Alum, (IPAEng: ˈæləm) refers to a specific chemical compound and a class of chemical compounds. The specific compound is the hydrated aluminum potassium sulfate with the formula KAl(SO₄)₂^.12H₂O. The wider class of compounds known as alums have the related stoichiometry, AB(SO₄)₂^.12H₂O.

Crystal chemistry of the alums

Double sulfates with the general formula A₂SO₄·B₂(SO₄)₃·24H₂O, are known where A is a monovalent cation such as sodium, potassium, rubidium, cesium, or thallium(I), or a compound cation such as ammonium (NH₄⁺), methylammonium (CH₃NH₃⁺), hydroxylammonium (HONH₃⁺) or hydrazinium (N₂H₅⁺), B is a trivalent metal ion, such as aluminium, chromium, titanium, manganese, vanadium, iron (III), cobalt(III), gallium, molybdenum, indium, ruthenium, rhodium, or iridium. The specific combinations of univalent cation, trivalent cation, and anion depends on the sizes of the ions. For example, unlike the other alkali metals the smallest one, lithium, does not form alums, and there is only one known sodium alum. In some cases, solid solutions of alums occur.

Alums crystallize in one of three different crystal structures. These classes are called α-, β- and γ-alums.

Applications

Alums are useful for a range of industrial processes. They are soluble in water; have an astringent, acid, and sweetish taste; react acid to litmus; and crystallize in regular octahedra. When heated they liquefy; and if the heating is continued, the water of crystallization is driven off, the salt froths and swells, and at last an amorphous powder remains.

Potassium alum is the common alum of commerce, although soda alum, ferric alum, and ammonium alum are manufactured.

Aluminium sulfate is sometimes called alum in informal contexts, but this usage is not regarded as technically correct. Its properties are quite different from those of the set of alums formally described above.

Alum in Antiquity - Pliny's writings

The word "alumen," which we translate "alum," occurs in Pliny's Natural History. In the 52th chapter of his 35th book he gives a detailed description of it. By comparing this with the account of stupteria given by Dioscorides in the 123rd chapter of his 5th book, it is obvious that the two are identical. Pliny informs us that alumen was found naturally in the earth. He calls it salsugoterrae. Different substances were distinguished by the name of "alumen"; but they were all characterized by a certain degree of astringency, and were all employed in dyeing and medicine, the light-colored alumen being useful in brilliant dyes, the dark-colored only in dyeing black or very dark colors. One species was a liquid, which was apt to be adulterated; but when pure it had the property of blackening when added to pomegranate juice. This property seems to characterize a solution of iron sulfate in water; a solution of ordinary (potassium) alum would possess no such property. Pliny says that there is another kind of alum that the Greeks call schistos. It forms in white threads upon the surface of certain stones. From the name schistos, and the mode of formation, there can be little doubt that this species was the salt which forms spontaneously on certain salty minerals, as alum slate and bituminous shale, and which consists chiefly of sulfates of iron and aluminium. Possibly in certain places the iron sulfate may have been nearly wanting, and then the salt would be white, and would answer, as Pliny says it did, for dyeing bright colors. Several other species of alumen are described by Pliny, but we are unable to make out to what minerals he alludes.