Page 299 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Theophilus's viride hispanicum (Spanish green) is as follows:

If you want to make Spanish Green, take some plates of copper that have been beaten thin,
carefully scrape them on each side, pour over them pure, warm vinegar, without honey and
salt, and put them in a small hollowed out piece of wood in the above way. After two weeks,
inspect and scrape them and do this until you have enough colour. 16

It is interesting that sodium chloride is specifically excluded in this recipe, which suggests
that many other recipes did use honey and salt. Banik replicated this recipe and found the prod­
uct to be a complex mixture of basic copper salts: besides copper acetates and carbonates, some
basic copper chlorides were also formed.
The Mappae clavicula, representing a varied body of knowledge compiled from the eighth
to the twelfth century (Smith and Hawthorne 1974), mentions seven different recipes for the
preparation of various copper corrosion products, not all of which would produce common
verdigris. Some examples of these recipes from the Mappae clavicula, together with relevant
passages from other medieval texts, are given in the following paragraphs.

[recipe iii] f you wish to make a different azure, take a flask of the purest copper and put
I
lime into it, half way up and then fill it with very strong vinegar. Cover it and seal it. Then
i
put the flask in a warm place for one month. Not as good as [recipe] i but it is suitable for
wood and plaster. (Smith and Hawthorne 1974:26)
Orna (1996) replicated this recipe, utilizing copper plates in a glass container. After
one month, mixed green, blue, and colorless crystals were found, as well as a blue reaction
product and unreacted starting materials. Elemental analysis of the crystalline product sug­
gested an empirical formula of Cu(CH 3 COO) 2 -Ca(CH 3 COO) 2 -6H 2 0, calcium copper ace­
tate hexahydrate, which is a water-soluble, deep blue crystalline solid that becomes very pale
blue when ground to a powder. This identification was confirmed by X-ray diffraction studies,
the data for which are shown in APPENDIX D, TABLE 17. The synthesis is described in APPEN­
DIX B, RECIPE 14.
Langs and Hare (1967) reported that calcium copper acetate hexahydrate can be prepared
by the slow evaporation of an equimolar solution of calcium acetate and cupric acetate. After the
initial pale green deposit was filtered, large deep-blue tetragonal prisms crystallized out of the
solution. Orna showed that these crystals had the same X-ray diffraction data as the product
i
from recipe ii of the Mappae clavicula. Calcium copper acetate hexahydrate may have been use­
ful as a pigment on wood or plaster but was probably not used for fine painting, and indeed the
Mappae clavicula text notes that it is not as good a pigment as that produced from recipe i (dis­
i
cussed later in this chapter).
Despite the observation by Wallert (1995) that the fifteenth-century manuscript Libro
Secondo de Diversi Colorie Sise da Mettere a Oro, known as the Simone manuscript, is not par-

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