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Alum, Allom [aluminium potassium sulfate], in chemistry, is a term given to the crystallized double sulfates of the typical formula M+2SO4M3+2(SO4)324H2O, where M+ is the sign of an alkali metal (lithium, sodium, potassium, rubidium, or caesium), and M3+ denotes one of the trivalent metal ions (typically aluminium, chromium, or iron (III)). The ammonium ion (NH4+) also occurs in the M+ position.

These salts are employed in dyeing and various other 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 both soda alum and ammonium alum are manufactured.

  • Alchemical and later discoveries and uses
  • The presence of sulfuric acid in potassium alum was known to the alchemists. J. H. Pott and A. S. 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.

  • Pliny's writings
  • The word "alumen," which we translate "alum," occurs in Pliny's Natural History. In the 15th chapter of his 35th book he gives a detailed description of it. 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. From the name schistos, and the mode of formation, there can be little doubt that this species was the salt which forms spontaneously on certain slaty minerals, as alum slate and bituminous shale, and which consists chiefly of sulfates of iron and aluminium. Possibly in certain places the iron sulfate may have been nearly 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 are unable to make out to what minerals he alludes.

    The alumen of the ancients, then, was not the same as the alum of the moderns. It was most commonly an iron sulfate, sometimes probably an aluminium sulfate, and usually a mixture of the two. But the ancients were unacquainted with our alum. They were acquainted with a crystallized iron sulfate, and distinguished it 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.

  • Early uses in industry
  • 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. 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.

  • 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 sulfa te (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
  • Potassium alum

    Potassium alumi, K2SO4Al2(SO4)324H2O, this type of alum is very soluble in water. Most commonly, alum crystallizes in regular octahedra form. The solution reddens litmus and is an astringent. When heated to nearly a red heat it gives a porous, friable mass which is known as "burnt alum." It fuses at 92 C in its own water of crystallization. "Neutral alum" is obtained by the addition of as much sodium carbonate to a solution of alum as will begin to cause the separation of alumina; it is much used in mordanting. Alum is used as a mordant, in the preparation of lakes for sizing hand-made paper and in the clarifying of turbid liquids. It has cosmetic uses as a deodorant and as an aftershave. See the mineral alunite.

    Soda alum

    Sodium alum, Na2SO4Al2(SO4)324H2O, 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.

    Chrome alum

    Chrome alum, K2SO4Cr2(SO4)312H2O, appears chiefly as a by-product in the manufacture of alizarin, and as a product of the reaction in dichromate batteries. Below 60oC it dissolves in water to give a purple solution, while above that temperature it gives a green solution, due to the hydrolysis of the chromium(III) ion.

    Ammonium alum

    Main article: Ammonium alum

    Ammonia alum, NH4Al(SO4)212H2O, 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.

    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.

  • Uses
  • Shaving alum is a powdered form of alum used as an astringent to prevent bleeding from small shaving cuts. The styptic pencils sold for this purpose contain aluminium sulfate or potassium aluminium sulfate. Similar products are also used on animals to prevent bleeding after nail-clipping. Alum in block form (usually potassium alum) is used as an aftershave, rubbed over the wet freshly shaved face.

    Crystal deodorant: Alum was used in the past as a natural underarm deodorant in Europe, Mexico, Thailand, and the Far East and in the Philippines where it is called Tawas. It is now commercially sold for this purpose in many countries, often in a plastic case that protects the crystal and makes it resemble other non-liquid deodorants. Typically potassium alum is used.

    Alum powder, found amongst spices at most grocery stores, is used in pickling recipes as a preservative, to maintain crispness, and as an ingredient in some play dough recipes. It is also commonly cited as a home remedy or pain relief for canker sores.

    Water treatment: Alum (aluminium sulfate) is used in water treatment. The addition of alum to raw water causes small particles and colloids to stick together and form heavier particles (floc) which will settle in water. This process is called coagulation or flocculation.

    Fire retardant: By soaking and then drying cloth and paper materials they can be made fireproof.

    Wax: Alum is used in the Middle East as a component in wax, compounded with other ingredients to create a hair-removal substance.

    Foamite: Alum is used to make foamite which is used in many fire extinguishers for chemical and oil fires.

    Adjuvant:Alum is used regularly as an adjuvant (enhances immune response to a given immunogen when given with it) in human immunizations.

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