Smalt

Synonyms:

Smalt, Royal Blue, Dumont's Blue, Zaffre, Bleu De Smalte, Smalte, Zaffer

Glass and vitreous mixtures containing cobalt, and of a rich blue colour, have been known for ages, although copper, not cobalt, was the colouring principle of most of the antique materials of this class. The smalt now made is a very deep-blue glass (appearing black in the lump), consisting essentially of cobalt and potassium silicate. It generally contains, in 100 parts, silica, 65 to 71; potash, 16 to 21; and cobalt oxide, 6 to 7 parts: a little alumina is always present. In the inferior varieties the oxides of iron and nickel always occur.

In order to prepare a pigment from the dark cobalt glass, it is fused, and then poured into cold water. After the disintegration thus effected, the glass is ground into a moderately fine powder, and submitted to the process called elutriation, or washing-over. The finest particles, which take longest to settle from the wash-waters, are the palest in colour; the larger particles, though of richer hue, are very difficult to use as a paint.

Smalt is rarely employed now as an artists' pigment, cobalt blue and artificial ultramarine having been very advantageously substituted for it. For not only is it a difficult pigment with which to work, in both water and oil, but it is gradually altered by moisture and by the carbonic acid of the air, becoming paler and greyer; moreover, the finer the state of division in which it exists, the more rapid is the change. It will be found that even spring or distilled water is competent to start the decomposition of smalt. For if a little of this pigment be placed on a piece of yellow turmeric-paper, and moistened with clean water, there will soon be formed a red stain beneath the smalt - proof of the liberation of some of the alkaline constituent (potash) of this blue glass.