Page 110 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

ingots from Nimrud, Assyria, and references in cuneiform glassmaking texts from the library
(
of King Assurbanipal 668-627 B.C.E.) that specify the use of crucibles with sealed lids, which
would have helped prevent oxidation (Oppenheim et al. 1970). Some of these instructions were
quite elaborate, for example:
[
[Y]ou place ten minas of "slow" copper compound in a clean dabtu pan. You put it] into
a hot chamber kiln and close the door. You keep a good and smokeless fire burning until the
copper compound glows red. You crush and grind finely ten minas of zuku glass. You open
the door of the kiln and throw the glass upon the copper compound. As soon as the zuku
glass becomes mixed into the surface of the copper compound you stir it a couple of times
with the rake until you see some drops of liquid glass form at the tip. When the "metal"
assumes the colour of ripe [red] grapes, you keep it boiling for a while, then you pour the
"metal" on a kiln-fired brick. This is called tersitu preparation. (Oppenheim et al. 1970:35)

Oppenheim and coworkers provide many different recipes for making glass colored by cop­
per, bronze, or copper compounds.
I RED GLAZES Freestone and Barber (1992) examined the red
glaze of a saucer/dish from the British Museum collections that proved to be a Qing dynasty
(1644-1911) imitation of an early Ming dynasty (1368-1644) vessel. The researchers discovered
that the red color was concentrated in the bottom layer of the glaze, which contained very fine
particles, less than 1 μιη in size. The layer itself was less than 100 μιη thick. Examination of this
layer with the transmission electron microscope revealed that the particles were metallic cop­
per, rather than cuprite, present in two size groups: fine polygonal crystals 0-100 nm in size
5
2
and coarser spherical droplets 00-300 nm diameter. These copper particles probably nucle­
ated and grew from cuprous oxide in the molten glaze. According to modern nanoparticle-
calculation theory, the initial particle size of the copper precipitate would be less than 50 μιη.
As the particles increase in size, less light would be transmitted through the glass, which would
become brownish, dull, and eventually opaque (Freestone and Barber 1992). The Chinese arti­
sans showed great skill in making this glaze, which depends on producing a very thin colored
layer containing reduced-copper particles beneath a colorless zone. Even more remarkable is
that there is not much difference in copper content between the colorless and red-colored lay­
ers in the glaze. It may be that the copper is still in the cuprous state in the colorless layer but
reduced to metallic copper in the bottom layer.
Ceramic objects colored with glazes containing reduced metallic copper are often referred
to as copper lusterware, exemplified by a type of iridescent ceramics from Spain. There is also
an English ceramic with a glaze in which the reduced copper metal imparts a natural copper
color. Gold foil on a brown-glazed ceramic ground was also used to create an effect similar to
that of bronzed copper (Savage and Newman 1985).

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