General Sources of Alum Salts. - Aluminium is very widely distributed in nature, clays being silicates of alumina. Two kinds of clay, kaolin and fuller's earth, being inert powders, are used as demulcents (pp. 347 and 446), and kaolin also as a pill-basis.

General Preparation. - It is prepared on a large scale from a kind of clay-slate called alum-schist. This contains a quantity of ferric sulphide. It is first roasted and moistened and exposed to air. The sulphur is thus converted into sulphuric acid, and ferrous sulphate and aluminium sulphate are formed. These are separated by lixiviation with water, and ammonium chloride is added. This forms ammonium sulphate, which combines with aluminium sulphate to form alum, ferrous chloride remaining in solution.

General Reactions of Alum Salts. - Salts of aluminium give a white gelatinous precipitate of hydrate with caustic potash or soda, soluble in excess; with ammonia a similar precipitate, insoluble in excess. The insolubility of the precipitate with ammonia in excess of the reagent readily distinguishes aluminium from zinc, which also gives a white precipitate with ammonium sulphide. Carbonates of potassium, sodium, and ammonium also precipitate the hydrate, which is insoluble in excess; ammonium sulphide also gives a white precipitate of hydrate.

General Impurities of Aluminium Salts. - The chief is sulphate of iron coming from the schist.

General Tests. - Alum should give no blue with either ferro- or ferri-cyanide of potassium.