ALUM, a whitish, astringent, saline substance; properly it is a double salt, being com posed of sulphate of potash and sulphate of alumina, which, along with a certain pro portion of water, crystallize together in octahedrons or in cubes. Its formula is A. is soluble in 18 times its weight of cold water, and in its own weight of hot water. The solution thus obtained has a peculiar astringent taste, and is strongly acid to colored test papers. When heated, the crystals melt in their water of crystallization; and when the water is completely driven off by heat, there is left a spongy white mass, called burnt A. or anhydrous A. A. is much used as a mordant in dyeing. This property it owes to the alumina in it, which has a strong attachment for textile tis sues, and also for coloring matters; the alumina thus becomes the means of fixing the color in the cloth. The manufacture of the colors or paints called lakes depends on this property of alumina to attach to itself certain coloring matters. Thus, if a solution of A. is colored with cochineal or madder, and ammonia or carbonate of soda is added, the alumina of the A. is precipitated with the color attached to it, and the liquid is left colorless. Alumina, the basis of pure clay—which is a silicate of alumina—derives its name from being first. extracted from A. A. is also used in the preparation of leather from skins, and, in medicine, as a powerful astringent for arresting bleeding and mucous discharges. Its use in the making of bread, to give a white appearance and more pleas ing consistence to bread made from indifferent flour, is highly objectionable. A. rarely occurs in nature, except in a few springs and in some extinct volcanoes, where it appears to be formed from the action of sulphurous acid vapors upon felspathic rocks. In this country, it is prepared artificially from A.-shale, obtained from coal-mines at Ilurlett and Campsie, near Glasgow; and A.-slate, which occurs at Whitby, in Yorkshire, and there forms precipitous cliffs, extending about 30 rn. along the e. coast of England. The A.-slate, shale, or schist, consists mainly of clay (silicate of alumina), iron pyrites (bisulphuret of iron), and coaly or bituminous matter. When the shale is exposed to the air—as it is in the old coal-wastes or mines from which the coal has been extracted—the oxygen of the air, assisted by moisture, effects a decided change upon it. The original hard stony substance begins to split up into thin leaves, and becomes studded over and interspersed with crystals. The latter are the result of the oxidation of the sulphur of the pyrites into sulphuric acid, and the iron into oxide of iron, both of which in part combine to form sulphate of iron,whilst the excess of the sulphuric acid unites with the alumina of the clay, and produces sulphate of alumina. When the A.-shale thus
weathered is digested in water, there dissolve out the sulphate of alumina (Al2022S03) and sulphate of iron (FeOS00; this solution is treated with chloride of potassium (K01), which decomposes the sulphate of iron, forming sulphate of potash (KOSO3) and chlo ride of iron (Feel). When this livid is evaporated to concentration, and allowed to cool, crystals of A. separate, consisting, of sulphate of alumina, sulphate of potash end water, thus, and the chloride of iron is left in the solution or mother-liquor. The crystals of A. obtained from the first crystallization are not free from iron, and hence require to be redissolved in water, reconcentrated, and recrystal lized. This operation is generally- repeated a third time before the A. is obtained pure.— As the preliminary weathering of the shale takes some years to proceed, a more expedi tious method is now largely resorted to. The shale is broken in fragments, and piled up over brushwood in long ridges, shaped .liko hugh potato•pits, and the brushwood being set fire to, the coaly matter of the shale begins to burn, and the whole ridge undergoes the process of roasting; the results of which are the same as that of the weathering oper ation—namely, the oxidation of the sulphur and iron, and the formation of sulphate of alumina and sulphate of •iron. This material is afterwards worked up as previously described. The roasting operation is so much more expeditious than the weathering pro cess, that months suffice for years. The A. made at Tolfa, near Civita Vecchia, is extracted from A.-stone, a mineral containing sulphate of potash and sulphate of alu mina, but united in such a form as'-to render them insoluble: When the mineral is cal tined, the sulphates become soluble, and are extracted by lixiviation. The A. thus man ufactured is prized, as being free of iron. The potash in A. can be replaced partly or altogether by soda or ammonia; the alumina by oxide of chromium or sesquioxide of or the sulphuric acid by chromic acid, or peroxide of iron, without altering the form of the crystals. There are thus soda, ammonia, chrome, etc., alums, forming a genus of salts of which common A. is only one of the species. The more important members of the class, expressed in symbols, arc: potash A.
soda A. ammonia A. chromic potash A. ferrous A.