Domestic Encyclopædia (1802)/Zaffre

ZAFFRE, is the oxyd or calx of cobalt, employed for imparting a blue colour to porcelain and pottery ware: it is prepared, according to Cronstedt, in the following manner.

When the cobalt is dug out of the mine, it is first broken into small pieces, and all heterogeneous matters are carefully separated.—The mineral is then submitted to the action of stamping mills, in wliiich it is reduced to a fine powder, that is sifted through brass wire sieves. Next, the lighter particles are carried off by water, and the cobalt is put into a reverberatory furnace, terminating in a long horizontal gallery; through which the arsenic, usually mixed with the mineral, sublimes: farther, the cobalt is frequently stirred with long iron hooks or rakes, till it ceases to emit any fumes; when it remains in the form of a dark grey calx, denominated Zaffre.

Considerable quantities of this oxyd, are manufactured from the cobalt dug out of the mines in the Mendip-hills, and also in Cornwall; beside which, there are large supplies annually imported from Saxony:—such zaffre, however, is seldom pure; being mixed with a considerable proportion of pulverized flints.

The blue of zaffre is the most permanent of the different colours employed in glass-works; as it resists, unchanged, the most intense heat: hence, it is also advantageously used for giving various shades of blue to enamels, and to the crystal glasses that are made in imitation of lapis lazuli, turquoise, sapphire, and other precious stones.