Page 282 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

GLAZED STONES Another method of producing small objects
was to carve soft stones into the desired shapes and then fire them, yielding so-called glazed
stones. A soft rock, such as steatite, was often used as the core material. Steatite consists princi­
pally of talc, Mg 3 Si 4 O 1 0 (OH) 2 , and has a fine-textured microstructure. On firing, the talc is con­
verted to ens ta ti te, Mg 2 Si 2 O e , and the body becomes harder. The glaze that results forms a
practically continuous surface about 100 μπι thick.
FRITS Frits of Egyptian blue are produced by firing a mixture
of quartz, lime, alkali, and copper colorant. These differ from faience in having significantly
higher percentages of lime (CaO content about 8-13%) and copper oxide (6-20%) throughout
the body of the frit.

Pigment A conundrum surrounds the origin of blue and green colors on
deterioration mystery wall paintings in Egyptian tombs from a range of dynasties. At
first, the colors were thought to be original copper carbonate
and copper silicate pigments, but then a peculiar deterioration process was proposed, involving
Egyptian blue and Egyptian green pigments as the possible source of the colors. The mystery,
however, has yet to be solved satisfactorily.
Lucas (1962) had reported that azurite, malachite, and chrysocolla were found in many
tombs from different dynasties, but this was discounted by El Goresy and coworkers (i986), who
found that basic copper chloride was the principal pigment used in tombs from the Fifth
Dynasty in Saqqara to the Twelfth Dynasty tomb of Khnum-hotep in Beni Hassan. These
researchers state, however, that some of these pigments consisted of granules of about 50 μπι in
diameter with a prominent concentric texture and cores composed of copper carbonate (mala­
chite). The copper chlorides formed a thick, fluffy mantle with a diameter of 10-20 μπι around
this carbonate core. The occurrence of the copper chlorides as thin films over carbonates and
also over silicates led them to state that "the copper chlorides are definitely synthetic products,
perhaps the oldest ever manufactured in Egyptian history" (El Goresy et al. 1986:28i).
Another much more probable explanation for the presence of these copper chlorides is that
they are alteration products of the original malachite pigment that had come in contact with
saline salt solutions percolating through the rock of the tombs. Even in a relatively dry tomb,
such as that of Queen Nefertari, long prismatic crystals of sodium chloride are found, and these
form slowly over many centuries. The ability of these salt solutions to create alteration crusts on
malachite particles is well supported.
In a later study, Schiegel, Weiner, and El Goresy (i989) appear to have reversed the 1986
conclusion by El Goresy and coworkers that the basic copper chlorides were original pigments.
The later study concludes that the copper chlorides were a form of cancer eating away at poly­
chrome wall paintings and Egyptian faience. This article, however, does not use the same

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