Page 109 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

of magnesium and potassium; the latter are thought to come from plant ash or possibly from
evaporated river water that could have been used as a source of alkali. The antimony con­
tent may be as high as 3%, but the colorant is cuprite, from 5% to 10% by weight. The first-
millennium glasses from Nimrud and Toprak Kale, Assyria, are fundamentally different and
contain about 25% lead as PbO. The concentrations of silica, lime, and soda are low compared
with those of the earlier samples, but this glass, too, is colored with about 10% of cuprous oxide.
These high-lead glasses are a lustrous opaque red and contain branched dendritic structures of
cuprite, along with common relict quartz grains, occasional large copper metal droplets, and
some diopside.
The cuprite in the low-lead glasses is much less well developed, and crystallites of cuprite
may be less than 10 μιη in size. Very small droplets of metallic copper may also be present. Some
of these glasses from Alalakh, Assyria, differ from the others in having metallic copper as a
significant phase in equant subangular grains up to 10 μιη in diameter. The major colorant in
these glasses is cuprite, although it has been suggested that the fine grains of metallic copper
could be responsible for the coloration of many other opaque red glasses from antiquity.
Growth of the very fine copper particles in ruby glasses that make the glass opaque also
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tend to dull the red color somewhat. n addition, a glass that precipitates metallic copper will
also enter the field of cuprite stability so that coloration of ruby glasses thought to be colored by
copper may actually be due to cuprite. Later work by Freestone and Barber (1992), however,
showed that Chinese glazes can also be colored by very fine copper deposits.
For glasses that are colored by cuprite, the size and shape of the particles can influence
the resulting color considerably. For example, f the particles are finely crystalline, the color
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of the cuprite can change to a yellow or reddish yellow. This alteration was investigated by
Stranmanis and Circulis (1935) in their article on yellow cuprous oxide. Typical microstructures
for some of these cuprite-colored red glasses include dendritic cellular cuprite zones outlined by
copper particles. In cross section, the cuprite appears red and the copper yellow.
An unusual example of the dichroism of cuprite, reported by Twilley, 9 is shown in
PLATE 16. The thin sections are of a fifteenth-century Chinese cloisonné enameled vessel from
the collections of the Los Angeles County Museum of Art. Dark red-brown cuprite crystals in
the red enamel are visible when viewed under crossed polars (see PLATE 16A); without bright
field plane polarized light, the dichroic character of cuprite is revealed, and the crystals now
appear blue (see PLATE 16G). This effect is not often observed for cuprite.
To make an opaque red glass colored by cuprite, care needed to be taken to keep the
copper in the cuprous state so it could form cuprous oxide rather than producing the glass in
the cupric state, which would discolor the final product. The most probable method of produc­
tion was to maintain the copper in the cuprous state during melting and heat treatment. This
approach is supported by archaeological finds, including a blanket of charcoal found on some

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