Page 309 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

This recipe will produce a mixture of the basic copper chlorides, principally atacamite, with
copper acetates, depending on the extent to which the "naushagar" (sal ammoniac) has acted to
convert the copper scraps to the chloride before the grape vinegar is added. Almost certainly
the final product will be a complex mixture of salts. A laboratory replication, described in
APPENDIX Β, RECIPE 23, produced neutral verdigris, rather than atacamite, with a little resid­
ual ammonium chloride.
Ancient Indian texts apparently do not mention recipes for verdigris because the pigment
was obtained from Persia during the early centuries C.E., according to Agrawal. 21
The Asrarul Khat gives another recipe for the preparation of ordinary verdigris: "Put the
specks of copper in a copper pot, crush them with curd and let the mixture be dried up for three
days and nights. The zangar will be ready for use" (Bukhari 1963 : iii).
Banik (i989) states that the use of sour milk, instead of vinegar, was also common in Russia
and cites Radosavljevic (1972), who noted that the use of sour milk or curd for preparing pig­
ment was quite common in some regions.
Purinton and Waiters (1991) briefly review some information on Persian green pigments
from the medieval period, although the data are somewhat incomplete. The most common green
was described as verdigris. Dickson and Welch (i98i) translate the canons of painting by the
medieval writer Sadiqi Bek, whose recipe for the preparation of verdigris calls for digging a pit
2 m deep and burying in it for a month copper plates that have been immersed in wine vinegar.
Rather than completely converting copper surfaces to verdigris, corrosion by plant or
animal materials — such as garlic or curd—could also be employed to alter the copper surface.
For example, the deliberate corrosive etching of copper plates used for oil painting was done
to make the imprimatura adhere thoroughly to the copper substrate. Palomino de Castro y
Velasco ([1715-23] 1944) recommends that the flat surface of the copper should be prepared by
rubbing it with garlic juice, which would dissolve the oxide layer and produce a scoured surface
for painting.
In a number of instances, the corrosion of copper to produce verdigris actually produces
one of the copper trihydroxychlorides (discussed in greater detail in CHAPTER 4). For example,
Naumova and Pisareva (1994) report a diversity of copper compounds used as pigments in Rus­
sian frescoes from the early sixteenth century. They conclude that most historical recipes do
indeed refer to the production of copper acetates, with a smaller number producing atacamite.
This was confirmed by Banik (i989), who reported paratacamite and other basic copper chlo­
rides as pigments from fifteenth- and sixteenth-century illuminated manuscripts. These basic
chlorides were mostly sampled from undeteriorated areas of the manuscripts. This is of particu­
lar interest because a common decomposition product of verdigris salts on interaction with
chlorides is one of the copper trihydroxychlorides. The fact that the chloride salts were found
in undeteriorated areas suggests that this is not the etiology in this case.

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