Page 159 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

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and patches of azurite on a bronze stove of Ramses I (reigned 1279-1213 B.C.E.) of the Nine­
teenth Dynasty. Nielsen (1977) examined a third-millennium B.C.E. Chinese bronze ax from the
ancient Iranian site of Hasanlu that contained—in addition to calumetite — malachite, ata­
camite, paratacamite, cuprite, tenorite, cerrusite (PbC0 3 ), and magnetite (Fe 3 0 4 ) in the corro­
sion crust. The conditions necessary for the formation and stabilization of calumetite remain to
be elucidated.
The ICDD files record a variant composition for the compound atacamite, namely,
Cu 7 Cl 4 (OH) 10 -H 2 O. This variety, which is listed as a synthetic compound rather than a min­
eral, needs further characterization to determine f the synthetic material is really different than
i
the natural mineral. The synthetic form of atacamite was reported by Selwyn and colleagues
(i996) on exposed bronzes in Ottawa, Canada, and by MacLeod (1991) during his studies of arti­
facts recovered from shipwreck sites off the Australian coast.

Anthonyite The rare copper chloride anthonyite, Cu(OH,Cl) 2 -3H 2 0, has
been identified on art objects only by Selwyn and colleagues
(i996) from their research on Ottawa bronzes. This elusive mineral is very soft, only 2 on the
Mohs hardness scale, and insoluble in water but readily soluble in cold, dilute acids. Like the
other hydrated basic chlorides, anthonyite is subject to dehydration.

Cumengite and mixed Gettens (i964) found cumengite, Pb 4 Cu 4 Cl 8 (OH) 8 , as a cor­
copper-lead chlorides rosion product in the form of deep blue, highly refracting crys­
tals on the stem of an ancient Persian lamp in the collections of
the Freer Gallery of Art. Cumengite is quite rare and in nature is often associated with boleite,
Pb 9 Cu 8 Ag 3 Cl 21 (OH) 16 -2H 2 0, or pseudoboleite, Pb 5 Cu 4 Cl 10 (OH) 8 -2H 2 O. All three minerals
are found together at the type deposit at Boleo, near Santa Rosalía, Baja California, Mexico
(Palache, Berman, and Frondel 1951), so it would not be surprising f the other two salts are even­
i
tually found as corrosion products as well.
Scott and Taniguchi (1998) examined a light blue corrosion product on a sixth-century
B.C.E. bronze-and-iron bed from Lydia, central Anatolia, in the collections of the J. Paul Getty
Museum. The light blue corrosion occurs as delicate patches on the outer surface of a malachite
patina, but these patches were quite difficult to identify. Analytical studies showed that copper,
lead, and oxygen are present, and the best match of the X-ray diffraction data was to diaboleite,
Pb 2 CuCl 2 (OH) 4 , although the presence of chlorine could not be found by scanning electron
microscopy-energy dispersive X-ray analysis.
Interestingly, these mixed copper-lead minerals have been found as alteration products
on ancient lead and silver slags from smelting operations in Lávrion (ancient Laurion), Greece
(Rewitzer and Hochleitner 1989). The mines were being used from Mycenean and Phoenician

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