Page 103 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

McKee and colleagues (i987) show interesting cross sections that reveal growth planes
in some redeposited copper globules in a thin film of patina on a Chinese bronze ritual ves­
sel from the Rewi Alley Collection at the Canterbury Museum in New Zealand. Scanning
electron microscopy indicates that the thin film contained copper, tin, sulfur, and lead. The
patina film may have formed during corrosion in a selective reaction of the alloying elements
tin and lead with sulfate or sulfide ions in the groundwater. This was followed perhaps by
renewed corrosive events in which copper redeposition occurred in the form of discrete par­
ticles within the patina.
McKee and coworkers also suggest that, in some cases, redeposited copper could fill the
spaces formerly occupied by lead globules in leaded tin-bronze alloys. The photomicrograph in
PLATE 13 shows redeposited copper at some depth in a ceremonial ax dated to about 500 B.G.E.
from the Luristan region of Iran. The sample was color etched in acidified thiosulfate etchant
(Scott 1991), revealing that corrosion had occurred in the alpha+delta eutectoid phase, which
resulted in deeply corroded channels within the otherwise sound alpha-phase matrix. As a result
of the corrosion of the tin-rich part of the alpha+delta eutectoid, redeposition of copper
occurred in regions of the ax many thousands of microns below the patina. Redeposited copper
was also found in the junction or space between the blade and the cast-on handle, indicating that
redeposited copper can fill voids as well as replace particular phases or regions.
Chase (1994) discusses an interesting example in which redeposited copper has replaced the
cuprite marker layer on a Chinese zodiacal mirror from the Sui dynasty (sei - 6I8) in the collec­
tions of the University of Michigan. This is a rare event, and only a few objects are known to
show this kind of pseudomorphic replacement by copper. Sometimes there is no pseudomor-
phic preservation of structure at all, particularly where the cuprite has formed euhedral crystals
or has developed as a spongy and sometimes porous layer.
I
I BANDING N CUPRITE PATINAS Cuprite corrosion may exhibit
banding. Sometimes this is impressively developed with multiple layers of cuprite and mala­
chite sandwiched together in a structure reminiscent of Liesegang phenomena, first reported by
the twentieth-century German chemist Raphael Eduard Liesegang in 1896. This phenomena
may be understood as follows: I f two reactants are mixed together, a banded or sequential
i
array of chemical reactions can be created n liquid-phase homogeneous systems that are
collectively known as the Belousov-Zhabotkinskii reactions. The possible reactions between liq­
uid and solid phases, which are classed as heterogeneous systems, can then give rise to periodic
precipitation of the products, which is commonly referred to as Liesegang phenomena (or
Liesegang rings).
Typical examples are shown in PLATE 14. The interlayered deposition of malachite and
cuprite from a corroded bronze pin in the collections of the Iran Bastian Museum, Tehran, is
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shown in PLATE 4 A . Some thick lines and many fine ones are visible in this photomicrograph,

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