Page 305 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

This recipe was replicated in the laboratory using distilled verdigris (ordinary verdigris dis­
solved in acetic acid and crystallized out) and freshly squeezed lemon juice. After mixing, the
solution was allowed to stand for twenty-four hours; it was then decanted into a dish to remove
the lemon-juice sediment. After a few hours, a light green crust began to crystallize out on the
surface of the dish, and this deposit was removed for analysis. Eventually, the remainder of
the solution solidified into a sticky green mass. Debye-Scherrer powder X-ray diffraction iden­
tified the light green crust from the dish surface as cupric citrate pentahydrate (ICDD 1-091),
Cu 4 [HOC(CH 2 COO) 2 (COO)] 2 . Citric acid, 2-hydroxy-i,2,3-propane tricarboxylic acid, is one
of the major components of lemon juice, and the transformation of distilled verdigris to cupric
citrate in this recipe is not unexpected. The powder X-ray diffraction data for this recipe are
given in APPENDIX D, TABLE 19.

Color measurements were made of samples of the distilled verdigris and of the cupric cit­
rate, both ground in gum arabic as pigment preparations. The color shift of the cupric citrate
toward green compared with that of distilled verdigris is seen in FIGURES 9.1 and 9.2. Verdigris
has different CIEL*a*b* coordinates; the peak maxima of copper citrate is around 520 nm while
neutral verdigris is at 495 nm, which is significantly more into the blue range of the spectrum.
The formula is therefore a workable recipe for preparing a green pigment from verdigris. To the
author's knowledge, no identifications of copper citrate used as a manuscript or painting pig­
ment have been reported. Positive identification of this pigment, however, would require
removal of a small sample, and this is rarely possible in manuscript studies, especially since the
identification of a copper citrate green from a very small sample could prove problematic.
These citrate and tartrate salts of copper have dubious stability. The GCI Museum Research
Laboratory's preparation of the copper (II) citrate appeared to be stable and granular for several
months, kept in a polyethylene bag exposed to ambient light levels in the laboratory. After about
a year, however, the larger mass of crystals began to turn much darker in color, and the outer
surfaces of the citrate developed into a viscous, dark green mass. Further study of these com­
pounds is required to evaluate their stability in glair or gum arabic. The pasta verde ( green paste)
referred to in the M s 992 recipe is actually a sap green added to the preparation to produce an
even brighter green color. The sap green may have other properties in terms of complexation of
copper ions and so could potentially act as a stabilizer of the prepared pigment.
Turner (i998) draws attention to the recipe collection of Johannes Alcherius, whose treatise
dates from around 1411. Alcherius gives a recipe not only for a copper citrate but also for copper
tartrates and for various combinations of copper greens mixed with organic dyestuffs (Merri-
field 1849:3-9, 45, 90, 95). The recipes compiled by Alcherius and used by illuminator Antoine
de Compeigne are of interest for their observations on the corrosive nature of these green pig­
ments. In fact, de Compeigne suggested neutralizing this property by adding gladiolus juice
(Merrifield 1849:284,286,288), which may have acted as a stabilizer similar to the sap green dis­
cussed earlier.

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