Silica 56·5 Alumina 19· Sulphuric acid (SO_{3}) 16·5 Potassa 4· Water 3· Loss 1· —————— 100· which exhibits an excess of about 3% of sulphuric acid, and about 14% of alumina, more than are requisite to form alum with the 4% of potassa; proportions which, therefore, require to be supplemented with a potassium salt during the process of manufacture. The alum-stone of Mont d’Or contains, according to Cordier, 1·4% of oxide of iron. The presence of lime in alum-ore is most prejudicial, owing to its affinity for sulphuric acid being greater than that of either alumina or iron. Ores containing it in any quantity are, therefore, unfitted for the manufacture of alum. Magnesia is also prejudicial; but in this case the sulphate of magnesia left in the mother-liquors is not wholly valueless, as it may be crystallised and sold as ‘Epsom-salt,’——a thing which is actually done in some English alum-works. The potash-salt employed by the alum-makers is either the sulphate or the chloride——chiefly the latter; its sources being the waste liquor of soap-works, saltpetre refineries, and glass-houses. Wood-ashes, although rich in potash, do not answer well unless freed by lixiviation from the large amount of carbonate of lime which is always present in them. 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.= (NH_{4})_{2}SO_{4} . Al_{2}(SO_{4})_{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.