Page 310 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Examination of small samples of verdigris from illuminated manuscripts always presents
the problem of how to obtain a sample without obvious damage to the manuscript. Banik
(i989) devised a sampling technique that avoided this difficulty. It used a polished resin block,
equipped with a fine copper grid, that was simply pressed against the surface being sampled. He
found that a sufficient number of pigment particles adhered to the resin without the use of any
additional adhesive. The adhesion was strong enough to prevent migration or dislocation of the
particles during analysis. The block loaded with the sample was viewed under a light micro­
scope; the copper grid ensured a convenient coordinate system for electron microscopy and for
quantitative analysis of the copper compound. Analytical studies showed that with increasing
degrees of surface damage, a corresponding decrease in the copper content of the remaining pig­
ment particles occurred, as low as 8-10% by weight, making it impossible to identify the origi­
nal pigment that was present.
Banik (i989) also discovered that a recipe for making verdigris, dating to the eighteenth cen­
tury, actually produced posnjakite, Cu 4 S0 4 (OH) 6 -H 2 0, rather than a carbonate or an acetate.

Problems with verdigris Copper acetates slowly change color in binding media: with res­
ins, they form copper résinâtes; with oils, copper oleates; and
with proteins, copper-protein compounds. The reaction of copper with oleic acid was reported
by Gates (1911), who found that when clean pieces of copper were placed in oleic acid, the solu­
tion gradually became a deep green color. Loss in weight of the chemical reaction in this case
probably resulted from the liberation of hydrogen, as described by the following:

Cu + 2C 1 7 H 3 3 COOH = (C 17 H 33 COO) 2 Cu + H 2 9.2

Both stearic and palmitic acids, when melted onto copper foil and fused for a minute, produced
green-colored pastes, showing that reaction had occurred between copper ions and the con­
tiguous fused solid, resulting in the formation of copper (II) stéarate and copper (II) palmitate.
The possibility of verdigris causing problems because of its solubility was well known for
centuries. Leonardo da Vinci (1452-1519), for example, remarks that verdigris ground in oil can
last only when varnished immediately after it has dried, otherwise "it not only fades, but may
be removed by a wet sponge, especially in humid weather. This is because of its saline nature;
it becomes deliquescent in a moist atmosphere" (Eastlake 1847:458).
This behavior would be particularly true with distilled verdigris, whereas some of the basic
salts or other products masquerading as verdigris would not necessarily be affected by very
humid weather.
Eastlake comments on Leonardo's observations on humid conditions by adding that "the
mode of locking up' verdigris may exemplify the means by which all colours liable to be
affected by damp, can be rendered durable. The colour was mixed either with a strong oleo-
resinous vehicle... or with varnish only" (Eastlake 1847:459).

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