Mr. J. Wilson's Researches into the Alum Manufacture. 421 



The second alum is dissolved for the last time in vessels of 

 lead into which steam is passed : these vessels are called roach- 

 ing pans. The solution is closely covered over and allowed to 

 settle, and is then run into the roaching-tubs, where it deposits 

 itself in crystals, shooting towards the centre of the tubs The 

 tubs are simply large wooden casks lined with lead : when the 

 alum has attained a sufficient degree of hardness, the staves are 

 removed, a hole is made near the bottom of the cask to allow the 

 mother-liquor to run off into the cellar for that purpose, and the 

 alum broken up into lumps or ground, as the case may be The 

 mother-liquor is again evaporated, as in the first process of alum 

 making, when it deposits a crop of first alum. The pure alum 

 contained — 



Sulphuric acid . . 35*57 



Alumina .... 11*40 



Peroxide of iron . . trace 



Ammonia .... 3-42 



Potash -83 



Water 48-78 



100 T ~' 



In preparing potash alum, the muriate is the salt generally 

 used ; hence small quantities of chlorine are often to be found 

 in the alum. By the process I have described, three parts of 

 shale produce one part of marketable alum. 



In 1836 and for several subsequent years, alum was made at 

 HurJet in the following way. Alum shale containing the small- 

 est amount of sulphur was calcined in a reverberatory furnace • 

 it was then ground and sifted, the powder was thrown into lead 

 cisterns and hot dilute sulphuric acid run upon it. A strong 

 action occurred, and the greater part of the shale dissolved The 

 solution was evaporated and the alkali applied in the usual way • 

 the sulphate of ammonia was at that time made by distilling sA 

 liquor into the mother-liquor of the alum ; oxide and sulpiride 

 of iron were thrown down, and the solution consisted of sulphate 

 of ammonia. 



In Sweden, where wood is the common fuel, and consequently 

 very expensive, the alum shale, which is very carbonaceous, is 

 used lor performing the calcination, an operation occupying 

 about a month or six weeks. The late Dr. Thomas Thomson 

 in his Travels in Sweden/ tells us that so great is the quantity 

 of combustible matter in the Swedish alum shale, that he saw 

 it employed for burning limestone. In the same way the shale 

 is used for the evaporation of the alum liquors, the shale being 

 calcined and the evaporation performed simultaneously. Wood 

 is also used iu Sweden for the calcination of the alum shale, as 



