ALUM 121 



The insoluble portion of the glass contains the original felspar, minus two-thirds of 

 its silica a light, porous substance, similar in composition to olaeolite. This is 

 boiled with sulphuric acid of 1'2 specific gravity. The intense heat needed has pre- 

 vented the success of this process. 



In 1 8-42 Kagenbusch proposed to cover the schist over -with a plastering of clay, 

 or mud, for several months, and wash with water ; then to have it burnt in kilns 

 fitted with air-holes. In this process turf is used, on which the schist rests. The 

 air-holes regulate the combustion, which lasts three days. He uses kelp to obtain the 

 alkali. 



In 1850 J. T. Wilson proposed a method of collecting the ammonia from smoke, 

 and using it in making ammonia-alum. What is wanting, he supplements by potash 

 salts, causing a mixed potash- and ammonia-alum to bo manufactured. 



In 1854 Richardson adds iron pyrites, to increase the amount of sulphur, and, con- 

 sequently, of sulphuric acid, in the shale ; but it does not seem to have been used. 



In 1855 Dr. Frankland precipitated the subsulphate of alumina, and added sul- 

 phuric acid, thus obtaining the base by a small expenditure of precipitant. 



In 1856 J. Metcalf makes a cake similar to the alum-cake described at p. 122 ; but 

 he uses coarse clay. 



In 1856 Henry Pease and Thomas Richardson mix clay with chloride of potassium 

 or with common salt; they convert both into sulphates; the muriatic acid set free 

 dissolves the alumina, and the chloride of aluminium formed is used as alum. 



In 1856 Spilsburg's patent purposed to make alum from cryolite. 



The Boghead Cannel-coal ash contains about 30 per cent, of alumina, which it has 

 been proposed to dissolve for making alum ; but it has not hitherto been found a con- 

 venient material. 



Among the modern methods adopted for manufacturing alum on the Continent, 

 some of the more interesting are those in which cryolite is the raw material (see 

 CRTOUTE). According to Thomson's method the cryolite is ignited with carbonate of 

 lime, whereby aluminate of soda and fluoride of calcium are produced ; the former 

 being soluble and the latter insoluble in water. The aluminate is, therefore, dissolved 

 out, and a current of carbonic acid gas transmitted through the solution. A gelatinous 

 precipitate of alumina is thus thrown down, while carbonate of soda remains in 

 solution. Or bicarbonate of soda may be added to the solution of the aluminate, and 

 the alumina thus precipitated in a compact form. The alumina obtained by either of 

 these methods is dissolved in dilute sulphuric acid, and the sulphate of alumina thus 

 formed is converted into alum. 



In another process, introduced by Sauerwein, the cryolite having been powdered, is 

 boiled with water and lime ; and, as before, aluminate of soda and fluoride of calcium 

 are obtained. The solution of aluminate is mixed with an excess of finely-powdered 

 cryolite, whereby alumina and fluoride of sodium are produced. The alumina is then 

 converted into sulphate, and this into alum, as previously described. 



Sulphate of alumina may also be obtained from cryolite by treating it directly with 

 sulphuric acid. 



Alum has likewise been prepared from the mineral called Bauxite, which, on igni- 

 tion with sulphate of soda, and charcoal, yields sulphate of alumina. Again, blast- 

 furnace slag maybe employed as the raw material; the slag, treated with hydrochloric 

 acid, yields chloride of aluminium, from which alumina may readily be precipitated. 



Alum has been made extensively in England and France from an artificial sulphate 

 of alumina, prepared from clays which are. chosen as free as possible from carbonate 

 of lime and oxide of iron. They are calcined in a roverberatory furnace, in order to 

 expel the water, to peroxidiso the iron, and to render the alumina more easily acted 

 on by the acid. The expulsion of the water renders the clay porous and capable of 

 absorbing the sulphuric acid by capillary attraction. The peroxidation of the iron 

 renders it less soluble in the sulphuric acid ; and the silica of the clay, by reacting on 

 the alumina, impairs its aggregation, and makes it more readily attracted by the acid. 

 The clay should therefore bo moderately calcined ; but not so as to indurate it like 

 pottery-ware, for certain combinations would then bo effected which would make it 

 resist the action of acids. The clay is usually calcined in a reverberatory furnace, 

 the flame of which serves afterwards to heat two evaporating pans and a basin for 

 containing a mixture of the calcined clay and sulphuric acid. As soon as the clay 

 has become friable in the furnace it is taken out, reduced to powder, and passed 

 through a fine sieve. With 100 parts of the pulverised clay, 45 parts of sulphuric 

 acid, of specific gravity 1'45, are well mixed, in a stone basin, arched over with brick- 

 work. The flame and hot air of the reverboratory furnace are made to play along tho 

 mixture, in tho same way as described for evaporating alum-schist liquors. (Soo 

 .Soj>A.) The mixture, being stirred from time to time, is at the end of a few days to 

 bo raked out, and to bo sot aside in a warm place, for the acid to work on the clay, 



