Page 139 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Chlorides and Basic Chlorides

C H A P T E R 4

The emerald green incrustations abound in the submuriate of copper,
and the red consist almost entirely of the protoxide of copper. These two
compounds I have never witnessed spread over the whole of a coin, but
more or less mixed with rusts of a different kind, studding the surface
in the form of little crystalline elevations.—JOHN DAVY 1

T H E C O P P E R C H L O R I D E S

One of the most troublesome group of minerals for bronze stability are the copper chlorides—
the "submuriate of copper" observed on copper coins by Davy (i826). The presence of cuprous
chloride, CuCl, as a corrosion product adjacent to the metallic surface can create long-term
problems for the stability of an object. The slow, progressive corrosion of bronzes, known as
"bronze disease" (discussed later in this chapter), is usually attributable to chlorides.
Some copper chlorides have been used as pigments; some also occur as pigment impurities
or components to other pigments and, in certain instances, may be present as a result of the
deterioration of an original pigment that has been replaced by one of the copper chlorides.
The most important copper chlorides in bronze corrosion are nantokite (cuprous chloride),
CuCl, and the copper trihydroxychlorides: atacamite, paratacamite, clinoatacamite, and botal-