Page 323 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

C O P P E R SALTS AS P I G M E N T S

Green copper pigments There are many different green pigments, some of which have
been described as "verdigris" but which are actually other types
of copper salts. The copper-arsenic pigments also have a place in this discussion.
I BRUNSWICK GREEN The pigment known as Brunswick
green, whose use goes back to antiquity, was considered a verdigris because it was prepared
from copper corroded in the presence of sodium chloride. This is a misnomer, however, since
Brunswick green is an example of one of the basic copper chlorides.
Riffault, Vergnand, and Toussaint explain that malachite was too expensive to be generally
used in painting in the late isoos and add that "it has been advantageously replaced, first, by the
greens of Brunswick and of Bremen, and afterwards, by that of.Schweinfurt and by Mittis green,
which are more durable than the two former" (Riffault, Vergnand, and Toussaint 1874:227).
Brunswick green was made by the action of hydrochloric acid on copper ores, which
produced a variety of copper trihydroxychlorides. Brunswick green, which began to be made
around 1795, was named after the town of Brunswick, Germany, where it was prepared by the
brothers Gravenhorst. They made the pigment by covering copper filings with a solution of
ammonium chloride and leaving the mixture in a closed container. The solid that formed was
then washed and dried. It was probably primarily atacamite or possibly an ammonium copper
chloride salt. This recipe was replicated in the laboratory using pure copper filings that were
kept partially covered with ammonium chloride for four days, as described in APPENDIX B,
RECIPE 24. The product that formed was analyzed by powder X-ray diffraction and matched the
reference data for atacamite, Cu 2 (OH) 3 Cl. APPENDIX D, TABLE 24, shows the data for this
preparation.
Brunswick green was used both for oil painting and for printing, but the extent of its use
remains obscure. By the 1920s, the name Brunswick green no longer referred to one of the cop­
per trihydroxychlorides but to a green prepared from a mixture of chrome yellow and Prussian
blue (ferric ferrocyanide) that included varying amounts of barytes, depending on the grade of
the pigment (Beam 1923). This mix of Prussian blue and chrome yellow was a very common
pigment at that time. Bremen green and Bremen blue were alternative names for green verditer
and blue verditer, respectively. Roy (1993) confirmed these descriptions, noting that many old
samples in the collections of the National Gallery of Art that are labeled "Brunswick green" are
really mixtures of Prussian blue with various yellow, brown, and black pigments, some of which
contain chrome yellow.
Other unusual copper pigments, related by name or tradition to these synthetic pigments,
were also produced during the nineteenth century. Riffault, Vergnand, and Toussaint (i874)
describe an interesting example of a copper blue derived from Bremen blue that used both cop­
per and zinc salts. The synthesis began with a solution of copper nitrate, which was heated and

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