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 mlls, in which 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 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.