110 ALUM 



In preparing alum from tho alum-stone, the ore is first sorted. The larger lumps 

 contain more or fewer flfnts disseminated through them, and are, according to their 

 quality, either picked out to make alum, or thrown away. Tho sorted pieces are 

 roasted or calcined, by which operation apparently tho hydrate of alumina, associated 

 with tho sulphate of alumina, loses its water and its affinity for alum. It becomes, 

 therefore, free ; and during the subsequent exposure to tho weather the stone gets 

 disintegrated, and tho alum becomes soluble in water. 



Tho calcination is performed in common limekilns in the ordinary way. In the 

 regulation of the fire it is requisite, here, as with gypsum, to prevent any fusion or 

 running together of the stones, or even any disengagement of sulphuric or sulphurous 

 acids, which would cause a corresponding diminution in the produce of alum. For this 

 reason tho contact of the ignited stones with carbonaceous matter ought to be avoided. 



The calcined alum-stones, piled in heaps from 2 to 3 feet high, are to be exposed 

 to the weather, and meanwhile they must be continually kept moist by sprinkling 

 them with water. As the water combines with the alum the stones crumble down, 

 and fall, eventually, into a pasty mass, which must be lixiviated with warm water, 

 and allowed to settle in a large cistern. The clear supernatant liquor, being drawn 

 off, is to be evaporated, and then crystallised. A second crystallisation finishes tho 

 process, and furnishes a marketable alum. Thus the Roman alum is made, which is 

 covered with a fine red film of peroxide of iron. 



Roman Alum crystallises partly in octahedrons, like other alums, partly in cubes. 

 If these cubes are dissolved in water of about 110 F., the evaporated liquid gives 

 crystals of common or octahedral alum. It was said that on heating, it deposited 

 subsulphato of alumina ; but Loewel says that such crystals were impure, and he finds 

 no real difference of composition. All that seems to bo known with certainty is, that 

 it is formed when there is a salt of alumina in solution with tho alum containing 

 more alumina than the neutral or common alum. This can very readily occur in tho 

 Roman alum, where there is a great excess of alumina in the alum-stone. Tho 

 Roman alum is prized for its great freedom from iron ; it was said by MM. Thcnard 

 and Hoard to contain only 5^0^ of sulphate of iron, whilst the ordinary alum contained 



II. Alum Manufacture from Alum-Schist. The greater portion of the alum found 

 in British commerce was until recently made from alum-slate and analogous substances, 

 This slate contains more or less iron pyrites, mixed with coaly or bituminous matter, 

 which is occasionally so abundant as to render the schist somewhat combustible. In 

 the strata of brown coal and bituminous wood, where the upper layers lie immediately 

 under clay beds, they consist of the coaly substance rendered impure with clay and 

 pyrites. This triple mixture constitutes the essence of all good alum-schists, and it 

 operates spontaneously towards tho production of sulphate of alumina. The coal, 

 besides burning, serves to make tho texture open, and to allow the air and moisture 

 to penetrate freely, so as to change tho sulphur and iron present into acid and oxide. 

 When these schists are exposed to a high temperature in contact with air, the pyrites 

 loses one-half of its sulphur, in the form of sublimed sulphur or of sulphurous acid, 

 and becomes a black sulphide of iron, which speedily attracts oxygen, and changes to 

 sulphate of iron, or green vitriol. The brown-coal schists contain, commonly, some 

 green vitriol crystals spontaneously formed in them. The sulphate of iron transfers its 

 acid to the clay, progressively, as the iron, by the action of the air with a little eleva- 

 tion of temperature, becomes peroxidised ; whereby sulphate of alumina is produced. 

 A portion of the green vitriol remains, however, undecomposed, and so much tho more 

 as there may happen to be less of other salifiablo bases present in the clay-slate. 

 Should a little magnesia or lime be present, tho vitriol gets more completely decom- 

 posed, and a portion of Epsom salt and gypsum is produced. 



The production of alum from alum-stone, in which the whole ingredients have been 

 found, has boon far from enough for the supply of tho world, and recourse has been 

 had to substances very different in composition, alum-shale, or schist, and clay. 

 Until within a few years the only supply of alum in Britain has been from the lias 

 shales of Whitby, and tho lower coal-measures of Campsie and Hurlet, near Glasgow, 

 and they are still the only places where it is manufactured from the ' ore,' as it is 

 called. 



Tho manufacture of alum from alum-schists may be described under the six follow- 

 ing heads: 1. Tho preparation of the alum-shale. 2. The lixiviation of the shale. 

 3. The evaporation of the lixivium. 4. The addition of the saline ingredients, or the 

 precipitation of the alum. 6. Tho washing of the aluminous salts ; and, 6. The 

 crystallisation. 



1. Preparation of the Alum-Shale, Some alum-shales are of such a nature that, 

 being piled in heaps in the open air, and moistened from time to time, they get spon- 

 taneously hot, and by degrees fall into a pulverulent mass, ready to bo lixiviated. The 



