Page 278 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

I PIGMENT PROPERTIES Elemental studies of Egyptian blue
show that the concentrations of the three principal components are 60-70% Si0 2 , 7-15% CaO,
and 10-20% CuO by weight. Most samples contain from 5% to 40% excess silica. Tite, Bimson,
and Cowell (i987) deduced that the relative amounts of CuO and CaO are critical to controlling
I
the nature of the frit produced. f the CuO content exceeds the CaO content, then Egyptian blue
I
crystals are formed, and the frit has an intense blue color. f the CaO content exceeds the CuO
content, however, Egyptian blue crystals do not occur; instead, excess lime is precipitated from
the glass, often as wollastonite, and the copper oxide remains dissolved in the glass to produce
a characteristic pale blue color. To determine the ancient methods used in fabricating Egyptian
blue, Tite, Bimson, and Cowell (i987) conducted a series of studies on a wide variety of pigments
with textures that varied from soft and friable to hard and semivitrified and with colors that
ranged from light to dark blue. Their samples included material from the Late Old Kingdom of
Egypt and from sites in Syro-Mesopotamia of the Late Bronze and Iron Ages. A ternary plot
of some compositional data was made by Tite (i984), using CaO, Si0 2 , and CuO as end mem­
bers. This ternary diagram is useful for plotting the stoichiometric composition of cuprori-
vaite. Riederer (1997) used the same plot to illustrate the compositional clusters for Egyptian
blue samples from Tell el Amarna, Egypt; Nimrud and Nineveh, Iraq; and from Roman-period
sites in Egypt. Riederer found that the glassy phase, which is usually present in the pigment,
provides long-range interconnection between the crystalline phases and that this causes the
hardness of the frit to increase with increasing alkali content. The alkali content is a primary
factor in controlling the microstructure of the pigment; it determines how "glassy" the tint or
pigment will be.
Neo-Assyrian objects examined by Tite, Bimson, and Cowell (i987) often had a low alkali
content, in the region of 0.5%, and were softer and more friable than earlier examples from
Egypt, which had a higher alkali content, from 1% to 5%. On the basis of their laboratory work,
the researchers decided that it was unnecessary to introduce the idea of multiple firings to
explain the production of coarse-textured Egyptian blue. A single firing at 900 °C was sufficient
for frits with a high alkali composition; a firing at 1000 °C was adequate for the low-alkali vari­
ety. This produces coarse-textured Egyptian blue, which is often quite dark with a hardness that
depends on the amount of the interconnecting glassy phase. The coarse frit could subsequently
be ground for use as a pigment with the range of color changed according to the degree of grind­
ing; the pigment becomes progressively paler with finer grinding, as also happens with azurite
and malachite.
Tite (1984) does suggest a two-stage firing cycle to produce small solid objects of fine-
textured material. The frit is ground and molded to shape between the first and second firings,
which produces Egyptian blue crystals uniformly interspersed among the unreacted quartz
grains. This second firing process probably was carried out between 850 °C and 950 °C. The end

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