An alum () is a type of chemical compound, usually a hydrated double sulfate salt of aluminium with the general formula , where is a monovalent cation such as potassium or ammonium. By itself, "alum" often refers to potassium alum, with the formula . Other alums are named after the monovalent ion, such as sodium alum and ammonium alum.
The name "alum" is also used, more generally, for salts with the same formula and structure, except that aluminium is replaced by another trivalent metal ion like chromium, and/or sulfur is replaced by another chalcogen like selenium. The most common of these analogs is chrome alum .
In most industries, the name "alum" (or "papermaker's alum") is used to refer to aluminium sulfate, , which is used for most industrial flocculation (the variable is an integer whose size depends on the amount of water absorbed into the alum). In medicine, "alum" may also refer to aluminium hydroxide gel used as a vaccine adjuvant.
History
Alum found at archaeological sites
The western desert of Egypt was a major source of alum substitutes in antiquity. These evaporites were mainly , , , and . The Ancient Greek Herodotus mentions Egyptian alum as a valuable commodity in The Histories.
The production of potassium alum from alunite is archaeologically attested on the island Lesbos. The site was abandoned in the 7th century CE, but dates back at least to the 2nd century CE. Native alumen from the island of Melos appears to have been a mixture mainly of alunogen () with potassium alum and other minor sulfates.
Alumen in Pliny and Dioscorides
A detailed description of a substance called alumen occurs in the Roman Pliny the Elder's Natural History.
By comparing Pliny's description with the account of stypteria (στυπτηρία) given by Dioscorides, it is obvious the two are identical. Pliny informs us that a form of alumen was found naturally in the earth, and calls it salsugoterrae.
Pliny wrote that different substances were distinguished by the name of alumen, but they were all characterised by a certain degree of astringency, and were all employed in dyeing and medicine. Pliny wrote that there is another kind of alum that the Greeks call schiston, and which "splits into filaments of a whitish colour". From the name schiston and the mode of formation, it appears that this kind was the salt that forms spontaneously on certain salty minerals, as alum slate and bituminous shale, and consists chiefly of sulfates of iron and aluminium. One kind of alumen 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. Contamination with iron sulfate was greatly disliked as this darkened and dulled dye colours. In some places the iron sulfate may have been lacking, so the salt would be white and would be suitable, according to Pliny, for dyeing bright colors.
Pliny describes several other types of alumen but it is not clear as to what these minerals are. The alumen of the ancients, then, was not always potassium alum, not even an alkali aluminum sulfate.
Alum described in medieval texts
Alum and green vitriol (iron sulfate) both have sweetish and astringent taste, and they had overlapping uses. Therefore, through the Middle Ages, alchemists and other writers do not seem to have distinguished the two salts accurately. In the writings of the alchemists we find the words misy, sory, and chalcanthum applied to either compound; and the name atramentum sutorium, which one might expect to belong exclusively to green vitriol, applied indiscriminately to both.
Alum was the most common mordant (substance used to set dyes on fabrics) used in the dye industry in the Islamic middle ages. It was the main export of the Chad region, from where it was transported to the markets of Egypt and Morocco, and then on to Europe. Less significant sources were found in Egypt and Yemen.
Modern understanding of the alums
In the early 1700s, G.E. Stahl claimed that reacting sulfuric acid with limestone produced a sort of alum. The error was soon corrected by J.H. Pott and A.S. Marggraf, who showed that the precipitate obtained when an alkali is poured into a solution of alum, namely alumina, is quite different from lime and chalk, and is one of the ingredients in common clay.