ALUM, the name given in commerce and by long popular usage to a double salt composed of aluminium sulphate and potas sium sulphate. In chemistry the name is applied to a whole series of sulphates, which form one of the most important examples of isomorphism (q.v.). The potassium of common alum may be replaced by ammonium, sodium, potassium, rubidium, silver, caes ium, and thallium (and possibly lithium). Further, the alumin ium itself, from which the substance derives its name, may also be replaced by other trivalent metals, titanium, vanadium, chro mium, manganese, iron, cobalt, gallium, rhodium, indium and irid ium, so that there are many possible alums containing neither aluminium or potassium. Also selenium may replace sulphur in the acid radical in some of the alums as, e.g., in aluminium ammonium selenate, (Se04) 3, (NH4) or in the sulphate-selen ate, An aluminium hydroxylamine alum, and alkyl-ammonium alum, may also be prepared. Thus the term covers the double sulphates or selenates of a univalent and a trivalent element or radicle, crystallizing in octahedra or cubes with 24 molecules of water of crystallization. Of the theoretically possible alum combinations a relatively small number only have been isolated; thus some 47 sulphate alums have been reported.
The alums crystallize in the regular system in octahedra with 24 molecules of water; from a basic solution of ordinary alum the cube tends to predominate. In a practically cold solution of the latter character an octahedron may gradually be grown into the cube, the composition of the crystal remaining unaltered. Owing to the symmetry of the crystal form, perfect single octahedra of large size may be obtained by slow crystallization in the cold. An octahedron weighing i icwts. has been grown in this way in a period of about two years. On the detailed structure of the alums as deduced from X-ray examination, see J. M. Cook (Phil. Mag., 1927, 4, p. 688) .
The aluminium sulphate alums have an astringent and acid yet sweetish taste. Potassium and ammonium alum crystals are per manent in the air; the sodium salt tends somewhat to effloresce. The alums melt in their crystal water between about 37° C. for ferric thallium alum, and I i7° C. for aluminium caesium alum. By careful heating, especially in a current of warm air, ordinary alum may ultimately be completely dehydrated at 100° C. If heated rapidly it intumesces and forms a very light bulky porous mass of "burnt alum." From the ammonium alum further heat ing at high temperature leaves the pure sesquioxide of aluminium.
The solubilities of the alums vary widely : the quantity dis solved by I oo parts of cold water (15° C.) may be illustrated by the following figures for some of the aluminium alums— caesium alum 0.35 part, potassium alum 9.59 parts, ammonium alum 12.66 parts, sodium alum 51 parts (i6° C.). Water will dissolve several times its own weight of potassium alum at 9o° C. The aqueous solutions act as weak acids. Aluminium alums find employment in a wide range of industries among which are dyeing, paper-making, tanning, water purification, pigment lakes, and plaster : chromium alum is used in dyeing and tanning.
Preparation.—Thevery early history of the manufacture of alum is unknown. Potash alum, aluminium sulphate, and ferrous and ferric sulphates all occur native as efflorescent salts or earthy impregnations in varying degree of admixture and purity, and the alumen of Pliny (Natural History, Bk. xxxv., Ch. 15) would appear to have covered a number of salts or their mixtures : he refers to methods of test by means of galls and pomegranate juice (the tannin of which would serve to indicate the presence of iron) to determine the qualities suitable for dyeing bright colours. In the 15th century the Turkish Mediterranean trade in dyed fabrics and wool, in which alum was employed as a mordant, amounted to a very large annual sum ; but the preparation of native salts by lixiviation and evaporation and, later, that of potassium alum from the mineral alunite, a basic sulphate of alumina and potash which is found in a number of places in the Near East, was probably practised much earlier.
