Page 355 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

• cuprous chloride (nantokite) • tin-oxide patina
• sound copper metal • lead carbonate (cerrusite)
• basic copper carbonates • sound bronze metal
(malachite, azurite) EH malachite, massive and fibrous
I 1 cuprite, with larger crystals O cuprite
below "marker layer" • eutectoid remnants in corrosion

F I G U R E 11. 3 Schematic diagram comparing the conventional type of corrosion pustule containing chloride corrosion
with the pustule observed on the Togati bronze shown in F I G U R E S 11.1 and 11.2 .

dioxide readily available but had no corrosion due to chloride ions. In this scenario, the corro­
sion products and pustules should show no evidence of chloride corrosion, nor should the pus­
tules be associated with cuprous chloride. This hypothesis was, in fact, confirmed by Geilmann's
analytical and microstructural studies. This corrosion environment was not deficient in carbon
dioxide, as seen from the extensive malachite deposits on the surface of the bronzes, the conver­
sion of the lead globules in the alloy to lead carbonates, and the presence of azurite crystals on
the Nike and the Ceres bronzes. In the case of the Ceres, azurite can be seen growing on indi­
vidual malachite fibers as tiny deep blue compact crystals. The outer surfaces of the corrosion
pustule on the Togati are also malachite rather than basic copper chlorides. This kind of warty
corrosion on all four of the bronzes studied is a strong argument that all of them were buried in
the same environment. Unlike warty corrosion associated with chloride ions, the kind seen on
the surface of these four Roman bronzes is chemically stable under storage or exhibition con­
ditions. Such objects do not require especially strict humidity control. The Roman bronzes had
been kept in a special case with the humidity regulated to below 40%, but since there is no chlo­
ride instability in the corrosion, such measures were unnecessary. During conservation treat­
ment, there is no need to remove warty corrosion unless it is justified for aesthetic reasons.

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