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For the colloquial term alum meaning "graduate," see Alumnus.
A crystal of alum

Alum (pronounced /ˈæləm/) is both a specific chemical compound and a class of chemical compounds. The specific compound is the hydrated aluminium 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.[1] 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.


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" occurs in Pliny's Natural History. In the 52nd chapter of his 35th book, he gives a detailed description.[2] 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. name schistos, and the mode of formation, it appears 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[vague] in certain places the iron sulfate may have been nearly[vague] 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[who?] are unable to make out to what minerals he alludes.

The alumen of the ancients, then, was not always the same as the alum of the moderns. They certainly knew how to produce alum from alunite as this process is archaeologically attested on the island Lesbos[3]. This site was abandoned in the 7th century but dates back at least to the 2nd century AD. Native alumen from Melos appears to have been a mixture mainly of alunogen (Al2(SO4)3.17H2O) with alum and other minor sulfates[4]. The western desert of Egypt was a major source of alum substitutes in antiquity. These evaporites were mainly FeAl2(SO4)4.22H2O, MgAl2(SO4)4.22H2O, NaAl(SO4)2.6H2O, MgSO4.7H2O and Al2(SO4)3.17H2O[5]. Any contamination with iron sulfate was greatly disliked as this darkened and dulled dye colours. They were acquainted with a variety of substances of varying degrees of purity by the names of misy, sory, and chalcanthum. As alum and green vitriol were applied to a variety of substances in common, and as both are distinguished by a sweetish and astringent taste, writers, even after the discovery of alum, do not seem to have discriminated the two salts accurately from each other. In the writings of the alchemists we find the words misy, sory, chalcanthum applied to alum as well as to iron sulfate; and the name atramentum sutorium, which ought to belong, one would suppose, exclusively to green vitriol, applied indifferently to both. Various minerals are employed in the manufacture of alum, the most important being alunite or alum-stone, alum schist, bauxite and cryolite.

Alchemical and later discoveries and uses

The presence of sulfuric acid in potassium alum was known to the alchemists. In the 18th century, J. H. Pott and Andreas Sigismund Marggraf demonstrated that alumina was another constituent. Pott in his Lithogeognosia showed that the precipitate obtained when an alkali is poured into a solution of alum is quite different from lime and chalk, with which it had been confounded by G.E. Stahl. Marggraf showed that alumina is one of the constituents of alum, but that this earth possesses peculiar properties, and is one of the ingredients in common clay. He also showed that crystals of alum cannot be obtained by dissolving alumina in sulfuric acid and evaporating the solutions, but when a solution of potash or ammonia is dropped into this liquid, it immediately deposits perfect crystals of alum.

Torbern Bergman also observed that the addition of potash or ammonia made the solution of alumina in sulfuric acid crystallize, but that the same effect was not produced by the addition of soda or of lime, and that potassium sulfate is frequently found in alum.

After M.H. Klaproth had discovered the presence of potassium in leucite and lepidolite, it occurred to L.N. Vauquelin that it was probably an ingredient likewise in many other minerals. Knowing that alum cannot be obtained in crystals without the addition of potash, he began to suspect that this alkali constituted an essential ingredient in the salt, and in 1797 he published a dissertation demonstrating that alum is a double salt, composed of sulfuric acid, alumina, and potash. Soon after, J.A. Chaptal published the analysis of four different kinds of alum, namely, Roman alum, Levant alum, British alum and alum manufactured by himself. This analysis led to the same result as Vauquelin.

Early uses in industry

Egyptians reportedly used the coagulant alum as early as 1500 B.C. to reduce the visible cloudiness (turbidity) in the water. Alum was imported into England mainly from the Middle East, and, from the late 15th century onwards, the Papal States for hundreds of years. Its use there was as a dye-fixer (mordant) for wool (which was one of England's primary industries, the value of which increased significantly if dyed). These sources were unreliable, however, and there was a push to develop a source in England especially as imports from the Papal States were ceased following the excommunication of King Henry VIII. With state financing, attempts were made throughout the 16th century, but without success until early on in the 17th century. An industry was founded in Yorkshire to process the shale which contained the key ingredient, aluminium sulfate, and made an important contribution to the Industrial Revolution. Alum (known as turti in local Indian languages) was also used for water treatment by Indians for hundreds of years.

