Alum (pron.: //) is both a specific chemical compound and a class of chemical compounds. The specific compound is the hydrated potassium aluminium sulfate (potassium alum) with the formula KAl(SO4)2·12H2O. More widely, alums are double sulphate salts, with the formula AM(SO4)2·12H2O, where A is a monovalent cation such as potassium or ammonium and M is a trivalent metal ion such as aluminium or chromium(III).
Chemical properties 
Alums are useful for a range of industrial processes. They are soluble in water; have a 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. They are astringent and acidic.
Industrial uses 
Alum has been used at least since Roman times for purification of drinking water and industrial process water. Between 30 and 40 ppm of alum for household wastewater, often more for industrial wastewater, is added to the water so that the negatively charged colloidal particles clump together into "flocs", which then float to the top of the liquid, settle to the bottom of the liquid, or can be more easily filtered from the liquid, prior to further filtration and disinfection of the water.
- Alum in block form (usually potassium alum) is used as a blood coagulant.
- Styptic pencils containing aluminium sulfate or potassium aluminium sulfate are used as astringents to prevent bleeding from small shaving cuts.
- 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 antiperspirant and 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 or ammonium alum is sold commercially for this purpose as a "deodorant crystal", often in a protective plastic case.
- 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 England during the 1800s to make bread whiter. The Sale of Food and Drugs Act 1875 prevented this and other adulterations.
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.
- Alum is also used in fire extinguishers to smother chemical and oil fires.
Chemical flocculant 
- Alum is used to clarify water by neutralizing the electrical double layer surrounding very fine suspended particles, allowing them to flocculate (stick together). After flocculation, the particles will be large enough to settle and can be removed.
- 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 is used in the tanning of animal hides to remove moisture, prevent rotting, and produce a type of leather.
In antiquity 
The word "alumen" occurs in Pliny's Natural History. In the 52nd chapter of his 35th book, he gives a detailed description. 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 characterised 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 schiston, and which "splits into filaments of a whitish colour", From the name schiston, and the mode of formation, it appears that this species 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. 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. This site was abandoned in the seventh century but dates back at least to the second century CE. Native alumen from Melos appears to have been a mixture mainly of alunogen (Al2(SO4)3·17H2O) with alum and other minor sulfates. 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. 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, alum schist, bauxite and cryolite.
Alchemical and later discoveries and uses 
In the 18th century, J. H. Pott and Andreas Sigismund Marggraf demonstrated that alumina was a 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 can be obtained by dissolving alumina in sulfuric acid and evaporating the solutions, and 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 BC 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 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. One of the oldest historic sites for the production of alum from shale and human urine are the Peak alum works in Ravenscar, North Yorkshire.
Alum (known as turti/sphatika in local languages) was also used for water treatment by Indians for thousands of years. Ayurveda describes sphatika as an astringent, haemostatic, antiseptic. It has anti-inflammatory, anti-pyretic and antibiotic properties. Sphatika’s use in treating tonsillitis has been referred in ancient Ayurvedic texts. Sphatika is used internally as well as externally.
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 and sulfuric acid, 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, 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.
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.
Many trivalent metals are capable of forming alums. The general form of an alum is AMIII(SO4)2·nH2O, where A is an alkali metal or ammonium, MIII is a trivalent metal, and n often is 12. In general, alums are easier formed when the alkali metal atom is larger. This rule was first stated by Locke in 1902:
Double sulfates with the general formula A2SO4·B2(SO4)3·24H2O, are known where A is a monovalent cation such as sodium, potassium, rubidium, caesium, or thallium(I), or a compound cation such as ammonium (NH+
4), methylammonium (CH3NH+
3), hydroxylammonium (HONH+
3) or hydrazinium (N2H+
5), B is a trivalent metal ion, such as aluminium, chromium, titanium, manganese, vanadium, iron(III), cobalt(III), gallium, molybdenum, indium, ruthenium, rhodium, or iridium. Analogous selenates also occur. 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.
Potash alum 
Aluminum potassium sulfate, potash alum, KAl(SO4)2·12H2O is used as an astringent and antisepsis in various food preparation processes such as pickling and fermentation and as a flocculant for water purification, among other things. A common method of producing potash alum is leaching of alumina from bauxite, which is then reacted with potassium sulfate. As a naturally occurring mineral, potash alum is known as alum-(K). Other potassium aluminium sulfate minerals are alunite (KAl(SO4)2·2Al(OH)3) and kalinite (KAl(SO4)2·11H2O).
Soda alum 
Soda alum, NaAl(SO4)2·12H2O, 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 
Ammonium alum, NH4Al(SO4)2·12H2O, a white crystalline double sulfate of aluminium, is used in water purification, in vegetable glues, in porcelain cements, in deodorants (though potassium alum is more commonly used), in tanning, dyeing and in fireproofing textiles.
Chrome alum 
Chrome alum, KCr(SO4)2·12H2O, a dark violet crystalline double sulfate of chromium and potassium, was used in tanning.
Selenate-containing alums 
Aluminium sulfate 
Aluminium sulfate is referred to as papermaker's alum. Although reference to this compound as alum is quite common in industrial communication, it is not regarded as technically correct. Its properties are quite different from those of the set of alums described above. Most industrial flocculation done with alum is actually aluminum sulfate.
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:
|T||Ammonium alum||Potassium alum||Rubidium alum||Cesium alum|
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 (CH6N+
3) and dimethylammonium ((CH3)2NH+
2) "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.
Double sulfates of the composition A2SO4·2BSO4, where A is a univalent cation and B is a divalent metal ion are referred to as langbeinites, after the prototypical potassium magnesium sulfate.
In popular culture 
Much use was made of the supposed properties of alum as a comedy gag in films, primarily in the 1920s and 1930s. In a typical situation it would be introduced by accident or intent into foodstuffs, with ingestion causing the victim's mouth to assume a tight pucker. Speech was usually difficult or impossible. Alum's use at the time as an astringent gargle for curing sore throats provided inspiration for the gag. In animation, cartoon physics could magnify the effect — a man or woman who ingested alum would learn that it caused his or her head to shrink and/or his or her voice to become several octaves higher. This gag was famously used on Sylvester Cat in "I Tawt I Taw a Puddy Tat", "Back Alley Oproar", and "Birds Anonymous", and by Bugs Bunny in the cartoon "Long-Haired Hare", as well as in Laurel and Hardy's Tit for Tat (1935) or The Three Stooges' No Census, No Feeling (1940).
See also 
- List of minerals
- Potash alum
- Soda alum
- Aluminium sulfate (Papermaker's alum)
- Gum bichromate photo prints and other similar processes use alums, sometimes as colloid (gelatin, albumen) hardeners
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