ALUM, in chemistry, a general name for a large class of substances, which may be defined as double sulphates or selenates, in which one of the bases is aluminum, chromium, manga nese, iron, indium or gallium, and the other is sodium, potassium, rubidium, caesium, am monium, silver or thallium. The alums all crystallize in cubes or octahedra, with 24 mole cules of water, and are all isomorphous, so that when in solution together they cannot be sepa rated by crystallization. In naming them alu minum is understood to be one of the metals present unless the contrary is expressly indi cated. Thus alum" is the alum whose formula is Al,(SO4).-1- KaS0.-1- 24H20. If aluminum is not present, the metal that re places it is stated; thus 9ron-sodium alum" is the alum whose formula is Fe,(SO4).± NasSO4+ 2411,0. When selenium replaces the sulphur in one or both of its positions, the alum is most clearly identified by giving its formula. In addition to the true alums a class of substances known as "pseudo-alums" ex ists. These also crystallize with 24 molecules of water, but they are not isomorphous with the true alums. Mti,(SO4) 2 + MgSO4-1- 24H,0 is an example of this class.
All the alums are soluble in water, and it is probable that all are resolved, at least partially, into their constituents, by solution. It is known that in certain cases (in silver alum, for ex ample) the separation is absolute. All of the alums give an acid reaction when in solution, all have an astringent taste and all lose their water of crystallization when heated.
The alum of commerce is assumed to be potash alum, the formula of which has been al ready given. Ammonia alum — (N1-14SO,A1, 03(SC:10,24H:0—is sometimes substituted, however, owing to the cheapness of sulphate of ammonia, which is now obtained as a by product in the manufacture of illuminating gas. Sodium alum is probably not substituted for potash alum to any great extent.
The alums are largely used in the arts, es pecially in dyeing and tanning, and in the puri fication of water. Potassium alum is also used with borax in waterproofing cloth and with lime in preserving timber; in the size used in paper making; and as a soap precipitant in the manufacture of glycerine. Bread made from flour containing a small amount of alum is said to be very white, and partly for this rea son and partly on account of the cheapness of the substance, ammonia alum has been largely used in the manufacture of baking-powders.
This practice has been condemned and pro nounced detrimental to health, however, by nearly all the authorities who have written on the subject.
Commercial alum is made from alum stone (see ALurirrE), from alum shale, or from bauxite and cryolite, generally by roasting. It is also made in greater purity by mixing solu tions of the sulphates of alumina and potassa and allowing the alum to crystallize out. In making ammonia alum, sulphate of ammonia is substituted for the potassium sulphate (see AturirrE). Some alums occur native. (See MENnozrrE; TSCH ER At xcnr..) The United State census of 1914 reported 19 establish ments making alums, with a total output of 313,712,000 pounds, valued at $3,467,969, an in crease over 1909 of 37,418,000 pounds, and $445,614. Of the 19 establishments, 5 were located in Pennsylvania, 3 each in Illinois, Mas sachusetts and New York, 2 in Michigan and 1 each in California, Connecticut and New Jersey. The output comprised 142,438,000 pounds reported as aluminum sulphate, valued at $1,277,836; 23,338,000 pounds of alum cake, valued at $251,186; 42,562,000 pounds of con centrated alum, valued at $450,730; 22,629,000 pounds of burnt alum, valued at $364,656; 12, 765,000 pounds of potash alum, valued at $219, 968; 13,995,000 pounds of soda alum, valued at $254,477, and 55,985,000 pounds of other alums, designated as porous alum, excelsior alum, pearl alum, ammonium alum and chloride of alumina, valued at $649,116.
Alum Poisoning.—This poisoning may be acute or chronic, the latter being much more common. In the acute variety, often the result of accidental drinking of a gargling solution, the symptoms are nausea, vomiting, purging, cold clammy skin, small thready pulse, thirst, muscular tremor, followed by a rise in tempera ture in those that have recovered. The treat ment pursued in the case of alum poisoning is to wash out the stomach and use the white of eggs as a chemical antidote.
Alum being so widely employed as a pre servative, as a means of clarifying water and as an adulterant in baking-powders, the ques tion of chronic alum poisoning becomes of great importance. The symptoms that have been most frequently observed in such poison ing are disturbances of digestion and consti pation. The question as to its harmful action on the kidneys is not yet decided, but it would seem to be harmful rather than the reverse.