The ammonia-salt used in making alum is generally the crude sulphate prepared from the ammoniacal liquor of gas-works, or that from the manufacture of sal-ammoniac by the destructive distillation of animal matter. Both these liquors may be used without previous conversion into sulphate of ammonia whenever there is an excess of sulphuric acid in the aluminous solution.
Soda-salts are seldom, if ever, used as precipitants in the manufacture of alum, on account of the easy solubility of the resulting SODA-ALUM—a property which unfits them for this purpose. See Alums, Ammonia, Dyeing, Mordants, Potash, Sulphuric Acid, &c. (also below).
Alum, Ammonia. (NH4)2SO4 . Al2(SO4)3 . 24 Aq. Syn. (Alumen; Alum; B. P.), Alu′men Ammonia′tum, L.; Alun d’ammoniaque, A. ammoniacal, Fr. This is an alum in which the sulphate of potassium is replaced by an equivalent of sulphate of ammonium. It is prepared by adding crude sulphate of ammonium to solution of sulphate of aluminum; or gas-liquor, putrid urine, &c., to the acid-sulphate.
Much of the common alum, especially that prepared on the Continent, contains both potassium and ammonium; and recently enormous works for its manufacture have been established in England. As an astringent, and as a source of alumina in dyeing, it resembles potash-alum (i. e. ordinary alum). It may, however, be readily distinguished from the latter by the fumes of ammonia which are evolved when it is moistened and triturated, or heated, with caustic potassa or quick-lime; and by the residuum of its exposure to a white heat being pure alumina. See Alum (antè).
Alum, Basic. A variety of alum found native at Tolfa. On calcination and subsequent lixiviation it yields ordinary alum. A like substance falls as a white powder, when newly precipitated alumina is boiled in a solution of alum.
Alum, Baumé’s. Alum-white. See White Pigments.
Alum, Dried; Alum, Burnt. Syn. Alu′men us′tum, A. exsicca′tum (B. P.); Alun Sec, Fr.; Gebrannter alaun, Ger.; Alume calcinato, Ital. Alum deprived of its water of crystallisation by heat.
Prep. Take of alum, 4 oz. Heat the alum in a porcelain dish or other suitable vessel, till it liquefies, then raise and continue the heat, not allowing it to exceed 400°, till aqueous vapour ceases to be disengaged, and the salt has lost 47 per cent. of its weight. Reduce the residue to powder, and preserve it in a well-stopped bottle.
Prop., &c. Similar to those of common alum, but it is rather more astringent, and is less soluble. When moistened, or placed in contact with water, it resumes its water of crystallisation with evolution of heat.—Dose, 10 to 20 gr.; in colic (especially painters’ colic), hæmoptysis, &c. It is chiefly used as an escharotic, to destroy ‘proud flesh,’ &c. It must be kept in a stoppered bottle.
Alum, Chrome. See Alums (in Chemistry).