The ores of zinc are the native carbonate, or common calamine, the oxide of zinc and blende, or the sulphuret of zinc. In the process for reducing the ore of zinc, it is first to be broken into small pieces, and the different impurities being separated, it is next calcined in a reverberatory furnace, at a moderate red heat; and if the ore be calamine, the carbonic acid is driven off, and if blende, it is deprived of its sulphur. After this it is washed, and the metallic oxide being separated from the earthy parts, it is dried, and carefully mixed with about one eighth of its weight of charcoal, by grinding the ingredients together in a mill, and is now ready for the smelting process. This is performed in a circular furnace, in which are fixed six large earthen pots, about four feet high, and nearly in the shape of oil jars. An iron tube is inserted into the bottom of each pot, and passing through the arched floor of the furnace, terminates in a vessel of water placed beneath, while the other end of the tube rises within the crucible to a few inches of the top. The crucibles are then filled with the mixture of the ore and charcoal, to the level of the tube; the cover of each is carefully luted on, and an intense heat is to be kept for several hours.
The zinc, as the process of reduction goes on, rises, in the form of vapour, to the top of the pot, but as it cannot escape, it descends through the iron tube, passes into the water, and is condensed in small drops. The globules are afterwards fused, and cast into the form of ingots, when it is fit for the market; but as common zinc contains a little of other metals, as copper, lead, arsenic, iron, and manganese, which impair its quality, these impurities are partially separated by melting the zinc in a crucible, and stirring into it, with a stick or earthen rod, a mixture of sulphur and fat; by the latter, the zinc is preserved from oxidation, and the sulphur combines with the other metals, except the zinc, and, converting them into sulphurets, they rise to the top in the form of scoriae, which may be removed. This process is to be repeated as long as any scoriae appear. See Zinc