Page 283 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

terminology as defined by Tite (i987) for the description of faience. Apparently, what is being
described is a frit—usually Egyptian blue or Egyptian green, composed of considerable amounts
of glassy material—although glazed faience tiles are also discussed.
Evidence is presented in the 1986 study by El Goresy's group that the deterioration begins
with the devitrification of the glass phase in Egyptian blue and green. Next, migrating chloride-
containing solutions react with the copper and carbonate phases — calcite within the stucco lay­
ers underlying the pigment layer containing dissolved copper (II) species, for example — to form
one of the basic copper trihydroxychlorides, usually atacamite or paratacamite. The same pro­
cess of deterioration was also evident on green areas of tiles from the south tomb at Saqqara that
dates to the Old Kingdom (2665-2155 B.C.E.). Where the surfaces were undeteriorated, the tiles
were a smooth, shiny blue. The green zones were rough and dull, with evidence for the for­
mation of atacamite or paratacamite on the surface. Several tiles had lost their glazed surface
almost completely, and the white quartzite cores were exposed.
Schiegel, Weiner, and El Goresy (i989) found that most Old Kingdom blue pigments were
severely attacked, with extensive alteration to green. The deterioration continued into the
Middle Kingdom and gradually lessened by the New Kingdom and Ptolemaic period, when the
extent of surface alteration was evaluated as being low medium.
These deteriorated surfaces indicative of copper chloride attack raise the possibility that the
frits or faience, which were described in the earlier literature as green, may originally have been
blue and are now discolored by chloride conversion. A1989 inspection by Schiegel, Weiner, and
El Goresy of objects in the Egyptian Museum in Cairo revealed that many objects —including
painted stone reliefs, decorated wooden objects, and glazed ware—were showing symptoms of
attack, with copper-containing silicate glasses or glazes also showing signs of deterioration. Fur­
ther research on this issue is clearly needed to ascertain the nature and the extent of the problem.

HAN BLUE AND HAN P U R P L E : S Y N T H E T I C PIGMENTS FROM C H I N A

During the Han dynasty (206 B.C.E.-220 C.E.) in China, mixtures of blue and purple silicates
were used as pigments for both ceramic and metallic objects. They were also formed into octago­
nal pigment sticks. The blue pigment, known as Han blue, was identified by Fitzhugh and
Zycherman (1992) as a synthetic barium copper silicate, BaCuSi 4 O 10 , which is a barium ana­
log of Egyptian blue (CaCuSi 4 0 10 ). The synthetic preparation of Han blue by Wiedemann
and Bayer (1997) shown in PLATE 52 illustrates the strongly colored crystalline particles in
bright-field illumination. The crystal structure of both the barium copper silicate and the cal­
cium copper silicate has been carefully studied by Pabst (1959), who showed that their structures
are similar to the gillespite group, BaFeSi 4 O 10 .
Fitzhugh and Zycherman (1992) propose the name "Han purple" for the purple pigment,
which was identified by X-ray diffraction as another barium copper silicate, BaCuSi 2 0 6 .

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