintings [1-2]. In addition, this pigment has been frequently used to obtain inks recommended to wash plans and cartographic documents [3-4]. The current English term verdigris comes from Middle English vertegrez. This last name derives from Old French vert de Grice, meaning «green of Greece». From antiquity to the 19th century the terminology associated to this pigment has varied, but in general, all names refer to a green substance.

Analyses of many works of art reveal that pictorial layers obtained with this pigment, frequently, present a very dark green colour. However, the reproduction of some recipes described in old texts produces corrosion products of blue colour and, on the other hand, some uses of verdigris are related with this colour. For example, this is the case of some inks recommended by M. Buchotte in the book published in 1743 "Les regles du Dessein et du Lavis". This author specifies that the ink used to represent water colour areas must have a sky-blue colour and, to prepare it, he mentions that a blue verd-de-gris has to be used [5]. A similar recommendation appears in a 19^th century text related to the materials used for the design and colouring of planes [6].

There are many different recipes for the preparation of this pigment. All of them produce compounds derived from the corrosion of copper or its alloys, but its composition is depending on the recipe and the process applied for obtaining them. The most important varieties are copper(II) acetate monohydrate and several copper(II) hydroxyacetates hydrated. The varieties consisting of copper(II) hydroxychlorides mixed with other substances are also significant. These diverse compounds produce pigments with different colorimetric characteristics [7] and the type of the medium used in applying them also affect these properties [8].

In this paper are presented the results corresponding to the colorimetric measurements of different varieties of verdigris obtained in our laboratory. The relationship between the colour of powder pigments and their compositions has been established. The resulting effect on the colour produced for the nature of the binder has also been studied.


2. Methodology

Pigments tested were previously synthesized in our laboratory according to recipes explained in old texts and the products obtained were identified using different analytical techniques (LM, SEM-DEX, FTIR-ATR, Raman spectroscopy and XRD). The varieties obtained are neutral verdigris [Cu(CH₃COO)₂·H₂O], basic verdigris [xCu(CH₃COO)₂·yCu(OH)₂·zH₂O], and the pigment known as "salt green" consists of a mixture of atacamite and paracamite, both idiomorphs of Cu₂Cl(OH)₃. These results have been published by the authors elsewhere [9, 10]. Obviously, these different compositions are linked to different colorimetric properties of powder and bound pigments.

Neutral verdigris (labelled as N-V) is a green-bluish pigment that has been obtained applying recipe 106 of the medieval manuscript Mappae Clavicula (12^th century) [11]; the procedure consists of submitting a copper sheet to acetic acid vapours from vinegar into a closed container which is maintained at 40ºC for 15 days. Basic verdigris (labelled as B-V) is a blue pigment and it was obtained from recipe V of Mappae Clavicula. The procedure was similar to that described above but, in this case, the container was maintained at 40ºC for six months. Finally, "salt green" (labelled as S-V) is a green pigment that was prepared applying recipe XXXVIII of Ms. Heraclius (10^th–13^th century) referred to as viridis color cum sale [12]. In this case, vinegar vapours act on a copper plate impregnated with honey and powdered NaCl into a glass container maintained at 40ºC for 25 days.

Figure 1 shows images corresponding to the pigments obtained. All of them fall within the greenish range but they present obvious chromatic differences that can be appreciated at naked eye.

All these pigments were bound with two types of binders, arabic gum and linseed oil, both of brand Talens. The procedure to prepare these paintings was similar to described by the authors in previous research [8]. Bonded paint with arabic gum was obtained by mixing 0.3g of each pigment with 0.5mL of arabic gum, 15uL of tensoactive (Triton X-100) and 0,3mL of water. Oil paintings were prepared by mixing 0.3g of each pigment with 0.5mL of linseed oil. Finally, in all cases, paint layers were applied on a white support (L*=94.06±0.1; a*=-0.26±0.0; b*=1.6±0.1) with a cylindrical hand applicator with the slot for a thickness of 120 um. The pictorial layers obtained are shown in figure 2.

Colour measurements were performed with a Konica Minolta CM 2600d spectrophotometer, measurement spot: 3 mm, SCE mode, CIELAB space, range 400nm to 700nm, step size of 10 nm, light source D65, standard observer 10º. These measurements were taken from powder pigments and from pictorial layers; in these several measurements were made at different times: after 1 day and 16 days for pigment bound with arabic gum and after 15 days and 380 days for pigment with linseed oil.

Left to right:
Figure 1. Pigments prepared in the laboratory according to old recipes.
Figure 2. Pictorial layers obtained from pigments bound with arabic gum and linseed oil.

The results obtained are listed in Table 1 and the corresponding plot in the CIE a*b* diagram and the corresponding reflectance spectra are shown in Figures 3-5.
 
