Page 123 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

in whiskerlike crystal growth. Some examples of these different forms are shown in PLATE 20.
Even on the same object, well-formed crystals may display a variety of crystalline habits. On the
cased Greek bronze mirror discussed in CHAPTER 1, for example (see CHAPTER 1, FIGURE 1.2),
crystals had signs of elongation that were optically positive or optically negative (Scott 1991).
Fibrous malachite, which has been found growing as curly, fiberlike crystals on bronze antiq­
uities (Scott 1991), has been observed to consist of parallel aggregates of finely elongated mala­
chite crystals, as shown in PLATES 21 and 22.

Malachite as a pigment There is no evidence that malachite or azurite was used as pig­
ment for Paleolithic rock art (Ucko and Rosenfeld 1967). The
minerals, however, were used extensively for painting in many different media from antiquity
until relatively recently.
Malachite and azurite must be ground quite coarsely to retain a good color for pigment use;
the minerals become progressively paler as the particle size is reduced. Malachite is most effec­
tive as a pigment in tempera. Although it is a bright green, malachite covers poorly in oil and
not very well in watercolor. Gettens and Fitzhugh (1966) describe the color, reflectance spectra,
infrared data, notable occurrences in paintings, and other characteristics of malachite used as a
pigment. Art and Archaeology Technical Abstracts {AAT A) 4 provides more than five hundred ref­
erences to the identification of malachite in paintings, especially in Asia, where its use in Japan,
China, and Korea has been extensive.

Malachite in bronze patinas Although malachite is only a minor constituent of patinas that
develop on exposed bronze objects, it can be a significant com­
ponent of patinas that develop during the corrosion of copper alloys buried in the soil, where
the first product to form adjacent to the metal is cuprite; malachite usually forms over this ini­
tial cuprite layer. The uniform growth of this corrosion accounts for the attractive patina seen
on many bronze antiquities.
The natural transition of metal to cuprite to malachite is very difficult to replicate in the
laboratory. In fact, most of the recipes for producing artificial green patinas on copper alloys,
such as those compiled by Hughes and Rowe (1982), do not result in malachite formation over
a cuprite layer. Consequently, the existence of this type of corrosion, supported by analytical
and metallographic studies, is a good indication of the authenticity of an artifact.
I BANDING A striking feature sometimes seen in polished
sections of corroded bronze antiquities is a corrosion crust with multiple, alternating layers
of malachite and cuprite. This structure is reminiscent of Liesegang phenomena described
in CHAPTER 2.
The processes behind the banding of malachite with cuprite were explored experimentally
by Krishnan and colleagues (i982), who examined the anomalous stratification of copper car-

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