Current uses

Alum is used in vaccines as an adjuvant. Alum is commonly used as a coagulant (usually termed a flocculant in this context) in water treatment and purification.


Alum from alunite

In order to obtain alum from alunite, it is calcined and then exposed to the action of air for a considerable time. During this exposure it is kept continually moistened with water, so that it ultimately falls to a very fine powder. This powder is then lixiviated with hot water, the liquor decanted, and the alum allowed to crystallize. The alum schists employed in the manufacture of alum are mixtures of iron pyrite, aluminium silicate and various bituminous substances, and are found in upper Bavaria, Bohemia, Belgium, and Scotland. These are either roasted or exposed to the weathering action of the air. In the roasting process, sulfuric acid is formed and acts on the clay to form aluminium sulfate, a similar condition of affairs being produced during weathering. The mass is now systematically extracted with water, and a solution of aluminium sulfate of specific gravity 1.16 is prepared. This solution is allowed to stand for some time (in order that any calcium sulfate and basic ferric sulfate may separate), and is then evaporated until ferrous sulfate crystallizes on cooling; it is then drawn off and evaporated until it attains a specific gravity of 1.40. It is now allowed to stand for some time, decanted from any sediment, and finally mixed with the calculated quantity of potassium sulfate (or if ammonium alum is required, with ammonium sulfate), well agitated, and the alum is thrown down as a finely-divided precipitate of alum meal. If much iron should be present in the shale then it is preferable to use potassium chloride in place of potassium sulfate.

Alum from clays or bauxite

In the preparation of alum from clays or from bauxite, the material is gently calcined, then mixed with sulfuric acid and heated gradually to boiling; it is allowed to stand for some time, the clear solution drawn off and mixed with acid potassium sulfate and allowed to crystallize. When cryolite is used for the preparation of alum, it is mixed with calcium carbonate and heated. By this means, sodium aluminate is formed; it is then extracted with water and precipitated either by sodium bicarbonate or by passing a current of carbon dioxide through the solution. The precipitate is then dissolved in sulfuric acid, the requisite amount of potassium sulfate added and the solution allowed to crystallize.

Types of alum

Alum crystal with small amount of chrome alum to give slight violet color

Soda alum

Sodium alum, Na2SO4·Al2(SO4)3·24H2O, mainly occurs in nature as the mineral mendozite. It is very soluble in water, and is extremely difficult to purify. In the preparation of this salt, it is preferable to mix the component solutions in the cold, and to evaporate them at a temperature not exceeding 60 °C. 100 parts of water dissolve 110 parts of sodium alum at 0 °C, and 51 parts at 16 °C. Soda alum is used in the acidulent of food as well as in the manufacture of baking powder.

Ammonium alum

Ammonia alum, NH4Al(SO4)2·12H2O, a white crystalline double sulfate of aluminium, is used in water purification, in vegetable glues, in porcelain cements, in natural deodorants (though potassium alum is more commonly used), in tanning, dyeing and in fireproofing textiles.

Chrome alum

Chrome alum, K2Cr(SO4)2·12H2O, a dark violet crystalline double sulfate of chromium and potassium, was used in tanning.

Alum solubility

The solubility of the various alums in water varies greatly, sodium alum being readily soluble in water, while caesium and rubidium alums are only sparingly soluble. The various solubilities are shown in the following table.

At temperature T, 100 parts water dissolve:


Ammonium Alum

Potassium Alum

Rubidium Alum

Caesium Alum

0 °C





10 °C





50 °C





80 °C





100 °C





Selenate-containing alums

Alums are also known that contain selenium in place of sulfur. They are called selenium- or selenate-alums.



  • Alum in block form (usually potassium alum) is used as an aftershave, rubbed over the wet, freshly shaved face.
  • Styptic pencils containing aluminium sulfate or potassium aluminium sulfate are used as astringents to prevent bleeding from small shaving cuts.
  • Alum was used as a base in skin whiteners and treatments during the late 16th Century. A recipe for one such compound was given thus :

    "For the Freckles which one getteth by the heat of the Sun: Take a little Allom beaten small, temper amonst it a well brayed white of an egg, put it on a milde fire, stirring it always about that it wax not hard, and when it casteth up the scum, then it is enough, wherewith anoint the Freckles the space of three dayes: if you will defend your self that you get no Freckles on the face, then anoint your face with the whites of eggs." —Christopher Wirzung, General Practise of Physicke, 1654.

