Natural Science in Archaeology

9.10 Silicates 219
centuries (Roy 1993). The use of ultramarine was rendered unnecessary by the
creation of a synthetic substitute in the early nineteenth century.
Egyptian Blue, Pompeii Blue/Blue Frit/Han Blue. Egyptian Blue is a glassy
manufactured material consisting of copper, calcium, and silica. It is a made by
fusing Ca, Cu, and SiO 2 using a sodium flux. The result corresponds to the mineral
cuprorivaite (see Fourestier 1999) with the formula CaCuSi 4 O 10 . According to
de Fourestier (1999), the mineral cuprorivaite (CaCuSi 4 O 10 ) has also been called
Egyptian Blue. Cuprorivaite is an azure-blue with a highly vitreous luster. It is best
known from Mount Vesuvius in Italy and from the Sattelberg volcanic cone, Eifel
district, Germany.
The earliest use of Egyptian Blue pigment dates to the third millennium BCE
(Tite et al. 1984). It was used as a paint pigment and for the manufacture of small
decorative objects such as beads, inlays, and talismans. It also was used as a pigment
in ancient Mesopotamia and in Etruscan wall paintings. Vitruvius reports that
the recipe was brought from Egypt. It was manufactured at Puzzuoli and was in
common use at Pompeii and Herculaneum (Canti and Heathcote 2002). The manufacture
of Egyptian Blue was similar to that of other ancient glasses. Egyptian Blue
consists of relatively large, coarse particles. Unlike minerals in general, most paint
pigments intensify in color as the pigment is ground to a finer consistency. This
is because finer particles are more easily dispersed throughout the medium. Conversely,
the more finely Egyptian Blue is ground, the less intense the color. It may
be that the crude product always contains significant amounts of remnant quartz and
glassy grains. The pigment may be improved by repeated sintering after washing
(Giovanoli 1969).
Egyptian Blue was used in frescoes at Pompeii. This material was known to the
Roman architectural writer Vitruvius, who wrote about its production from sand,
natron, and metallic copper. There has been some confusion about the term Egyptian
Blue that Onoratini et al. (1987) have attempted to clarify. These authors distinguish
four separate products: (1) a sodium copper silicate “Hubert Blue”, (2)
a calcium copper silicate “true Egyptian Blue”, (3) a ceramic containing “Hubert
Blue”, quartz, and tenorite, and (4) an “Antique Blue” ceramic which has “Egyptian
Blue” as one component. This latter material corresponds with the caerulea of
Vitruvius. Closely related to Egyptian Blue, with the substitution of barium for calcium,
are the Chinese Han Blue (BaCuSi 4 O 10 ) and Han Purple (BaCuSi 2 O 6 ) (Berke
2007). Han purple (really a lavender color) was used on some of the Qin Dynasty
terracotta warriors. Purple has often been called the color of royalty, partly because
good quality purple pigment was rare.
Smalt. Smalt is the most recently developed pigment that is of any interest to the
archaeomineralogist. Smalt is an artificial potash silicate colored with cobalt oxide.
The modern European use of smalt began in the fifteenth century. However, smalt
has been identified on Asian paintings predating European use (Roy 1993).
European smalt manufacture may represent the rediscovery of a forgotten technique.
There is some speculation that smalt may have been known and used by the
ancient Egyptians. Inconclusive evidence suggests that they could have used cobalt
ores in antiquity as a colorant for glass objects (Riederer 1974). However, there