Page 324 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

then decomposed with a clear solution of potassium carbonate. As soon as the resulting effer­
vescence slowed down, additional potassium carbonate solution was added in small amounts.
The precipitated green copper carbonate was then added to a new solution of copper nitrate to
which a potassium zincate solution was also added. This solution was made by dissolving clip­
pings of metallic zinc in a solution of potassium hydroxide. When the solution cleared, it was
added to the solution of copper carbonate in copper nitrate. This precipitated a light blue syn­
thetic pigment that is sometimes called light Bremen blue.
I SCHEELE'S GREEN Scheele's green is copper arsenite,
CuHAs0 3 , a pigment discovered by the Swedish chemist Carl Wilhelm Scheele (1742-86) in
3 2
1775 while working on arsenic compounds. Scheele was well aware of its poisonous properties
and warned others of the danger in using the compound as a pigment. The formula for Scheele's
green is variable, depending on the preparation conditions. A synthesis by Schweizer and
Mühlethaler (i968) suggests that copper diarsenite, 2CuO-As 2 0 3 -2H 2 0, forms in the cold;
copper orthoarsenite, 3CuO-As 2 0 3 -2H 2 0, forms in hot solutions; and copper metaarsenite,
CuO'As 2 0 3 , forms when the arsenic content is increased.
These arsenic-containing pigments were often referred to as "arsenates" in older literature,
but this is not correct. The arsenates are based on As0 4 ~, and these pigments are based on
3
3 ~, which makes them arsenites. Some of the arsenites may be polymeric; for example, the
As0 3
metaarsenites are composed of extended anionic chains formed by oxygen-bridged, pyramidal
units (King 1996). This helps to explain some of the difficulties with identification some­
As0 3
times encountered with this group of pigments. Work by Stavenhagen (i895) suggested that the
composition of a particular copper arsenite depended on the method of synthesis. Both cop­
per diarensite and orthoarsenite are a lighter shade of green than the color usually associated
with Scheele's green. All of these factors make it difficult to identify the pigment from the usual
microsample.
Scheele's green has a deep green color and reasonable covering power as a pigment,
although the extent of its use as a pigment is not entirely clear. A process for its production was
patented in England in I812, and the pigment was then sold as "patent green." The original
preparation mixed potash and a pulverized material called "white arsenic" (apparently arsenic
sulfide, As 2 S 3 , rather than arsenious acid, As 2 0 3 ) in water. The solution was heated and slowly
added to a warm solution of copper sulfate. It then was allowed to stand until a green precipi­
tate formed that was then washed and dried.
A preparation by Liebig (1823) began with dissolving verdigris in warm vinegar, then
adding an aqueous solution of arsenic sulfide to obtain a dark green precipitate. Additional vine­
gar was added to redissolve the precipitate, and the solution was then boiled and cooled to pro­
duce the brighter green of the copper arsenite.

T H E ORGANI C SALT S O F C O P P E R
307