Attending to the attribute chromatic values, sample N-V (neutral verdigris) has a bluish-green hue (h_ab»194º) and its chroma (C*_ab»43) and lightness (L*»62) are high. These characteristics are confirmed by the corresponding reflectance spectra; in figure 2 it can be appreciated that the maximum percent reflectance wavelength is around 492nm. However, these characteristics change when the pigment is bound (Fig. 3). If the medium is arabic gum, the pictorial layer obtained shows a hue angle (h_ab*»192) and a lightness (L*»60) values similar to the corresponding to powder pigment, but chroma value is slightly higher (C*_ab»49). The reflectance spectrum is also coincident. These changes are more pronounced after 16 days (L*»58; C*_ab»35; h_ab»183) and the pictorial layer presents a green colour, which is confirmed by the reflectance spectrum where the maximum percent reflectance wavelength is around 497nm. Nevertheless, if this same pigment is bound with linseed oil, the changes appreciated are much more important (L*»40; C*_ab»38; h_ab*»170); so, its chromatic values corresponding to a green-yellowish hue and these changes are more significant after 380 days (Lab*»34; C*_ab»27; h_ab»135). Moreover, the lightness and chroma decrease significantly. In relation to the reflectance spectra, when measurements are taken after 15 days, the maximum reflectance wavelength is around 508nm, but this value is less defined after 380 days and it presents a wide range of values (510-560nm).

The chromatic values of sample labelled as B-V (basic verdigris) correspond to a blue colour which presents a high lightness (L*»69; C*_ab»33; hab»224). When this pigment is bound with arabic gum, the pictorial layer obtained is also blue (L*»75; C*_ab»31; h_ab»216), but its colour changes over time towards a green-bluish colour (L*»66; C*_ab»36; hab»203). The maximum percent reflectance wavelengths are 482nm and 486nm, respectively. However, when the medium used is linseed oil, the colour obtained is very different (Fig. 4). These changes are appreciated after 15 days and the chromatic attributes correspond to a green colour (L*»25; C*_ab»28; h_ab»154). Measurements made after 380 days show evidences that more important changes regarding to lightness (L*»16), chroma (C*_ab»18) and hue (h_ab»141) are produced. These values correspond to a very dark green-yellowish colour. In both cases, the maximum percent reflectance wavelength is not defined and it gives a wide range of values, 495-550nm and 515-555nm, respectively.

Left to right:
Fig. 3. Neutral verdigris [Cu(CH3COO)2·H2O]. Colourimetric measurements. Powder and bound pigment: a) Diagram CIE a*b*; b) Reflectance spectra.
Fig. 4. Basic verdigris [xCu(CH3COO)2·yCu(OH)2·zH2O]. Colourimetric measurements. Powder and bound pigment: a) Diagram CIE a*b*; b) Reflectance spectra
Fig. 5. Salt Green (Viridis colour cum sale) [Cu2Cl(OH)3]. Colourimetric measurements. Powder and bound pigment: a) Diagram CIE a*b*; b) Reflectance spectra.

The powder pigment labelled as S-V ("salt green") has a green colour (L*»65; C*_ab»26; h_ab»174) and its maximum percent reflectance wavelength is into the range 490-525nm (Fig. 5). When it is bound with arabic gum, not very important changes are appreciated after 1 day (L*»60; C*_ab»36; h_ab»167) and also after 16 days (L*»60; C*_ab»36; h_ab»166). Moreover, in both cases the chromatic values are similar and the peak of reflectance wavelength is around 510nm. However, these changes are very important when this pigment is bound with linseed oil. Paint layer obtained with this last medium has a green-yellowish colour. Furthermore, in this case the drying time of the paint has a clear influence; after 15 days hue value is hab=129 and after 380 days it is hab=114. On the other hand, lightness and chroma values show changes (L*»42; C*_ab»39) and (L*»35; C*_ab»33), respectively. Regarding to the maximum percent reflectance wavelengths, they are into the range of 510-530nm and 525-545nm, after 16 and 380 days, respectively.

From these results, it is found that pigments bound with arabic gum provide paintings with much more lightness than those obtained with linseed oil. This effect is produced because of the difference between the refractive indexes of the pigment and the surrounding medium. This difference increases the scattering of the incident light on the painting and, therefore, the lightness of painting increases. On the other hand, when the refractive indexes approach each other, the scattering effect is reduced. This increases the penetration of the light onto the pictorial layer, the absorption also enhances, so the colour shows less lightness.

The arabic gum has a refractive index of 1.34, while in the case of linseed oil, its initial value is 1.48, but it achieves 1.57 over time. On the other hand, the pigments tested have the next values: neutral verdigris [Cu(CH₃COO)₂·H₂O] (n_a=1.53 and n_g=1.56) and "salt green" [Cu₂Cl(OH)₃] (n_a=1.831; n_b=1.861 and n_y=1.880). With regards basic verdigris [xCu(CH₃COO)₂·yCu(OH)₂·zH₂O], because its exact composition could not be established, values regarding this property are approximated (1.54 - 1.66) [13]. So, for the three pigments there exists a larger refractive index difference between the pigment and the medium when arabic gum is used. As a consequence, paintings bound with this binder have more lightness. However, when linseed oil is used, the paintings obtained are darker and this characteristic increases over time.


