Page 303 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

chemical species made by this recipe is an interesting problem, which future research will prob­
ably resolve, although the utility of the blue pigment produced remains doubtful.
Orna, Low, and Julian (i985) carefully examined the recipes for blue pigments requiring sil­
ver as a starting material. They conclusively showed that medieval silver contained some cop­
per and that this copper content was responsible for the formation of the required substances.
In describing how to make the best azure, the Mappae clavicula instructs:

[recipe ii] [T ]ake a new pot that has never been used for any work and set in it sheets of the
purest silver, as many as you want, and then cover the pot and seal it. Set the pot in the must
that is discarded from a wine press, and there cover it well with the must and keep it well
for fifteen days. Then uncover the pot and shake the efflorescence that surrounds the sheets
of silver into a bowl. (Smith and Hawthorne 1974:26)
Pure silver strips did not react during laboratory trials of this recipe using 5 M acetic acid,
while silver alloys with some copper produced cupric acetate monohydrate, neutral verdigris.
Alloy samples treated with acetic acid and horse dung also produced cupric acetate but appar­
ently of a different structure—larger and more perfectly formed—than the product formed with
acetic acid alone. Single crystal X-ray crystallography work showed that the structure of this
cupric acetate was identical with Cu 2 (CH 3 COO) 4 -2H 2 0, tetra^-acetato-bisdiaquocopper(II),
a dinuclear species. Orna (i996) states that this compound may form because of the slower reac­
tion rate for the silver-copper strip exposed to the acetic acid and horse dung compared with
exposure to acetic acid alone. This is a little perplexing, since many copper compounds, such as
copper acetates, are actually dimeric in structure. The formula shown here for the dinuclear spe­
cies is, in fact, simply neutral copper acetate monohydrate stated in a more complicated way;
there is really no significant difference (de Meester, Fletcher, and Skspski 1973).
Merrifield (i849) records a great deal of relevant information about pigments from another
Paduan manuscript, Ricitte per Far Ogni Sorte di Colon, MS 992 in the Library of the University
of Padua. This manuscript apparently originated in Venice and probably dates to the middle or
late seventeenth century. By this time, the chemical procedures had become quite complex and
are sometimes difficult to interpret, as shown by the following recipes:

[recipe 16] To make a good green of verderame —Take 10 parts of verdigris, 2 of corrosive
sublimate, VI a part of saffron, % of galls of Istria, and % of sal ammoniac, grind them up with
very strong vinegar (distilled vinegar is the best), put them into a glass vase, and when the
vinegar is clear and coloured, let it be decanted and evaporated in a glazed vase. Then pour
fresh vinegar on the remainder, mix again and do as before, until the part which settles
i
ceases to colour the vinegar, and f you pour the coloured vinegar into shallow open vases,
it will dry much quicker either in the sun or in the shade. When dry remove the colour

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