Page 281 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

is obtained. The green frit is also produced, according to Ullrich, f the ratio of CuO to CaO
i
(lime) is not ι : 1. This is not quite the same conclusion reached by Tite (i987), however, who
found that f the CuO content exceeded that of the CaO, Egyptian blue would form and the frit
i
would assume a deep blue color.

Terminology The terms used to discuss the interesting developments in the
manufacture of blue-colored faience, glazed steatite, Egyptian
blue, Egyptian green, and pale blue frits can be quite tangled. Tite (i987) makes the following
distinctions among faience, glazed stones, and frits.
I FAIENCE Faience, as found in ancient objects, is composed
of a modeled core of relatively coarse quartz grains that are bonded together by varying amounts
of glass (Tite and Bimson 1986). The core material, or "body," is usually covered by a glaze free
of quartz, although the glaze may consist of quartz sand, lime, alkali, and a copper colorant that
is applied to the quartz body.
The glaze can be applied by efflorescence, cementation, or direct application (Tite, Free­
stone, and Bimson 1983; Vandiver 1982). In the efflorescence application, the glazing mixture —
comprising an alkali, lime, and copper compound—is mixed with some of the moistened quartz
material used to produce the body. As the form dries, components of the glaze are carried to the
surface by efflorescence. When fired, these components react with the quartz to form the bright
blue glaze that is characteristic of blue faience.
In the cementation process, the quartz body is buried in the glazing mixture — consisting
of quartz, alkali, lime, and copper compound—which, when fired, reacts with the body to form
a glaze. In the direct process, raw or partially fired glazing mixture is applied to the surface of
the quartz body prior to firing (Tite et al. 1999). The efflorescence method is characterized by
extensive interstitial glass and a thick interaction layer, while minimal interstitial glass is
formed during the cementation reaction; the cores remain white and friable under their green
blue glaze. The direct application technique is characterized by minimal interstitial glass and
a thin interaction layer but with a thick glaze layer (Tite and Bimson 1986). The faience, of
crushed quartz, often has natron (soda-rich ash) as the alkali. The lime used in the formulation
is thought to be derived from plant ash. Further details about faience can be found in Tite (i983),
Tite and Bimson (1986), and Tite and coworkers (1999).
Mao (1999) recently studied Egyptian faience artifacts from the Ptolemaic period (300-
30 B.C.E.) in the collections of the Walters Art Gallery, Baltimore. Mao found a consider­
able quantity of lead in the outer glazed layer. The lead ranged from 2.5 to 8.5 weight % PbO.
Although lead glazes are known from earlier periods, the use of a lead-glaze coating on blue
Ptolemaic faience figures is an interesting discovery because it suggests that samples of Egyp­
tian blue from this period may also contain lead.

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