About the year 146o, John di Castro learned the art at Con stantinople and began to make alum at Tolfa from an impor tant local deposit of alunite which has remained in use to within the present century. The introduction of the manufacture into Italy (where it shortly afterwards became a papal monopoly) was announced as a great victory over the Turks. In England Sir Thomas Chaloner began the preparation of alum from the schists on his Yorkshire estate about the year 1600. Despite papal denunciation, the manufacture extended considerably, and the method continued largely unchanged down to about the middle of last century in the neighbourhood of Whitby and Guisborough, employing the Lias schists, and, at Hurlet and Campsie, with mineral of very similar character from the base of the much older carboniferous limestone series of Scotland. This type of deposit also occurs in Belgium, Germany, the former Austrian empire and other countries, and has been worked for centuries for the same purpose in a number of places on the Continent.
Both the Italian method of manufacture from alunite and that from the alum schists were carried on, of necessity, in the absence of any extraneous supply of sulphuric acid, the production of which only assumed importance towards the end of the 18th and early part of the I gth centuries. When alunite [ if pure] is calcined at a temperature below that at which would be driven off, a part of the alumina together with the major portion of the sulphuric acid and potash is rendered soluble in water, although much of the alumina remains insoluble. The solution obtained was basic and gave rise to a largely cubic alum which, although tending to be tinted by a very small amount of insoluble iron oxide, was otherwise of a very high degree of purity and was known as Roman alum.
Similarly the value of the Lias and other alum schists depended mainly upon their self-contained source of sulphuric acid. Much finely divided pyrites is present and also a varying proportion of carbonaceous matter and when subjected to slow combustion in very large heaps, over a period of many months, the pyrites gradu ally oxidized with the production of sulphurous and sulphuric acids and concurrent formation of the sulphates of iron and aluminium. The calcined material was systematically lixiviated in large stone or brick-built tanks, the liquor settled, concentrated by the aid of surface heat, and any excessive proportion of iron allowed to crystallize out as copperas. Very little natural potash is present in these schists and the addition of a potash salt, derived from kelp, wood ashes or other source, was necessary. The fine meal of alum octahedra first obtained was submitted to a second crystal lization to purify it. In spite of the fact that from 8o-13o tons of schist were required to produce one ton of alum, the process continued in successful competition with Roman alum for be tween two and three hundred years.
In 1845 the manufacture of alum in England was radically altered by the substitution, by Peter Spence, of the alum schists by aluminous shales from the coal-measures. These shales con tained more alumina, less iron and some potash and, of ter regu lated calcination, were found to be reasonably soluble in sulphuric acid. The weight of aluminous material required per ton of alum was reduced from about I oo tons to two-thirds of a ton, the time required for the process was shortened very greatly, and the old method of manufacture gradually died out. This process, in which the by-product ammonia from the coal-gas industry largely sub stitutes the potash salts hitherto employed, has since accounted for the major proportion of the world's output. Bauxite has also been used as a source of the alumina component.
Potash alum is again the principal alum of commerce and is largely obtained from the Italian mineral leucite, a double silicate of alumina and potash or soda. In the lava flows from certain extinct volcanoes in the Apennines, the leucite is almost entirely potassic and constitutes some 3o-4o% of the whole. The mineral is crushed, graded and subjected to electromagnetic treatment for the separation of the gangue. The substantially pure leucite meal or grain is then treated with sulphuric acid in which its alumina and potash are soluble and the solution, which already contains these oxides in almost equi-molecular ratio, is crystallized and purified in the ordinary way.
Iron alum (ferric ammonium sulphate) is prepared technically to a small extent, but the only other alum of commercial impor tance is the chromium potassium sulphate salt which is obtained as a by-product from the reduction of potassium bichromate in the manufacture of alizarin.
BIBLIOGRAPHY.-For the older methods of obtaining alum: Ure's Bibliography.-For the older methods of obtaining alum: Ure's Dictionary of Arts, Manufactures and Mines (7th ed., 1878) ; also . Geschwind's Manufacture of Alum and the Sulphates of Alumina and Iron (igoi) . (H. Se.)