Page 332 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

deposit was thoroughly washed when the green color acquired a bluish tinge. A more yellowish
hue was apparently obtained by increasing the proportion of yellow wood.

Brown copper pigments A number of rather obscure mixtures for brown pigment have
also been produced with copper compounds. Osborn (i854) and
Field (1835) mention a brown made with "hydrocyanate of copper" (hydrocyanic or prussic acid
and copper), which had been recommended "for beauty and intensity by an English chemist of
the name Hatchett" (Field 1835: 31). There are two copper cyanides, Cu(I)CN and Cu(II)(CN) 2 ;
it is the latter cupric form that would have been produced. As this is a very poisonous pigment,
the extent to which it was put to practical use remains unknown.
Standage (1887) records a pigment known as "copper brown," which is described as being
a "ferrocyanide of copper." This is, in fact, a copper analog of Prussian blue, with the iron
replaced by copper. Copper (II) hexakis-cyanoferrate, otherwise known as cupric ferrocyanide,
Cu 2 Fe(CN) 6 , is a good brown color but is awaiting identification as a pigment, as is copper
borate, Cu x B(OH) 3 , a pale green mentioned by Salter (i869). Richter (i988) describes the iden­
tification of a potassium copper chloride, KCuCl, that is also very obscure.

Copperphthalocyanine Copper phthalocyanine, CuN 8 C 3 2 H 1 6 , was discovered by
Arthur Gilbert Dandridge and others in 1928 and was first intro­
duced as a pigment under the trade name Monastral blue at an exhibition in London in 1935. 34
Copper phthalocyanine is a tetraazatetrabenzo derivative of porphyrin; it is made from the con­
densation reaction between four molecules of phthalonitrile, which reacts at 200 °C with one
atom of copper to form the phthalocyanine salt (Roberts and Caserío 1964). It is a bright blue
microcrystalline solid with a purple luster that is practically insoluble in water, ethanol, and
hydrocarbons. Many of the phthalocyanine group of pigments have exceptionally good light-
fastness. Dahlen (1939) describes the crystalline salt as a deep blue with a strong bronze
reflection, but the dry dispersed pigment is bright blue with little or no bronze reflection. The
dispersed pigment is very close to an ideal pure blue, absorbing light almost completely in the
red and yellow, and reflecting only green and blue. The tinting strength is very strong, about
twice the strength of Prussian blue and twenty to forty times that of ultramarine. A microscope
preparation of the pigment is shown in PLATE 66.
Other varieties of the phthalocyanine group include a chlorinated copper phthalocyanine,
which produces a green dye, as well as a pigment with properties similar to those of the blue
variety (Gettens and Stout 1966). Copper phthalocyanine has been extensively used as a pigment
for inks and paints, although it is not as popular as it once was and has declined in use in recent
years. The solid can exist in two forms: the thermodynamically less stable, redder alpha form is
a better pigment than the more stable, greener, beta form {Merck Index 1983). In the presence of
aromatic solvents, heat, or high shear stress, the alpha form may convert to the beta variety.

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