120 ALUM 



Macquer, Fourcroy, and Vauquelin having discovered tho component parts of alum, 

 Chaptal made it from its elements, using clay. Ho says, ' Pure clay upon which the 

 sulphuric acid is digested is dissolved with difficulty.' He then says, 'I calcine my 

 clays, and reduce them into small pieces, which I spread on tho floor of my leaden 

 chambers. The sulphuric acid, which is formed by combustion of a mixture of 

 sulphur and saltpetre, expands, itself in tho cavity of these chambers, and exists for a 

 certain time in the vaporous form. In this form it .has a stronger action than when 

 it has been weakened by the mixture of a quantity of water more or less considerable, 

 so that it seizes the earths, combines with them, causes tjiem to increase in bulk by 

 the effervescence which takes place, and at the end of several days the whole 

 surface exposed to the vapour is converted into alum. Caro is taken to stir these 

 earths from time to time, that they may successively present all their surfaces to the 

 action of the acid.' ' But whatever process may bo used to combine the acid with 

 clay, it is necessary to expose the aluminised earths to the air during a greater or 

 less space of time, in order that the combination may bo more accurate, and the satu- 

 ration more complete." This is, in fact, the mode of making the sulphate of alumina. 

 It was then dissolved in water, drawn off clear, to free it from the silica and undis- 

 solved matter, mixed with sulphate of potash, evaporated, and crystallised. 



The manufacture of the alum from clay seems to have been a good deal used in 

 France. Their method at present, according to Regnault, is as follows: 'They 

 choose clays, such as kaolins, which contain little iron. The clays are then calcined 

 at a low red heat in a furnace ; they are ground to powder in a mill, and mixed with 

 half their weight of sulphuric acid of 1*45 specific gravity. The mixture is then 

 heated in another furnace until the sulphuric acid begins to evaporate. It is then 

 taken out, and left to stand for several days.' After some time the combination 

 becomes intimate, and the usual method of removing the sulphate of alumina from 

 the insoluble matter is resorted to, and the potash, or ammonia-salt, is added, to 

 convert it into alum. 



The most usual method has been to allow it to stand some weeks, or months, until 

 the combination has been effected. This has partially arisen from a supposition of 

 the necessity of giving it as much time as is needful with the shales, as it was not 

 known until lately how completely tho acid may decompose the clays. 



A patent was obtained in November, 1839, by Mr. William Wiesmann, of Dues- 

 burg, for improvements in the manufacture of alum. He subjects potters' clay 

 to a moderate red heat, grinds it, and subjects the powder, in leaden pans, to tho 

 action of concentrated sulphuric acid (f>6 B.), taking care to use excess of clay 

 and a moderate heat. The mixture is to be stirred till it is dry, then treated with 

 boiling water, in order to dissolve the sulphate of alumina formed. So far the pro- 

 cess is old and well known. The novelty consists in freeing the saline sdlution from 

 iron by ferrocyanido of potassium (prussiate of potash). When the iron has been all 

 thrown down in the form of Prussian-blue, the liquor is allowed to settle, the super- 

 natant pure sulphate is drawn off, and evaporated till it forms, on cooling, a concrete 

 mass, which may be moulded into the shape of bricks, &c., for the convenience of 

 packing. This was manufactured at Lee-Moor, near Plympton, on a small scale ; 

 but it is no longer made in this country. Dr. Muspratt's analysis made it a basic 

 sulphate = 2APO'. 6SO*+33 Aq. ; and he adds that manufacturers objected to it 

 because it was impossible to judge of its purity by its merely physical appearance. 

 Mohr's analysis gave 



Alumina . . . . . . . 13-91 



Sulphuric acid . . . . . . . 36*24 



Water 49-60 



Sulphate of potash . . . . . 1'50 



By having an excess of -clay, Wiesmann intended to have all his acid saturated. 

 He found that he could not dissolve all the alumina- by using only its equivalent of 

 acid ; he preferred, therefore, to lose some of the alumina, as in the other processes. 



.Hervey's patent of 1839. Clay is dried, ground, and sieved ; it is then mixed 

 with sulphuric acid of from 10 to 80 T\vad.,and from to an equal quantity of clay, 

 ' used according to its quality. The mixture is then well stirred; a great ebullition 

 .ensues, and after ebullition it is again stirred. This is the formation of the sulphate 

 of alumina, which is washed out, and made into alum in the ordinary way. 

 In 1842 Mr. Turner patented a method, said to bo originally Sprengel's proposal, of 

 extracting the alumina and potash from felspar to make alum. The felspar is heated 

 with sulphate of potash to melting, then carbonate of potash is added. This gives a 

 soluble, glass, which, in boiling water, takes up two-thirds of the silica and as much 

 potash as was added to the felspar. This being heated with carbonic acid, gives a 

 gelatinous mass of silica. When dried, tho carbonate of potash may bo washed out. 



