The Meta-Encyclopedia

Alum

2 of the 6 encyclopedias on this shelf carry an entry for Alum. Both are reproduced below, so you can see where they agree and where they differ.

Aiton's Encyclopedia (1910)

a substance which is essentially a double sulphate of aluminium and some other element, especially an alkali metal, combined with 24 molecules of water. Ammonium takes the place of an alkali metal in forming one of the alums. The most important alums of commerce are potassium alum and ammonium alum. Potassium alum, the well-known common alum, is made by roasting certain easily disintegrated rocks, containing aluminium silicate, then treating them with sulphuric acid. To the solution thus obtained is added potassium sulphate, and the alum is crystallized out. The crystals are large, white, and transparent, and are soluble in about ten parts of water, giving an acid reaction to the solution. When heated strongly, they lose their water of crystallization and form a substance known as "burnt alum." Alum is strongly astringent, and produces a puckering sensation when applied to the lining of the mouth. All alums crystallize from solution very perfectly, forming exceedingly pure crystals. Largely on account of this property the alums are extensively useful in the arts. Alum is used in the tanning of leather, in the preparation of size to be used in the manufacture of paper, and as the basis of the mordant or material used for making a permanent dye in the coloring of cloth. It is also one of the essential ingredients in many of the artificial yeasts, or so-called baking powders. The use of alum for the purpose of improving the appearance of inferior or slightly damaged flour was at one time commonly practiced in Europe, both by millers and bakers. Now, however, it is rarely if ever used for this purpose. The presence of notable quantities of alum or alum residue in bread is likely due to the use of an alum baking powder . Alum is sometimes added to the salt solution used to produce hardness and crispness in pickles. It has been occasionally mentioned as a preservative, but its use for that purpose has not found a wide application. Alum is employed to some extent as a medicine. It is an antidote in acute cases of lead poisoning, and, on account of its astringent action, it is applied to slight cuts to check the flow of blood. Very large doses produce symptoms of poisoning. The white of a raw egg is an effective antidote, as, in coagulating, it combines with the alum and permits of its removal from the stomach by the aid of an emetic. Alum crystals

Collier's New Encyclopedia (1921)

the name given to double salts of sulphate of aluminum with sulphates of potassium, sodium, ammonium, or of other monatomic metals, as silver, thallium cæsium, rubidium. They crystallize in octohedra. Potash alum, m, Al2K2 Al₂K2(SO4). +24H2O, is prepared by the decomposition of a shale containing iron pyrites. Alum has a sweet astringent taste, reddens litmus paper, and dissolves in its own weight of boiling water. Sodium alum is very soluble. Ammonia alum is often prepared by adding the ammonia liquor of gas-works instead of potash. Alum is used in dyeing and in preparing skins, etc. Alum is used in medicine as an astringent in doses of 10 to 20 grains. Ammonia alum, a mineral, called also tschermigite. Feather alum, a mineral, called also halotrichite. Iron alum, a mineral, called also halotrichite. Magnesia alum, alum, a mineral, called also pickeringite. Manganese alum, a mineral, in shipbuilding, especially for torpedocalled also apjohnite. Native alum, a boats. And as it is especially suitable mineral, called also kalinite. Soda alum, a mineral, called also mendozite. Saccharine alum, a composition made of common alum, with rose-water and the white of eggs boiled together to the consistence of a paste, and thus capable of being molded at ple pleasure. As it cools it grows as hard as an ordinary stone.

What Has Changed in the Last Century

The chemistry in both accounts still holds: alum is a double sulphate of aluminium with potassium, ammonium or sodium and 24 molecules of water, and the crystals are still prized for their near-perfect regularity — they were among the earliest substances examined by X-ray crystallography after 1912. What has moved is the medicine and the manufacture. Collier's 1921 dose of "10 to 20 grains" as an internal astringent, and Aiton's claim that alum is "an antidote in acute cases of lead poisoning" treated with raw egg white and an emetic, are both long abandoned; modern lead poisoning is treated with chelating agents such as EDTA or succimer, and inducing vomiting is no longer standard first aid. Alum is no longer roasted out of alum shale in quantity either — it is now made from purified aluminium hydroxide derived from bauxite — but the uses Aiton lists survive almost intact: styptic pencils, crystal deodorants, crisp pickles, paper sizing, and the aluminium salts that flocculate the impurities out of municipal drinking water.

Written for this edition, 2026.