Calcination is the reduction of solid bodies by lire, to a state of powder, or ashes : a process which is attended with a change of their quality, and is es-sentially different from comminution, or mechanical trituration.

Having, under the article ASHES treated of the burning of vegetable and animal matters, we shall con-line our account, in this place, to metals.

To calcine such metals as melt before ignition, they must be kept in fusion for some time; nor will this operation succeed, without a free admission of air; the surface of the metal must therefore be kept clear of the calx. Should any part be excluded from the air, no such change of quality will take place: and if any coal, or unctu-ous, inflammable matter, be suffered to fall into the vessel, it Would reduce even the quantity, already calcined, to its former metallic state. The continuance of fusion causes the increase of the particles of fire; which, as they penetrate every pore, decompose the whole so completely, that the fluidity can no longer subsist. The body is thus left porous, extremely brittle, and easily reducible to the finest powder. This accounts for the parts of the body calcined being much broken and rarefied, and specifically lighter than in their original state.

The calcination of metals, gold, silver, and mercury excepted, is much promoted by nitre; the process of which is usually termed de-flagration; or detonation.

Fusion with any vegetable, or animal inflammable matter, will restore all calces and scoricc to their natural metallic state. They are, however, more difficult of fusion than the metals themselves, and scarcely any but those of antimony, lead, or bismuth, can be melted, without some additional flux, consisting chiefly of alkaline salt, in the strongest fire that can be furnished by the common furnaces. The reducing flux, which is a mixture of alkaline salt inflammable matter, brings the calx into fusion, and revives it into metal. The common preparation of such a mixture, consists of two parts of water, and one part of nitre, well ground together, .then set on lire, and covering the deflagrating powder. with sowevessel, till it be changed into a black alkaline, coaly mass. This is called, by chemists, the Mack flux, a double quantity of which, in proportion to the calces, or scoriar, exposed to a proper heat in a crucible, closely covered, will male them melt, and resume their metallic form.