This section is from the book "Cassell's Cyclopaedia Of Mechanics", by Paul N. Hasluck. Also available from Amazon: Cassell's Cyclopaedia Of Mechanics.
Zinc (Zn),a bluish-white and highly crystalline metal, is very malleable when pure, but impure commercial zinc is inclined to be brittle. It melts at 773 P. and has a specific gravity varying from 6'86 in the cast state to 7.21 when rolled or forged. Cast zinc is named spelter, only the rolled metal being known as zinc, as a rule. Zinc oxidises at a red heat, but the rolled metal will form a film of grey suboxide at an ordinary temperature if in a damp situation. Zinc is hardened by rolling, and is annealed at a low heat to make it malleable again. Pure zinc is dissolved by nitric acid and alkalies, but not by hydrochloric or sulphuric acid, although the commercial metal is readily dissolved by either of these latter acids. Zinc is much used as a pure metal, and also in alloys. "Galvanised iron" is sheet-steel coated with zinc. The chief ores of zinc are zincite (red oxide of zinc), a white ore when pure, but usually of a reddish colour owing to the presence of oxide of manganese: blende or " black jack, a sulphide which is a black or yellowish-black ore, with sometimes a reddish tinge imparted by galena: calamine, a carbonate; and electric calamine, a silicate. Zinc is very volatile, and thus has to be extracted from its ores by distillation.
In reducing blende, it is first oxidised and then treated with carbon and carbonic oxide, or by hydrogen and hydrocarbons. The powdered blende is roasted in a reverberatory furnace until most of the sulphur has disappeared, and the zinc oxide remaining is heated in fireclay retorts to a temperature of about 1832° F. (1000°C), and the vapours are condensed.