158 PAINTING MATERIALS
cobalt minerals such as cobaltite (CoAsS) and smaltite (CoAsg) were discovered
on the borders of Saxony and Bohemia. At the time, the nature of these minerals
was not known and, since they gave the miners a certain amount of trouble,
they were called * cobalt' for spirits or ' kobolds ' which were thought to haunt
the mines. The Bohemian glass makers soon found, however, that these strange
minerals had the property of coloring glass blue. Beckman, who treats the early
history of smalt (I, 478-487) relates (p. 483) that information about the early
discovery has come through a certain Christian Lehmann (d. 1688) who wrote an
historical work on the mines in Misnia. Lehmann said that the color mills, at the
time when he wrote, were about a hundred years old, and Beckman, therefore,
places the invention at about 1540-1560. Lehmann added that smalt was dis-
covered by Christian Schurer (or Christoph Schiirer [see Rose, p. 277]), a glass
maker of Flatten in Bohemia, who found that when cobalt mineral was added to
a molten, vitreous mass, he obtained a fine blue glass. At first, he prepared it only
for the use of potters, but in the course of time it was carried as an article of
merchandise to Nuremberg and thence to Holland. Later it came to be manu-
factured in Holland and in better quality than that previously prepared by the
Saxons.
As has been said, smalt was first prepared by roasting native cobalt minerals
like cobaltite and smaltite to form cobaltous oxide, CoO. * Zaffer,' it is called by
Beckman. Rose mentions (p. 278) that Biringuccio, in his Pyrotechnia (1540),
calls it by that name. The oxide was then added to a mass of molten glass and,
when thoroughly combined, the molten mass was poured into cold water to break
it into fine particles. It was further ground and then washed and allowed to settle.
The finer particles, which settled last, gave only a pale blue pigment, but the
coarser particles gave a deep, rich, purple-blue. Smalt may contain in 100 parts,
65 to 71 parts of silica (Si02), 16 to 21 of potash (K20), and 6 to 7 parts of cobalt
oxide (CoO), as well as some alumina (see Church, p. 224). In an analysis given by
Beam (p. 92), the cobalt oxide content is much lower, only 0.8 per cent. Farns-
worth and Ritchie say (he. «/., p. 160) that in a modern cobalt glass, a strong
blue is imparted by 0.15 per cent of cobalt oxide and a quite perceptible blue by
0.006 per cent. Blue glasses, however, with such low cobalt content, when ground
to a powder, have little or no color.
Smalt, since it is a glass and is transparent, has very poor covering power and,
for this reason, it has had to be used very coarsely ground. This has led to certain
difficulties: a tendency to settle in the paint film; poor spreading qualities, and a
tendency to streak down the surface of a painting. Various tricks were employed
by early painters to overcome this fault (see De Wild, p. 25).
Smalt can usually be recognized easily by its microscopic appearance. The
glassy character and conchoidal fracture are seen even at low magnifications.
Quite characteristic are particles with square and angular corners and others with
thin, flat edges; some are shaped like sharp splinters; tiny air bubbles in the