Alumina, the most abundant of the Earths (q.v.), is the oxide of the metal Aluminium (q.v.), the formula being . It occurs in nature abundantly in combination with silica, associated with other bases. The most familiar of its native compounds is feldspar, a silicate of alumina and potash, . This is one of the constituents of granite, and of several other igneous rocks. Certain varieties of these, by exposure to the atmosphere, become completely disintegrated, passing from the state of hard, solid rock, such as we are accustomed to see in building-granite, into soft, crumbling, earthy masses. It is the feldspar which undergoes the change; and it appears to be owing to the action of rain-water charged with carbonic acid, which dissolves the potash and some of the silica of the feldspar, leaving the excess of silica and the alumina still united. It is not known, however, why certain specimens of granite are rapidly corroded and crumbled down, whilst others have resisted for ages the same causes of decay. By such a process of disintegration as we have described, the clays of our arable soils are produced. Clay consists of silica and alumina in a state of chemical combination. It never is pure alumina, but the quantity of silica united to the latter is variable. When it is pure, clay is quite white, as we see in the porcelain clay of Devonshire and Cornwall, which is derived from colourless feldspar. More frequently, clay is red, owing to the presence of oxide of iron; or black, from the diffusion through it of vegetable matter.
From alum, alumina is prepared by adding to a solution of the former, water of ammonia, as long as it occasions a precipitate. The alumina appears as a voluminous, white, gelatinous substance, consisting of the oxide of the metal combined with water. When alumina is precipitated from a solution containing colouring matter, such as logwood, &c., it carries down the colour chemically united to the flocculent precipitate; in this way are formed the coloured earths called Lakes (q.v.). Alumina in the state of precipitate, after being gently dried, is readily soluble in acids and in alkalies; but if strongly heated, at a certain temperature it presents an appearance of sudden incandescence, it loses the associated water, contracts greatly in bulk, and now forms a white, soft powder, not at all gritty, and with difficulty soluble in alkalies and acids. Alumina, as generally prepared, whether hydrated or anhydrous, is insoluble in water, possesses no taste, and does not alter colouring matters; but it has also been obtained in an allotropic hydrated form, which, in the presence of a very small proportion of acetic acid, is largely soluble in water, from which a minute trace of sulphuric acid precipitates it. It is quite different, therefore, in properties from the alkaline earths, and is a much weaker base. In the anhydrous state it absorbs water with great readiness without combining with it, so that it adheres to the tongue, and is felt to parch it. Clay retains this property; and the ends of tobacco-pipes are often glazed, to prevent adhesion to the lips or tongue. Alumina is not fusible by a forge or furnace heat, but it melts before the oxyhydrogen blow-pipe into a clear globule, possessing great hardness. It occurs in nature in a similar state. The more coarsely crystallised specimens form the emery which is used for polishing; the transparent crystals, when of a blue colour, owing to a trace of metallic oxide, constitute the precious gem the sapphire, and, when red, the ruby. Alumina, in common with other sesquioxides, is a feeble base.
Aluminium—sym. Al, eq. 27—is one of the metals present in clay, feldspar, slate, and many more rocks and minerals. It was named about 1812 by Davy, who dissolved alumina, but failed to isolate the metal. It was isolated by Wöhler in 1828, and was re-examined by him in 1846, when he obtained the metal in minute globules or beads, by heating a mixture of chloride of aluminium and sodium. In 1855, the French chemist Deville showed, as the result of a series of experiments, that aluminium could be prepared on a large scale and in a compact form without much difficulty. The mineral cryolite found in Greenland, which is a double fluoride of aluminium and sodium, was the ore first used for its manufacture; but bauxite, a clay first found at Les Baux, near Arles, consisting chiefly of alumina or oxide of aluminium, and oxide of iron, has recently been employed as a convenient source of the metal. An aluminate of soda is first obtained by heating the bauxite with soda ash in a furnace, and separating it (the aluminate) from the insoluble portions by lixiviation. When carbonic acid is passed through the solution, pure alumina is thrown down. The alumina is then formed into balls with common salt and charcoal, which are heated in an earthenware retort through which chlorine gas is passed. In this part of the process, the charcoal combines with the oxygen, and the chlorine with the aluminium; the latter sublimes over with the common salt (chloride of sodium), and is collected as a double chloride of aluminium and sodium. When this double chloride is heated in a reverberatory furnace with fluxes and metallic sodium, the latter seizes the chlorine combined with the aluminium, which is then set free, and falls to the bottom ready to be cast into ingots for use. Aluminium has also been made from alum. As far back as 1854 Bunsen accomplished the manufacture by a somewhat troublesome and expensive electrolysis. But since 1890 electrolytic methods, of which one of the most successful is that of Messrs Cowles of Milton, near Stoke-upon-Trent, and of Lockport in New York, have largely superseded the chemical processes. In the Cowles apparatus a mixture of clay (such as bauxite) and charcoal is subjected to the heat of the electric arc and decomposed. Advantage is largely taken of the electric force generated by the utilisation of waterfalls—as at Neuhausen, near the Schaffhausen falls of the Rhine. The Niagara Falls are being employed in the same way; and bauxite from the north of Ireland is the material worked on at the aluminium works below the Falls of Foyers. The fall of water available at Foyers is 350 feet, supplying seven turbines, each of 700 horse-power. In 1898 the price of the metal was 1s. 3d. per lb.
