Alum, a white, saline substance, with a sweetish, astringent taste, is, properly speaking, a double salt, being composed of sulphate of potash and sulphate of alumina, which, uniting together along with a certain proportion of water, crystallise in octahedrons or in cubes. Its formula is . Alum is soluble in eighteen times its weight of cold water, and in its own weight of hot water. The solution thus obtained is strongly acid to coloured test-papers. When heated, the crystals melt in their water of crystallisation; and when the water is completely driven off by heat, there is left a spongy white mass, called burnt alum or anhydrous alum. Alum is much used as a mordant in dyeing. This property it owes to the alumina in it, which has a strong attraction for textile tissues, and also for colouring matters; the alumina thus becomes the means of fixing the colour in the cloth. The manufacture of the colours or paints called lakes depends on this property of alumina to attach to itself certain colouring matters. Thus, if a solution of alum is coloured with cochineal or madder, and ammonia or carbonate of soda is added, the alumina of the alum is precipitated with the colour attached to it, and the liquid is left colourless. Alumina, the basis of pure clay—which is a silicate of alumina—derives its name from being first extracted from alum. Alum 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 to impart whiteness to bread made from poor flour is highly objectionable.
Alum 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 vapours upon felspathic rocks. In this country it is prepared artificially from alum-shale, obtained from coal-mines at Hurlet and Campsie, near Glasgow; and from alum-slate, which occurs at Whitby, in Yorkshire. The alum-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 alum-shale thus weathered is digested in water, there dissolve out the sulphate of alumina, , and sulphate of iron, ; this solution is treated with chloride of potassium, , which decomposes the sulphate of iron, forming sulphate of potash, , and protochloride of iron, . When this liquid is evaporated to concentration, and allowed to cool, crystals of alum, leaving the composition above described, separate out, and the protochloride of iron is left in the solution or mother liquor. The crystals of alum obtained from the first crystallisation are not free from iron, and hence require to be redissolved in water, reconcentrated, and recrystallised. This operation is generally repeated a third time before the alum is obtained pure.—As the preliminary weathering of the shale takes some years to complete, a more expeditious method is now largely resorted to. The shale is broken in fragments, and piled up over brushwood in long ridges shaped like huge 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 operation—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 process, that months suffice instead of years. The alum made at Tolta, near Civita Vecchia, is extracted from alum-stone, a mineral containing sulphate of potash and sulphate of alumina, but united in such a form as to render them insoluble. When the mineral is calcined, the sulphates become soluble, and are extracted by lixiviation. The alum thus manufactured crystallises in opaque cubes, having a reddish tint, due to the presence of iron, and goes by the name of Roman alum. The potash in alum can be replaced partly or altogether by soda or ammonia; the alumina by oxide of chromium or sesquioxide of manganese; or the sulphuric acid by chromic acid, or peroxide of iron, without altering the form of the crystals. There are thus soda, ammonia, chrome, iron, &c. alums, forming a genus of salts of which common alum (potash alum) is only one of the species. The more important members of the class, expressed in symbols, are:
, potash alum.
, soda alum.
, ammonia alum.
, chrome potash alum.
, iron alum.