Page 273 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

straightforward: under oxidizing conditions, the cupric ion is formed, while under reducing
conditions, cuprous ions —and even reduced copper—can form (see CHAPTER 2).
The use of copper compounds in glassmaking may have been influenced by the availability
of copper-containing slags in the Bronze Age. This may also apply to red opaque glasses made
in the early medieval period when slags produced during the refining of silver coinage debased
with copper could have been put to practical use (Mass, Stone, and Wypyski 1997).
Copper was an important colorant for blue glazes applied to faience (as discussed later
in this chapter) and steatite (soapstone) from the fourth millennium B.C.E. and later. It was
used to make Egyptian blue pigment and pale turquoise frits (both discussed in the following
section). From the Eighteenth Dynasty of Egypt onward (1550-1307 B.C.E.), the introduction of
glass marked the beginning of the use of cobalt as a blue colorant (Tite et al. 1999). Used in a vit­
reous faience, the colorant corresponded to a product Lucas (1962) calls Lucas variant D.
Vandiver, Fenn, and Holland (1992) examined the glaze on a third-millennium B.C.E.
quartz bead from Tell es-S weyhat in Syria. Microprobe analysis and replicate melts showed that
the composition was 60% quartz, 20% CuO, and 20% flux that was not precisely determined but
was thought to be either soda or potassia. The presence of a flux had to be inferred in this study
because it had weathered away; only with this amount of flux can a glass with such a high
copper oxide content be molten at 800-1000 °C. Very few glass and glaze compositions are
known from this time period, and none has such a high copper content as this glaze. Techno­
logically innovative processes were being established during this time in the Near East. Since
the successful smelting of copper had already been established by this time, it is natural that cop­
per compounds were finding use in blue-green glazes or other colored products, such as the
opaque-red glasses, which started around the second or first millennium B.C.E. and continued
to the medieval period.

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