Zinc (sym. Zn, eq. 65, sp. gr. 6.9 to 7.15) is for many purposes a valuable metal. Some coins struck by the Romans early in the Christian era are of brass (an alloy of copper and zinc), but as zinc was not then known as a separate metal, it is believed that this brass had been produced by adding to melted copper the mineral calamine, which we now know is a zinc ore. Paracelsus (q.v.) pointed out that zinc was a metal; but the exact nature of this metal and of its ores was not distinctly known till the 18th century. But zinc appears to have been used in China, and perhaps also in India, from an early period.
Zinc is a bluish-white metal, breaking with a crystalline fracture. When chemically pure it is malleable and ductile at ordinary temperatures, but all commercial zinc is somewhat brittle until it is raised to the temperature of 212° F. or preferably to 300° F. (149° C.), when it can be hammered, drawn, or bent. It is rather remarkable that it does not readily become again brittle when cooled, although workmen manipulating zinc out of doors in winter generally take the precaution to previously heat it over a fire. If zinc is raised to a temperature as high as 400° F. it becomes so brittle that it can easily be reduced to powder. The melting-point of this metal is 773° F. (412° C.), and its boiling-point is 1904° F. (1040° C.). It burns in the air at a high red heat with a bright, greenish-white flame, emitting dense white fumes. It is only slightly acted on by air and moisture, but when exposed to these it rapidly loses its bright metallic lustre, taking on a thin grayish film which protects the metal beneath from the further action of oxygen and carbonic acid. Pure zinc is only very slowly attacked by mineral acids, but all commercial kinds dissolve in them very readily with evolution of hydrogen. Ordinary zinc will also dissolve in a warm solution of potash or soda. A slight crackling sound is produced when the metal is bent after fusion, and even with sheet-zinc prepared for use this sound is sometimes detected by artisans shaping it, in which case they anneal the piece before sharply bending it. Zinc, though much harder than lead or tin, is softer than ordinary brass. It dirties, or at least changes in appearance, even indoors, sooner than most common metals, and it does not keep on a coat of paint so well as any of them.
Oxide of Zinc, ZnO.—When the finely-divided metal is held in the flame of a Bunsen burner it takes fire, giving off abundance of white fumes which are zinc-oxide. These fumes were called philosopher's wool by the alchemists. On a large scale oxide of zinc is manufactured by burning the vapour of the metal issuing from the necks of clay retorts in chambers through which an air current is passing. The vapour employed for the purpose is that produced from a retort charged with calcined ore and coke, and the oxide is deposited as a loose, white powder, forming the paint known as zinc-white. It has only about half the density of white lead and has much less 'body,' but it does not darken like the lead pigment (see WHITE PIGMENTS). This substance is also used as an ingredient in pottery colours. Oxide of zinc is readily soluble in acids, forming colourless salts. Heated in a flame it shows a yellow colour which turns white on cooling—a characteristic test for the metal. Zinc hydroxide, , is thrown down as a bulky white precipitate from solutions of zinc salts by potash or soda, avoiding excess of alkali. It dries to a white powder.
Zinc Sulphate, .—This salt, which, as shown by the formula, crystallises with seven atoms of water, used to be known as white vitriol. It crystallises in four-sided rhombic prisms isomorphous with Epsom salts, and, as sold, has usually the same appearance. It dissolves readily in water, and has a nauseous metallic taste. Sulphate of zinc is obtained when hydrogen is prepared by the action of sulphuric acid on zinc, but is made in larger quantities by roasting the native sulphide of zinc (blende) at a low red heat in the air until the necessary oxygen is absorbed. The product is thrown into water while hot, giving a solution from which the crystals are deposited. Sulphate of zinc so obtained is purified from arsenic and some other metals by treating its acidified solution with sulphuretted hydrogen. Iron and manganese have to be separated from the filtered solution by sodium hypochlorite. The salt is used in calico-printing, in making varnishes, and in medicine.