4. Conclusions

Variants of verdigris obtained from old recipes present different colorimetric properties that are related to their composition. Neutral verdigris [Cu(CH₃COO)₂·H₂O] is a green-bluish substance (h_ab»194), basic verdigris [xCu(CH₃COO)₂·yCu(OH)₂·zH₂O] is a blue pigment (h_ab»224), and the pigment known as "salt green" consists of a mixture of atacamite and paracamite, both idiomorphs of Cu₂Cl(OH)₃ is green (h_ab»175). All of them show high lightness but their chroma values are different. Neutral verdigris present higher chroma value (C_ab*»43º) and salt green has the lower value (C_ab*»26º).

In the three cases, their colorimetric characteristics are modified when they are binded and the changes observed depend on the medium used to prepare the painting. In the three pigments, paint layers bound with arabic gum have more lightness than those obtained with linseed oil. These differences are related to the refractive index of binders.

When linseed oil is used, all paintings obtained show green colour and reflectance spectra displace towards longer wavelength. In general, the chroma values decrease and this effect grows over time.

Finally, it has to be noted that basic verdigris is the only pigment that provide blue colour paintings. This colour is obtained by mixing this type of verdigris with arabic gum, so it is possible that this was the pigment recommended in old texts to represent water colour areas in maps and planes.

5. Acknowledgments

This work has been supported by the Project Geomateriales2-CM (ref. S2013/MIT-2914) and has been made in the Laboratory of Materials [LabMat] from Universidad Complutense of Madrid.


6. References

[1] H. Kühn, Verdigris and Copper Resinate, in: A. Roy (editor), Artists´ Pigments. A Handbook of their History and Characteristics, Vol. II., Oxford University Press, Oxford, 1993, p 131-158

[2] N. Eastaugh, V. Walsh, T. Chaplin, R. Siddall, Pigment Compendium. A Dictionary of Historical Pigments, Elsevier Butterworth-Heinemann, 2004, p 385-386

[3] N. Sancho, M. San Andrés, Verdigris ink. Compilation and interpretation of recipes extracted from mediaeval manuscripts, in: R. Córdoba de la Llave (editor), Craft Treatises and Handbooks. The Dissemination of Technical Knowledge in the Middle Ages. De Diversis Artibus (DDA 91), Brepols Publisher, Turnhout, 2013, pp. 115-138

[4] M. Giménez, M. San Andrés, J.M. de la Roja, El color y su significado en los documentos cartográficos del Cuerpo de Ingenieros Militares del s. XVIII, Ge-Conservación 0, 2009 pp.142-160

[5] M. Buchotte, Les Règles Les Règles du Dessein et du Lavis puor les plans des ouvrages et des bâtimens, tant de l’Architecture militaire que civile, Imprimé par A. Jombert, París, 1743

[6] M.C.Hidalgo Brinquis,  Hallazgo de un breve tratado del siglo XIX con recetas para barnizar papel y una descripción detallada de los materiales usados para el diseño y coloración de los planos, in: Actas II Congreso de Conservación de Bienes Culturales, 1978, pp. 225-230

[7] M. San Andrés, J.M. De la Roja, V.G. Baonza, Verdigris, a pigment with different hues. Relation between chemical composition an colour, in: Proceedings of the 2^nd Latin-American Symposium on Physical and Chemical Methods in Archaeology, Art and Cultural Heritage Conservation, Universidad Autónoma de Mexico, 2010, pp. 4-9

[8] J.M. De la Roja, M. San Andrés, N. Sancho, S. Santos, 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 Research and Application 32, 2007, pp. 414–423

[9] M. San Andrés, J.M. De la Roja, V.G. Baonza, N. Sancho, Verdigris pigment: a mixture of compounds, Input from Raman spectroscopy, Journal of Raman Spectroscopy 41, 2010, 41, pp.1178-1186

[10] J.M. De la Roja, M. San Andrés, N. Sancho, S. Santos, V.G. Baonza, Varieties of verdigris containing copper(II) hydroxychlorides and other compounds: preliminary results, in: D. Saunders, M. Strlic, C. Korenberg, N. Luxford, K. Birkhölzer (editors), Lasers in the conservation of artworks IX, Archetype Publications, 2013, pp. 194-197

[11] C. S. Smith, J. G. Hawthorne, Mappae Clavicula. A little key to the world of medieval techniques, Transactions of the American Philosophical Society 64, 1974, pp. 26-121

[12] M.P. Merrifield, Original treatises on the arts of painting, Vol. II, Dover Publications, New York, 1967, pp. 236-239

[13] N. Eastaugh, V. Walsh, T. Chaplin, R. Siddall, Pigment Compendium. Optical microscopy of Historical Pigments, Elsevier Butterworth-Heinemann, 2004, pp. 78-83