Page 294 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

The moist verdigris was scraped from the copper strips and prepared in semidarkness to
protect the crop from strong light. Since it is known that many copper reactions are influenced
by light, it is intriguing that the new crop of verdigris should have been protected from it, since
the action of light in altering the path or sequence of reactions in this system had not yet been
elucidated by modern chemistry.
Once dried, the verdigris was pulverized and then taken to an assessor to be certified. Prod­
uct of an acceptable quality was sold to merchants who exported most of it, primarily to Hol­
land where it was used principally as a painting pigment. There are many examples of verdigris
being used as a pigment from the thirteenth to the nineteenth century, ranging, for example,
from a thirteenth-century German altarpiece to an 1877 painting by the Russian artist Ernst von
Liphart (i847-i934) (Kühn 1993).
Laurie (1914) was not able to identify verdigris with certainty on any manuscript before
1400, while manuscripts from 1419 onward showed brilliant greens that were found to reveal the
crystalline structure of verdigris. For example, on a manuscript from the Burgundy Breviary 8
Laurie identified finely crystalline verdigris that he considers characteristic of fifteenth-century
continental manuscripts. He describes these greens as partially composed of doubly refracting
crystals with a refractive index below that of oil of Cassia (RI = 1.602). They are a pale emerald
green, not matched by the blue green usually associated with verdigris. Laurie proposed the fol­
lowing explanation for the color difference: Because verdigris is partially soluble in water, its
direct use on manuscripts was objectionable. Instead, the crude product of basic verdigris, first
leached with water, could have been employed for manuscript illumination. This preparation
would have greater stability in gum arabic solutions. The author's studies of a pale green verdi­
gris on Barlaam undJosaphat, the German illuminated manuscript described earlier, confirmed
that the verdigris was composed of pale emerald green crystals matching verdigris sample Β4
in the X-ray diffraction data shown in APPENDIX D, TABLE 15. This particular basic verdi­
gris salt is completely soluble in water, however, so it is unlikely that Laurie's suggestion can
be correct here.
From the Coram Rege Rolls (folio 1013) dated 1500-1700, Laurie identified a verdigris green,
but it was mixed with particles of azurite. This suggests that the verdigris was not prepared by
the corrosion of copper but by the dissolution of azurite in vinegar. Laurie also found blue par­
ticles in some of the copper resínate greens from manuscripts, and this may also support his the­
ory. In this case, however, a more likely explanation may be the presence of undissolved crystals
of verdigris used in making the copper resínate green.
I ANCIENT MEDICINAL USES Verdigris and other copper com­
pounds were appreciated at a very early period for their medicinal qualities (see CHAPTER 6).
The biochemical basis for the use of copper salts in medicine is discussed by Weser (i987), who
draws attention to a Middle Kingdom Egyptian papyrus from about 1536 B.C.E. This document,

T H E ORGANI C SALT S O F C O P P E R
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