The properties of aluminium are, that it is a white metal, somewhat resembling silver, but possessing a bluish hue, which reminds one of zinc. This bluish colour can be whitened by hydrofluoric and phosphoric acids, and also by a heated solution of potash. It is very malleable and ductile, in tenacity it approaches iron, and it takes a high polish. It fuses at about 1292° F. (700 C.), and can then be cast in moulds into ingots. Exposed to dry or moist air, it is unalterable, and does not oxidise or tarnish like most common metals. Neither cold nor hot water has any action upon it. Sulphuretted hydrogen, the gas which so readily tarnishes the silver in households, does not act on aluminium, which is found to preserve its appearance under all ordinary circumstances as perfectly as gold does. When cast into moulds, it is a soft metal like pure silver, and has a density of 2.56; but when hammered or rolled, it becomes as hard as iron, and its density increases to 2.67. It is therefore a very light metal, being lighter than glass, and only one-fourth as heavy as silver. Aluminium is very sonorous, a bar of it when struck giving out a very sweet clear ringing sound. It is a good conductor of heat and electricity.
It is somewhat difficult to understand why a metal with so many valuable properties should have hitherto been so little in demand. Since about 1850, such articles as coins, medals, statuettes, personal ornaments, keys, helmets, sabresheaths, mounts for furniture, and culinary vessels of aluminium have been tried, and failed to take the market. It is used for optical, surgical, and chemical instruments and apparatus. Aluminium leaf and wire may be employed with great advantage in place of silver leaf for decoration, or silver wire for embroidery. Of late it has come to be used in shipbuilding, especially for torpedo-boats, and boats meant to be sent in pieces to African lakes, &c., its hardness and lightness and non-corrosiveness being in its favour. Bicycles have also been made of it. And as it is specially suitable for cooking-vessels, efforts to cast it for pots and pans have often been made, but unsuccessfully till 1895, when aluminium (at 1s. 6d. per lb.) was, weight for weight, three times the price of copper, but bulk for bulk, the cheaper metal.
Aluminium Bronze.—Aluminium forms, with copper, several light, very hard, white alloys; also a yellow alloy, which, though much lighter than gold, is very similar to it in colour. This gold-like alloy, which is ordinary aluminium bronze, contains from 5 to 10 per cent. of aluminium, and is very strong, was discovered by Dr Percy of London. For many years it has been manufactured into watch chains, pencil-cases, and other small ornamental articles. More lately it has been made on a limited scale into such articles as table-plate and carriage mountings, which have an attractive appearance. This bronze, which can be made with a tensile strength equal to steel, has certain advantages for field-guns. Its anti-friction and wearing qualities make it well adapted for bearings of shafts; but its price, considerably in excess of ordinary bronze, somewhat hinders its use on a large scale for objects of utility.
An alloy of aluminium and tin is used for optical instruments, and from another of aluminium and silver called 'Tiers Argent,' excellent spoons and forks are made. See works on aluminium by Richards (2d ed. 1890), a work by Menel (Paris, 1892), and one by Anton (Leip. 1892).