PIGMENTS 157
phous or cryptocrystalline (chalcedonic) form of silica also used for wood fillers
and in paints (see Ladoo, pp. 641-651). Diatomaceous earth (see Diatomaceous
Earth) is a fossil form of silica. Silicic acid or precipitated silica, SiCV#H2O, pre-
pared by the action of an acid on an alkali silicate, is a pure white, amorphous
powder which has special uses as a filler and extender. Silica in all its forms is
extremely inert. It is unaffected by heat and is insoluble in strong acids (except
hydrofluoric), but it is slowly attacked by strong alkalis. Quartz, or crystalline
silica, can be recognized by its optical properties. It has medium refractive index
(o> = 1.544)3 and is only moderately birefracting. Particles of quartz are often seen
as an impurity in mineral pigments and other natural products. Sand particles are
usually rounded and frosty in appearance as a result of the wearing action of wind
and wave.
Silver Leaf and Silver Powder were used occasionally in mediaeval paintings,
but their very great tendency to tarnish and to blacken limited their effectiveness.
This fault was known very early and Cennino Cennini warns against silver for
that reason (see Thompson, The Craftsman's Handbook, p. 60). Laurie speaks of
Byzantine manuscripts (The Pigments and Mediums of the Old Masters, pp. 78-79)
where not only the silver but the mordant also has become black and appears
to have stained through the manuscript page. There seems to be no connection
between the discolorations of the two. When protected with a good varnish coat-
ing, however, this metal may retain its lustre for years. Silver leaf was used for
rendering armor in battle scenes and pageants (Thompson, The Materials of
Medieval Painting, p. 190). In some early paintings, it was used for a background
like gold leaf. Methods of application were much the same as those for gold.
Smalt was the earliest of the cobalt pigments. It is artificial, in the nature of
glass, a potash silicate strongly colored with cobalt oxide and reduced to a powder.
The origin is obscure. For years there has been much debate concerning whether
or not cobalt was used by the Egyptians and by other peoples of classical times
to color glass. Marie Farnsworth and P. D. Ritchie have shown recently (c Spec-
troscopic Studies on Ancient Glass,' Technical Studies, VI [1938], pp. 155-173)
that cobalt was definitely present along with copper in much Egyptian blue glass,
but only in amounts of the order of o.i to 0.2 per cent. They assume that the
cobalt may have been used intentionally with full knowledge of its properties
for that purpose. There is no evidence as yet, however, that any powdered cobalt
glass was ever used as a painter's pigment in ancient times. When cobalt was first
employed in Europe for glass making is not known, but probably the Venetian
glass makers knew of its properties. B. Neuman (* An tike Glazer/ Zeitschrift
fur Angewandte Chemie, XXXVIII [1925], p. 863) remarks that it was first used
by them in 1443, but he does not give his source of information. According to
Laurie (The Pigments and Mediums of the Old Masters, pp. 12-16), the word,
smalto, was used as early as 1492, and a glass pigment under the name, azzurro
di smalto, was described in 1584, About the middle of the XV century, certain