Cobalt Fluosilicate, CoSiF₆

Cobalt Fluosilicate, CoSiF₆.6H₂O, results when cobalt carbonate is dissolved in aqueous hydrofluosilicic acid and the solution allowed to evaporate. It crystallises out in red rhombohedra.

Smalt has been known for several centuries, having been discovered by Scheurer in 1540, who sold his secret to England. It is prepared by fusing cobalt oxide with silica (powdered quartz) and potassium carbonate, and is thus really a cobalt glass or silicate. The cobalt oxide employed is in the form of zaffre or safflower; that is, partially roasted sulphide or arsenide ore, the roasting being continued until the cobalt is mostly present as oxide and only sufficient arsenic is left to combine with such impurities as copper and nickel. The quartz is obtained by heating to redness and then disintegrating by plunging into water, after which it is powdered in a mill.

A mixture of zaffre, quartz, and potassium carbonate is now introduced into the small furnace, together with a little white arsenic (arsenious oxide), which combines with the iron, copper, and nickel, etc., present as impurities, causing them to separate as a heavy regulus below the liquid mass of glass, at the bottom of the melting-pots. After being kept at a white heat for some time, the glass is ladled out into cold water, which effects its disintegration. The smalt is then ground and levigated.

Smalt is an extremely permanent and inert pigment of a deep blue colour. Its composition varies considerably, but usually within the following limits:

Silica	66.2-72.11 per cent.
Cobalt monoxide	6.75- 1.95 per cent.
Potash	16.31- 1.80 per cent.
Alumina	8.64-20.04 per cent.

It is interesting to note that one part of cobaltous oxide will impart a distinctly blue colour to 250 parts of glass.