Alum

An alum (/ˈæləm/) is a type of chemical compound, usually a hydrated double sulfate salt of aluminium with the general formula XAl(SO4)2·12 H2O, such that X is a monovalent cation such as potassium or ammonium.

Native alumen from the island of Melos appears to have been a mixture mainly of alunogen (Al2(SO4)3·17 H2O) with potassium alum and other minor sulfates.

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 for dyeing and medicine.

Pliny wrote that there is another kind of alum that the ancient Greeks term schiston, and which "splits into filaments of a whitish colour".

[8] From the name schiston and the mode of formation, it seems that this kind was the salt that forms spontaneously on certain salty minerals, as alum slate and bituminous shale, and consists mainly of sulfates of iron and aluminium.

[citation needed] 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.

[12][a][b][13] The error was soon corrected by Johann Heinrich Pott and Andreas Sigismund 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.

[14][c][15]: 41–66 Marggraf also showed that perfect crystals with properties of alum can be obtained by dissolving alumina in sulfuric acid and adding potash or ammonia to the concentrated solution.

As soon as Martin Klaproth discovered the presence of potassium in leucite and lepidolite,[17][18][e] Vauquelin demonstrated that common alum is a double salt, composed of sulfuric acid, alumina, and potash.

They are soluble in water, have a sweetish taste, react as acid by turning blue litmus to red, and crystallize in regular octahedra.

The first X-ray crystal structures of alums were reported in 1927 by James M. Cork and Lawrence Bragg, and were used to develop the phase retrieval technique isomorphous replacement.

It is still widely used in water treatment, for medicine, for cosmetics (in deodorant), for food preparation (in baking powder and pickling), and to fire-proof paper and cloth.

The amount of alum present in each loaf of bread could reach concentrations that would be toxic to humans and cause chronic diarrhea, which could result in the death of young children.

[26] Alum is used as a mordant in traditional textiles;[27] and in Indonesia and the Philippines, solutions of tawas, salt, borax, and organic pigments were used to change the color of gold ornaments.

Alum in the form of potassium aluminium sulphate or ammonium aluminium sulfate in a concentrated bath of hot water is regularly used by jewelers and machinists to dissolve hardened steel drill bits that have broken off in items made of aluminum, copper, brass, gold (any karat), silver (both sterling and fine) and stainless steel.

The most important example is chrome alum, KCr(SO4)2·12 H2O, a dark violet crystalline double sulfate of chromium and potassium, was used in tanning.

In addition to the alums, which are dodecahydrates, double sulfates and selenates of univalent and trivalent cations occur with other degrees of hydration.

A pseudo alum is a double sulfate of the typical formula XSO4·Y2(SO4)3·22 H2O, such that Double sulfates with the general formula X2SO4·Y2(SO4)3·24 H2O are also known, where X is a monovalent cation such as sodium, potassium, rubidium, caesium, thalliumI, ammonium, or (NH+4), methylammonium (CH3NH+3), hydroxylammonium (HONH+3) or hydrazinium (N2H+5) and Y is a trivalent metal ion, such as aluminium, chromium, titanium, manganese, vanadium, ironIII, cobalt, gallium, molybdenum, indium, ruthenium, rhodium, or iridium.