  • Alum may be used in depilatory waxes used for the removal of body hair, or applied to freshly waxed skin as a soothing agent.
  • In the 1950s, men sporting crewcut or flattop hairstyles sometimes applied alum to their front short hairs as an alternative to pomade. When the hair dried, it would stay up all day.
  • Alum's antibacterial properties contribute to its traditional use as an underarm deodorant. It has been used for this purpose in Europe; Mexico; Thailand, where it is called Sarn-Som; throughout Asia; and in the Philippines, where it is called Tawas. Today, potassium alum is sold commercially for this purpose as a "deodorant crystal," often in a protective plastic case.


  • Alum is used in many subunit vaccines as an adjuvant to enhance the body's response to immunogens. Such vaccines include hepatitis A, hepatitis B and DTaP.
  • Alum in powder or crystal form, or in styptic pencils, is sometimes applied to cuts to prevent or treat infection.
  • Powdered alum is commonly cited as a home remedy for canker sores.
  • Preparations containing alum are used by pet owners to stem bleeding associated with animal injuries caused by improper nail clipping.


  • Alum powder, found in the spice section of many grocery stores, may be used in pickling recipes as a preservative to maintain fruit and vegetable crispness.
  • Alum is used as the acidic component of some commercial baking powders.
  • Alum was used by bakers in the England during the 1800s to make bread whiter[6]. White bread was demanded by the middle class. In 1875, the Sale of Food and Drugs Act prevented this and other adulturations. [7]

As a Flame Retardant

  • Solutions containing alum may be used to treat cloth, wood and paper materials to increase their resistance to fire.
  • Alum is also a component of foamite, used in fire extinguishers to smother chemical and oil fires.

As a Chemical Flocculant

  • Alum is used to clarify water by catching the very fine suspended particles in a gel-like precipitate of aluminium hydroxide. This sinks to the bottom of the containing vessel and can be removed in a variety of ways.
  • Alum may be used to increase the viscosity of a ceramic glaze suspension; this makes the glaze more readily adherent and slows its rate of sedimentation.
  • Alum is an ingredient in some recipes for homemade modeling compounds intended for use by children. (These are often called "play clay" or "play dough" for their similarity to "Play-Doh", a trademarked product marketed by American toy manufacturer Hasbro).

Alum in comedy

Much use was made of the supposed properties of Alum as a comedy gag in films, primarily in the 1920's and 1930's. In a typical situation it would be introduced into foodstuffs (by accident or intent) with ingestion causing the victim's mouth to assume a tight pucker. Speech was usually difficult or impossible. In animation Cartoon Physics could magnify the effect- the victim's head might shrink and/or the voice alter to a shrill squeak.

Related compounds

In addition to the alums, which are dodecahydrates, double sulfates and selenates of univalent and trivalent cations occur with other degrees of hydration. These materials may also be referred to as alums, including the undecahydrates such as mendozite and kalinite, hexahydrates such as guanidinium (CH6N3+) and dimethylammonium ((CH3)2NH2+) "alums", tetrahydrates such as goldichite, monohydrates such as thallium plutonium sulfate and anhydrous alums (yavapaiites). These classes include differing, but overlapping, combinations of ions.

A pseudo alum is a double sulfate of the typical formula ASO4·B2(SO4)3·22H2O, where A is a divalent metal ion, such as cobalt (wupatkiite), manganese (apjohnite), magnesium (pickingerite) or iron (halotrichite or feather alum), and B is a trivalent metal ion.

A Tutton salt is a double sulfate of the typical formula A2SO4·BSO4·6H2O, where A is a univalent cation, and B a divalent metal ion.


  • List of minerals
  • Potash alum
  • Soda alum


  1. ^ Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.
  2. ^
  3. ^ A. Archontidou 2005 Un atelier de preparation de l'alun a partir de l'alunite dans l'isle de Lesbos in L'alun de Mediterranee ed P.Borgard et al.
  4. ^ A.J.Hall & E. Photos-Jones The nature of Melian alumen and its potential for exploitation in Antiquity in Bogard
  5. ^ M.Picon et al. 2005 L'alun des oasis occidentales d'Egypte: researches sur terrain et recherches en laboratiore in Bogard
  6. ^,M1
  7. ^,M1

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