ALUM. (Fit., Alun ; GER., Alaun.)—The name alum is applied in science and the arts to a class of double salts containing sulphate of alumina (see Alumina), which plays the part of an acid, in combination with an alkaline sulphate, representing the base. The salts are composed of one equivalent of each of these constituents, together with 21 equivalents of water of crystal lization, and are represented by the following formulas:— Potash alum A1B + Ammonia alum .. Al SO, + 12 Other alums exist in which the acid is represented by the oxides of ohromium, iron, and man ganese, which are isomorphous with alumina ; and besides potash and ammonia, the base may be constituted by soda, alumina, or the oxides of iron and chromium. These acids and bases are found to replace each other singly, and also, in combination with one another, to form alums of more or less complexity. In eaeh oase, however, 12 equivalents of water are required for the constitution of the crystal.
The only alums known in commerce are those of potash and ammonia, the latter being now manufactured very extensively. Ordinary potash alum, commonly called " alum," to distinguish it from ammonia alum, consists of white, diaphanous, octahedral crystals, of the following percentage composition : Potash ... 9.95 Sulphuric acid .. Alumina .. 1083 I Water 45.51 Tho crystals have a specific gravity of 1.71 ; they are slightly efflorescent in the air, have an acid, astringent taste and an acid reaction. One hundred parts of water at dissolve about 4 parts, and at 100° about 360 parts of the salt. When heated, the crystals melt in their water of crystal lization, the solid residue left on cooling being called rock-alum. Calcined at a low red heat, alum loses the sulphuric acid combined with the alumina, the latter remaining behind with the sulphate of potash. If the heat be raised to whiteness, the sulphate Of potash is decomposed also, the residue consisting of a mixture of potash and alumina. A neutral variety of this alum, commonly called cubical or Roman alum, on account of the cubical form of the crystal, is made by boiling 12 parts of ordinary alum with one part of slaked lime in water. It is preferred to the common variety for some dyeing and printing operations, as it does not affect certain colours. It is prepared in the neighbourhood of Rome from a mineral called alumite. According to Schmidt, its percentage composition is:— Potash Sulphuric acid .. Alumina .. 11.48 I Water .. Ammonia alum possesses many of the properties characteristic of ordinary alum, and may be applied to all the purposes for which the latter is used. When heated to redness, both the sulphuric acid and the ammonia disappear, nothing but pure alumina being left; this latter sub stance is often prepared on a large scale by this method (see Alumina). One hundred parts of water at 0° dissolve about 5 parts of this alum, and at 100°, 420 parts. Its percentage composition is Ammonia .. Sulphuric acid .. 36.10
Alumina I1.90 I Water .. Of the remaining alums, the most important is soda alum ; it is, however, not yet largely used in industrial operations, on account of the difficulty experienced in obtaining the crystals in a pure state; it is analogous in constitution to the two above-mentioned alums.
Alum is found native, like saltpetre and carbonate of soda, in volcanic districts in the form of a white incrustation upon rocks and stones. In this form it occurs in the neighbourhood of Naples ; in the Solfatara ; in Sicily, and in the south of France. In these districts it has long been the custom to collect the white efflorescence and dissolve it in water ; this solution is allowed to stand in order that mechanicdl impurities may settle out, and it is then evaporated in leaden pans by the natural volcanic heat of the soil, without the necessity of having recourse to fuel. The residue recrystallized affords a very pure product, which was for many centuries the only alum known in commerce. At the present time, native alum forms only a very small portion of that consumed in this country. The chief source is a bituminous clay called "alum shale," found in Norway, Bohemia, and the Hartz ; in England, near Whitby ; and in Scotland, near Glasgow. The shale undergoes a series of processes by which the sulphate of alumina is extracted and combined with sulphate of potash or sulphate of ammonia, in solution, as the case may be, the resulting mixture being evaporated down to obtain crystals of alum. In the neighbourhood of Manchester, large quantities of the coal-shales are employed for this purpose. Another important source of alum is the alum rock or alum stone, found in volcanic districts, and produced by the action of sulphurous vapours upon aluminiferous rocks. The mineral is calcined in large kilns, and then ]ixiviated with boiling water, the lye being evaporated down and crystallized out ; this process is only employed in the volcanic districts, where the rock is extensively found. Other sources are clays of different kinds, notably fireclay and pipeclay ; the minerals cryolite and bauxite are also used, and various mineral phosphates. These contain alumina only, and require the addition of both sulphuric acid and an alkaline sulphate in order to produce aluin. A11 shales and clays selected for the manufac ture of alum should be as free as possible from carbonate of lime and from iron.
Owing to their extensive application as mordants in the processes of dyeing and calico-printing, and to the comparatively economical methods which have been introduced from time to time for their preparation, potash and ammonia alum have risen to a position of much commercial importance during the last thirty years. There are also other applications of this useful substance to be men tioned later, in which the quantity annually consumed is rapidly increasing.