Encyclopædia Britannica 1797/XVIII

Encyclopædia Britannica; or, a Dictionary of Arts, Sciences, and Miscellaneous Literature, Constructed on a Plan, by which the different Sciences and Arts Are digested into the Form of Distinct Treatises or Systems, comprehending The History, Theory, and Practice, of each, according to the Latest Discoveries and Improvements, and full Explanations given of the various detached Parts of Knowledge… The third Edition… XVIII, Edinburgh [Andrew Bell – Colin Macfarquhar] 1797.

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Zaffre

ZAFFRE, is the oxyd of cobalt, employed for painting pottery ware and porcelain of a blue colour. The method of preparing it is as follows : The cobalt taken out of the mine is broken with hammers into pieces about the size of an hen’s egg; and the stony involucrum, with such other heterogeneous matters as are distinguishable by the eye, are separated as much as possible. The chosen mineral is then pounded in stamping mills, and sifted through brass wire sieves. The lighter parts are washed off by water, and it is afterwards put into a large flat-bottomed arched furnace, resembling a baking oven, where the flame of the wood reverberates upon the ore; which is occasionally stirred and turned with long handled iron hooks or rakes; and the process is continued till it ceases to emit any fumes. The oven or furnace is terminated by a long horizontal gallery, which serves for a chimney; in which the arsenic, naturally mixed with the ore, sublimes. If the ore contains a little bismuth, as this semimetal is very fusible, it is collected at the bottom of the furnace. The cobalt remains in the state of a dark grey oxyd, and is called zaffre. One hundred pounds of the cobalt ore lose 20 and even 30 per cent. during this operation, which is continued 4 or even 9 hours, according to the quality of the ore. The roasted ore being taken out from the furnace, such parts as are concreted into lumps are pounded and sifted afresh. Zaffre, in commerce, is never pure, being mixed with two or rather three parts of powdered flints. A proper quantity of the best sort of these, after being ignited in a furnace, are thrown into water to render them friable, and more easily reduced to powder; which, being sifted, is mixed with the zaffre, according to the before-mentioned dose; and the mixture is put into casks, after being moistened with water. This oxyd, fused with three parts of sand and one of pot-ash, forms a blue glass; which, when pounded, sifted, and afterwards ground in mills, included in large casks, forms smalt.

The blue of zaffre is the most solid and fixed of all the colours that can be employed in vitrification. It suffers no change from the most violent fire. It is successfully employed to give shades of blue to enamels, and to the crystal-glasses made in imitation of some opaque and transparent precious stones, as the lapis lazuli, the turquois, the sapphire, and others of this kind.