Page 101 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Sulfide inclusions originate from impurities in smelted copper ores, which are frequently
derived from sulfidic ore bodies or copper-sulfarsenide ores. Cuprite can originate from reac­
tions that occur when copper is melted, since copper absorbs both oxygen from the air and
hydrogen from the decomposition of water vapor. When the copper cools and solidifies, the
oxygen is precipitated in the form of cuprite and the hydrogen expelled. The expelled hydro­
gen may then react with the cuprous oxide, producing steam that may become entrapped in the
metal. Wooden poles were once used to stir molten copper to deoxidize it, which left about
0.05-0.06% oxygen in the metal to form cuprite. In this way, the production of steam balanced
the natural shrinkage of the metal, and a casting pipe was avoided (Smithells 1937).
Cuprite can also be formed as a result of interaction with organic compounds that can be
easily oxidized, such as aldehydes, in Benedict's and Fehling's solution (a classic use of cuprite
in a chemical test), and so on. This may have an indirect bearing on possible reactions with
decaying organic matter in burial, where the organic material may be oxidized and the copper
salt reduced to cuprite.
In PLATE 10, a photomicrograph of a typical bronze patina viewed under polarized
light illustrates the pleochroic colors of the corrosion crust. The red-and-yellow cuprite
layer is clearly visible under a malachite crust containing soil minerals, with sound metal
below the cuprite.
PLATE 11 shows a polished cross section of a Han dynasty (206 B.C.E.-220 C.E.) buckle
viewed under polarized light. The fine pseudomorphic retention of the dendritic structure of
the cast bronze is visible in the cuprite corrosion that has replaced the metal. A faint dendrite
pattern can even be seen in the malachite crust over this cuprite layer and appears to be excep­
tionally well preserved in this example. It is uncommon for malachite to preserve pseudomor­
phic structural detail, and few examples of such structures have been published. They do occur,
however, especially in bronzes from China, where the burial conditions appear to be favorable
for this kind of preservation.
PLATE 12 shows a cuprite crust on a small Bronze Age ax from Ireland. The cuprite layer
has preserved fine lines that represent the strain lines of the original copper grains. Such strain
lines result from heavy cold-working of the ax to harden its edge, which was a common fabri­
cation technique. Other comparable metallographic studies confirm that this edge hardening is
due to deliberate cold-working on the part of the metalsmiths and not from hardening that
occurs during use (Allen, Britton, and Coghlan 1970).
Infrared spectroscopy reveals that metal oxides are usually nonabsorbing above 1000 cm" 1
but show at least one strong absorption band and several weaker ones in the 300-900 c m - 1
region. Cuprite shows a very strong absorption band at 615 c - 1 due to lattice vibrations
(O'Keeffe 1963; Carabatos, Diffine, and Sieskind 1968). Giangrande (i987), who confirmed that

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