PIGMENTS 159
particles are common. Like all true glasses, the pigment is isotropic; the refractive
index is low. Large particles are purple-blue by transmitted light, but small
particles are clear blue. Smalt, like other glasses, is ordinarily considered to be
quite stable. It is commonly seen on pictures two or three centuries old, without
sign of alteration. While most samples are insoluble, even in strong acids, a few
have been seen which are quite readily soluble, even in dilute hydrochloric acid.
Church says (p. 224) that it is gradually altered by moisture and by carbonic acid
of the air, becoming paler and grayer, and Doerner also speaks (p. 80) of this
tendency. These unstable pigments may be high in potash and not true glasses.
In Europe smalt seems to have had its beginnings as a pigment certainly as
early as the late XVI century. In the early XVII century it came into general use
in oil painting. There is evidence, however, that smalt was known in Asia even
earlier than in Europe. It has been identified on Chinese wall paintings: one from
Kara Khoto in Central Asia and dated perhaps as early as theXI-XIII centuries;
another, a l Seated Buddha ' of the Ming Dynasty (Fogg Museum, no. 1933.190).
Farnsworth and Ritchie say (loc. tit., p. 160):' Some of the finest blue underglazes
in Chinese blue-and-white pottery, occurring about the middle of the Ming
Dynasty, were made by means of cobalt ores imported from some Islamic area
west of China, and are always described as ' Mohammedan blue.'
The use of smalt as an artist's pigment was discontinued around the beginning
of the XIX century. This was because of its shortcomings as mentioned above,
and also because its place was taken by the more satisfactory cobalt aluminate
(cobalt blue) and by artificial ultramarine. It is still available, however, and is
used to some extent by sign-painters for strewing on a background for gold letter-
ing. Smalt is quite frequently found on old paintings. It was identified on a
portrait by Hans Holbein the Younger (1497-1543) of Sir William Butts, and this
indicates that it was known before the time which Beckman suggests for its
discovery (1540-1560). Sixteen specimens of blue pigment were identified as smalt
by De Wild on XVII and XVIII century paintings, one by Rubens, dated 1620,
being the earliest. Laurie records (New Light on Old Masters, p. 126) thatTeniers
used smalt in his skies and that Velasquez (op. cit., p. 129) used a mixture of smalt
and azurite in the green drapery of the Rokeby * Venus.'
Soapstone (see Talc).
Steatite (see Talc).
Strontium Yellow (lemon yellow) (see also Barium Yellow) is strontium chro-
mate, SrCr04. It is prepared much like barium yellow except that strontium
chloride replaces barium chloride. The finely divided, crystalline precipitate that
is formed must be thoroughly washed to be useful for pigment purposes. It is a
little deeper and brighter in lemon hue and has greater hiding power than barium
chromate. It takes the form of blades or needles which are pale yellow by trans-
mitted light and are strongly birefracting. Strontium chromate is slightly soluble
in water and soluble in alkalis, in dilute mineral acids and in acetic acid; it some-