Aluminium (symbol Al, melting point varying from 1,050 to 1,292° F., specific gravity 2'6), when of 98.5 per cent, purity, is bright white in colour, somewhat resembling silver, though its appearance depends much on the temperature at which it has been worked. It is capable of taking a high polish. Its melting point may he increased greatly if impurities are present or if it is alloyed with another metal. Aluminium is only slightly elastic; it is, however, fairly malleable and ductile, but these latter properties are impaired by the presence of its two chief impurities, silicate and iron.
If of more than 99 per cent, purity, it can be rolled, it is said, into leaves 1/40000in. thick, in this respect being inferior only to gold. Aluminium has a tensile strength of 7 tons to the square inch. "When pure, it is non-corrosive and resists the oxidising action of the atmosphere, but this advantage has to be partly sacrificed to obtain increased hardness and elasticity by adding small quantities of copper, nickel, or zinc. It dissolves in hydrochloric acid and in most solutions of the alkalies, but it is only slightly affected by dilute sulphuric acid, and not at all by nitric acid. Rolled or forged metal breaks with a fine silky fracture. Aluminium is not found in a metallic state, but when in combination with oxygen, various alkalies, fluorine, silicon, and acids, it is the base of many clays and soils. Common compounds of aluminium are felspar, mica, gneiss, and trachyte, whilst other aluminium compounds, classed as precious stones, are the ruby, sap-pbire, garnet, turquoise, lazulite, topaz, etc. The ores from which aluminium is commercially reduced are bauxite, cryolite, and corundum. In reducing bauxite, it is mixed with soda ash in a furnace, an aluminate of soda being obtained afterwards, and the insoluble substances are separated bylixiviation.
Bypassing carbonic acid gas through the solution, pure alumina is precipitated, and this is formed with salt and charcoal into balls, which are heated in an earthenware retort through which chlorine gas is passed, the result being that the charcoal combines with the oxygen, and the chlorine with the aluminium; the aluminium chloride sublimes over with the salt (sodium chloride) and forms a double chloride of aluminium and sodium. This double chloride is heated in a reverberatory furnace with suitable fluxes and with metallic sodium; the sodium combines with the chlorine and leaves the aluminium free to fall to the bottom and to be drawn off into ingot moulds. The chemical method of producing aluminium now has been superseded by the cheaper and more satisfactory electrical process. The three best known electrical methods are the Cowles, the Hall, and the Herault, the first-named depending on the heating effect of the electric current and producing aluminium alloys only, whereas by the other two methods aluminium salts are submitted to electrolytic action at a high temperature, pure metal being in these cases produced.