This metal has not been found native, but alumina, its oxide, Al2O3 (known also as argillaceous earth), is widely diffused as a silicate in clay, slate, granite, etc., and occurs nearly pure in the sapphire and ruby. The metal itself is of steel-gray color and is not readily oxidized: sp. gr. 2.67.

Alumen, the officinal alum, NH1,Al,(SO4)212H1O, is a double sulphate of alumina and ammonia with a large amount of water of crystallization: it occurs native sometimes in mineral waters, and in efflorescence on stone. (There are many similar "alums," or double sulphates of an alkaline base, and a metal: thus there is a potash - alum - formerly officinal, but more expensive than ammonia-alum - a soda-alum, etc. The same name is applied even when no alumina is present, as to the double sulphate of iron and ammonia - iron-alum - and to similar compounds of manganese and chromium.)


The officinal alum is prepared by oxidation of aluminous schist, sulphates of alumina and iron being formed then dissolved in water, and treated by sulphate of ammonia: on concentrating the solution, alum crystallizes out.

Characters And Tests

Alum crystallizes in octahedral, sometimes in cubic, forms, but is generally met with in irregular lumps, translucent and colorless when fresh, efflorescent and covered with small crystals after exposure. It has an acid reaction and a strongly astringent, subacid taste; is insoluble in alcohol, soluble in eighteen parts of cold, and half its weight of boiling water. Heated, it dissolves in its water of crystallization, and when this has been driven off, alum remains as a dry, white, spongy mass (alumen exsiccatum vel ustum - dried, or burnt alum). This has very astringent, somewhat caustic properties; it readily absorbs moisture, but is sparingly soluble: heated beyond 400° F. it is decomposed, and alumina, the oxide, Al2O3, remains. This oxide is insoluble in water, and when alum solutions are decomposed, separates as precipitate; hence the use of alum for clearing: turbid water, for when it is added, the alkaline and earthy salts present in the water combine with the sulphuric acid of the alum, and the alumina which precipitates, carries with it most of the impurities present: it has also special disinfecting powers.

The acetate of alumina (argilla acetica), the chloride (aluminium chlo-ratum), and the single sulphate (argilla sulphurica), though not officinal, are in occasional use; they are all soluble salts, of characteristic styptic taste. The sulphate has been found native, though not quite pure; it is more acid than the ordinary double sulphate, so that it blunts steel instruments and corrodes linen.

1. The bisulphide of ammonium (NH1HS), when added to solution of the salts of aluminium hydrate, gives a white gelatinous precipitate of ammonium hydrate. 2. The caustic alkalies and their carbonates give a white precipitate with aluminous solutions, soluble in excess of the former. 3. Solutions of the aluminous salts should not give a blue color on the addition of ferrocyanide and ferricyanide of potassium, showing freedom from iron. 4. Alum, when heated with caustic soda or potash, evolves ammonia.

Absorption And Elimination

Taken into the mouth alum exerts the local action presently to be described, and its first sweetish taste is followed by a peculiar feeling of constriction, and abundant flow of saliva: after reaching the stomach, combined, more or less, with albumen, some of it becomes absorbed though slowly. Orfila detected it in the urine and viscera of dogs after large doses (Annales d'Hygiene pub., i., 235 - 9,v.), and Krauss found the urine become very acid under its use. The greater part of the alum taken combines to form insoluble compounds with the bile and other organic products, and is eliminated with the faeces. It is remarkable, that although alumina is so common a constituent of vegetable food, it is not found in the human organism, showing how completely it passes out.

Physiological Action (External)

Alum acts as a typical simple astringent, contracting the arterioles and muscular fibres of the part touched by it, and rendering the surface pale and dry. It combines with albuminous secretions forming whitish flakes, or membranous films, insoluble in water, but soluble in acetic and hydrochloric acids (Mitscher-lich). If there be not enough fluid present to saturate the alum, it affects the deeper tissues in a somewhat caustic manner: this is especially the case with the dried salt. Strong or long-continued applications excite irritation with some degree of inflammation, and, under such circumstances, discharge from an affected part - e.g., the conjunctiva, or the vaginal mucous membrane - may be increased rather than diminished.

The acetate of alumina and chlor-aluminium have marked disinfectant powers, preserving organic tissue, and hindering putrefaction. Burow (185?) found that the acetate, mixed with fresh blood, formed a brown syrupy mass, in which the shape of the corpuscles was not retained, but which remained, without decomposing, for many months. Albumen, treated with the same salt, continued clear, and did not coagulate much on boiling: 0.5 per cent, prevented putrefaction of urine and of meat, and 2 per cent. sterilized bacteria.

The chloride of aluminium, chlor-alum, was introduced (mainly by Mr. John Gamgee) as a disinfectant free from poisonous or corrosive properties; it not only prevents decomposition, but removes its products by absorbing gas, etc.: it serves best, perhaps, for the disinfection of closets, drains, etc.: in the post-mortem room it is useful, but locally applied renders the muscular tissue pale (Lund: Medical Times, 1873).

Physiological Action (Internal)