Page 181 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

sculpture was recoated. This finish also darkened and was replaced with another sprayed-on
brass coating after the previous one was removed by sandblasting.
In 1994 the Getty Conservation Institute and the Swedish Corrosion Institute were com­
missioned to measure the environmental parameters for the Infinite Column; to advise on the
selection of a new metallic coating to be thermally sprayed over the restored cast-iron modules,
which suffered from corrosion beneath the initial zinc coating; and to advise on a polymer coat­
ing that would best protect the newly sprayed surface.
Alloy coupons used in the testing regime, together with different organic coatings, included
89Cu5Zn5AllSn and 93Cu5Al2Ni, which gave a superior performance during outdoor weath­
ering trials compared with binary brasses of 80Cu20Zn or 63Cu37Zn. The 89Cu5Zn5AllSn
alloy had been developed as a golden-colored coinage alloy in Sweden and is thought to be
partially protected from corrosion by formation of a mixed aluminum-tin oxide on the metal
surface. This alloy effectively resists general tarnishing indoors and performed well outdoors,
remaining untarnished during three years of exposure trials. During this time, brass samples
became almost black, and bronze samples were badly discolored. A Finnish coinage alloy,
93Cu5Al2Ni, was not as effective as the Swedish alloy but is preferable to any binary brass
composition.

S U L F A T E D E P O S I T I O N IN B U R I A L E N V I R O N M E N T S
In burial environments, malachite is the usual compound that forms from the interactions of
copper-containing solutions and calcite. In laboratory studies of the same corrosion chemistry,
however, the preferred products are sulfates and chlorides. In an attempt to understand this dis­
crepancy, Garrels and Dreyer (1952) examined the conditions for the precipitation of basic sul­
i
fates and chlorides on calcite. These authors surmised that f the concentration pH curves for
-
the precipitation of brochantite and atacamite are known, it is possible to explain why these
products form in the laboratory in preference to malachite.
A copper sulfate solution is acid because of hydrolysis:
= Cu + + S0 4 " 5.6
CuS0 4
+ +
2 H 2 0 = 2 0 H " 2H +
II I I

Cu(OH) 2 H 2 S 0 4
When calcite is placed in such a solution, it begins to dissolve, and the excess hydrogen ions of
the solution react with the carbonate ions from the calcite. An equilibrium is probably reached
between carbonate ions, bicarbonate ions, and carbonic acid. This reaction then reduces the
hydrogen ion concentration at the solid surface where there are also hydrogen ions, hydroxyl

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