Page 107 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

of the British Museum are discussed by Craddock and Giumlia-Mair (1993). As research pro­
gresses, numerous other examples are being discovered, such as the mid-nineteenth-century
B.c.E. statuette of Amenemhet II from al-Fayyum, Egypt, and a bronze crocodile from the same
I
period in the Ägyptische Sammlung Museum, Munich (Giumlia-Mair and Lehr 1998).
I SURFACE TREATMENTS TO INFLUENCE PATINA DEVELOPMENT
A variety of surface treatment techniques were used in antiquity to influence the kind of patina
that would develop on an object. Giumlia-Mair and Lehr report a Greek process for patination
that calls for quenching red-hot copper alloy in aqueous solutions, for example. The principal
patina expected from this kind of heat treatment and quenching would be one of copper oxides,
either cuprite or tenorite.
Using a verdigris, copper sulfate, and alum mixture to chemically treat copper alloys con­
taining small amounts of added gold produces a lustrous black cuprite patina containing small
amounts of gold; pure copper treated in this way would show only the red coloration of cuprite.
The use of this kind of patinating solution in Japan, where it is known as nikomi-chakushoku, can
be traced back at least six hundred years. Replication work for this technique is described in
APPENDIX B, RECIPE 1.
Allusions by Plutarch (ca. 46 — ca. 120 C.E.) to black-patinated bronzes were not understood
for many years. This passage from his Mor alia is much easier to grasp today:
[H]e did however, admire the patina of the bronze, for it bore no resemblance to verdigris
or rust, but the bronze was smooth and shining with a deep blue tinge [I]t was not by
art, as they say, but by accident that the Corinthian bronze acquired its beauty of colour; a
fire consumed a house containing some gold and silver and a great store of copper, and
when these were melted and fused together, the great mass of copper furnished a name
because of its preponderance. 4

PLATE 15 shows an example of a black-surfaced Egyptian bronze of the god Ptah in the col­
lections of the Ashmolean Museum, Oxford. Its attractive red color indicates that cuprite may
have been deliberately prepared from copper for such uses, and others of a more practical
nature, as this extract from Pliny the Elder suggests:

The flower of copper also is useful as a medicine. It is made by fusing copper and then trans­
ferring it to other furnaces, where a faster use of the bellows makes the metal give off lay­
ers like scales of millet, which are called the flower. However when the sheets of copper are
cooled off in water they shed off other scales of copper of a similar red hue—this scale is
called by the Greek word meaning "husk." 5

Pliny also describes burning corroded copper and pounding it in a mortar of Theban stone,
washing it with rainwater, pounding again with more water, leaving it until it setdes, then

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