Page 106 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

made from copper containing from 1% to 3% gold. X-ray diffraction studies show that the black
patina is cuprite, but transmission electron microscopy studies indicate that gold atoms are also
present in the cuprite layer, forming dispersed clusters. These aggregates of gold atoms, in a
superlattice-type structure, must be responsible for the alteration of cuprite from red to black,
although precisely how is not understood. Murakami (1993) suggests that the fine gold particles
or clusters may be of a critical size to absorb the red portion of the visible spectrum, resulting
in a black color. Murakami notes that a variant of the Japanese term shakudo can be translated
as "crow gold" or "black gold." The type of shakudo used in the Edo period (16OO-1868), how­
ever, sometimes contained small amounts of silver instead of gold, indicating that silver was also
used as a deliberate alloying addition.
A copper alloy containing more than 10% gold is known as shi-kin, "purple gold." This par­
ticular alloy may be related to a Chinese copper alloy, called "purple sheen gold," that contains
small deliberate additions of gold, often 1-3% (Needham 1974). Like the Japanese shakudo, these
Chinese alloys also have an artificially patinated surface that is principally of cuprite.
One of the earliest publications describing experimental work on Japanese shakudo is that
of Uno (1929), who had determined that copper with about 5% gold could produce these black
patinas. Uno prepared a large number of test alloys and treated them with several different pati-
nation solutions. His laboratory work showed that various minor alloying elements, such as
arsenic or antimony, could also influence the patinated color. Giumlia-Mair and Lehr (1998)
investigated the effects of small additions of gold, silver, arsenic, iron, tin, and lead on the sur­
face color of the alloy after a number of different patination procedures. Giumlia-Mair (1996)
encountered lustrous, dark patinas on copper objects from ancient Egypt that did not contain
any gold but rather small amounts of arsenic, tin, iron, and lead.
i
It is difficult to know f certain alloys were produced deliberately with trace elements to
influence surface coloration. Many ancient bronzes with low tin content might qualify as a spe­
cial composition, since iron, arsenic, lead, and antimony are the trace impurities most com­
monly found in copper alloys. For example, the magnificent Chalcolithic bronze hoard found
at Nahal Mishmar, Israel, includes many objects made in a copper alloy with high amounts
of arsenic and antimony (Shalov and Northover 1993). The objects display a lustrous, dark plum-
colored patina. Because of their ancient origins, however, no one would suggest that these alloys
had been deliberately made with a special composition to influence the coloration.
There is strong evidence, however, that alloys with different compositions were sometimes
used to manipulate the surface color of objects. Gold may always be viewed as a deliberate addi­
tion. For example, Giumlia-Mair and Quirke (1997) examined a Syro-Palestinian sword from
Balata Shechem, Israel, now in the Ägyptische Sammlung Museum in Munich, that had a blade
made of bronze with about 12% tin, and a blue-black midrib of copper alloy containing about
0.5% gold and 3% arsenic. Examples of black-surfaced Egyptian bronzes from the collections

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