Page 417 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

single-bond distance. Practically all cupric salts are blue or green. Exceptions are caused by
strong ultraviolet absorption bands, charge-transfer bands, trailing off into the blue end of the
spectrum and causing the salts to appear red or brown. For example, cupric oxide (tenorite),
CuO, and some of the copper sulfides are almost black.
Some cupric salts, such as cupric chloride, dissolve easily in water. The basic compounds,
however, such as the most common basic sulfate, brochantite, Cu 4 S0 4 (OH) 6 , or the common
basic chloride, paratacamite, Cu 2 (OH) 3 Cl, are practically insoluble. In water, the aquo ion of
copper can be written as Cu(H 2 0) 6 ] , with two of the water molecules farther from the metal
[
2 +
atom than the other four. When ligands are added, some of these water molecules may be dis­
placed. A classic example is ammonia. This ligand can displace up to five water molecules to
make attractive dark blue solutions capable of dissolving cellulose.
Some ligands that coordinate through oxygen form a large number of cupric complexes
that are difficult to characterize. For example, cleaning copper objects with citric acid or tartaric
acid solutions may form polynuclear complexes of complicated or unknown structure. Oxalate,
glycerol, and various thio compounds form cupric complexes as well, hence the ability of alka­
line glycerol to dissolve copper salts when used to clean copper objects.
Cupric salts play an important role as catalysts in many systems where C u - C u 2 +
+
oxidation-reduction cycles are involved. Copper occurs in several enzymes, such as phenolase
and lacease, and as cuprous copper in hemocyanin, an essential respiratory function in many
invertebrate animals, such as crayfish.

BRONZE

Bronze is an alloy of copper with tin as the primary component; minor ones can be zinc, nickel,
or lead. To understand the corrosion of bronze alloys, it is necessary to know something about
the various phases that may be present in the alloy. These phases may corrode differently,
depending on their composition and their environmental contexts. In the phase diagram for the
copper-tin system, many reference books show a low-tempera ture region that contains an alpha
solid solution and an epsilon phase. For ancient and historic bronze alloys with a composition
of less than 28% tin, the epsilon phase is of no importance.
In the phase diagrams for the copper-tin system, which apply to practical alloys, the low-
temperature-phase field of the alpha+epsilon is ignored in two sections of the diagrams. This
is because the epsilon phase never appears in bronzes containing up to around 28% tin that have
been manufactured by conventional means because it would require thousands of hours of
annealing, which is unrealistic even for modern bronzes. Further information about the com­
position of the relevant phases and types of alloys can be found in Cottrell (1975), Scott (1991),
Samans (i963), and Lechtman (i996).

A P P E N D I X A
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