110 Chemical Manufactures. 



water, heat and fall into powder ; bnt most of them require to 

 be roasted. If the bituminous matter they contain is too little 

 for this purpose, small coal, or some other combustible must be 

 added. The roasting deprives any pyrites that is present of 

 half its sulphur, which passes off, either free or as sulphurous 

 acid, sulphuret of iron being formed. This sulphuret, by at- 

 tracting oxygen, becomes sulphate, which, as the temperature 

 rises,, becomes peroxidated, and yields its acid to the alumina. 

 If, in roasting the schist, the heat is allowed to become too 

 great — and, when the mass is very large, this is very likely to 

 be the case — the sulphuret of iron will form a slag with the 

 earth, or too much sulphur will pass off, and, in either case, 

 waste will occur ; the more moderate the heat, within certain 

 limits, the more abundant the alum product. After calcination 

 the mass is so porous, that the air can circulate freely through 

 it ; to facilitate this, and also to remove the sulphate of alu- 

 mina, water is sprinkled upon it. When all the alumina is ob- 

 tained in solution as a sulphate, the liquid is concentrated by 

 evaporation : and, on adding an alkali, alum precipitates as a 

 crystalline powder, which is purified by washing with cold 

 water, in which it is nearly insoluble : being then dissolved 

 in just enough of water raised to the boiling point, the solution 

 is run into casks with moveable staves. When the whole has 

 cooled, the staves are taken asunder, and the alum appears to 

 be a solid mass : but on making an aperture in it, the mother 

 liquor flows out. 



As the supply of shales is confined to certain localities, alum 

 is manufactured also from clay. The difference of the process 

 employed, in this case, consists in adding sulphuric acid after 

 calcination, and removing the iron — generally as Prussian blue, 

 with ferrocyanide of potassium ; the alkali is applied in the usual 

 way. When ammonia is used as the alkali, it is obtained from 

 gas liquor. Ammonia alum has the advantage of precipitat- 

 ing from a less concentrated solution. Spence manufactures it 

 on a very large scale, his products being, on an average, more 

 than three hundred tons per week. He uses the carbonaceous 

 shale of the coal-measures : and his process is so effective that 

 instead of one ton of alum being made from sixty tons of oohtic 

 shale, sixty- five tons of alum are made from fifty tons. He cal- 

 cines in the ordinary manner, but with great care ; and adds 

 sulphuric acid to make up for the deficiency of that produced 

 from the pyrites, introducing along with it the ammonia and 

 its volatile salts contained in gas liquor, the fixed ammoniacal 

 salt being subsequently liberated by lime. The precipitated 

 alum powder is dissolved, not by boiling water, but by steam : 

 and any basic alum (a subsulphate) subsides from the solution, 

 which is crystallized in the usual way. The masses of alum, 



