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



times small particles of the pyrites are visible when it is frac- 

 tured, but in general this is not the case. There is another 

 seam of alum ore which is called by the miners the Duff bed, 

 lying above the limestone. This mineral, although not so suit- 

 able for making alum as the other, is still of great value for that 

 purpose. It has almost the same composition as the ordinary 

 alum ore. 



The sketch of a portion of a sec- 

 tion of a Hurlet pit shows the rela- f ce i 

 tive positions of the minerals men- Ironstone - 

 tioned. After the coal has been dug Duff bed. 1 

 out, leaving pillars at regular inter- 

 vals tO Support the roof, the alum Limestone. 4 



shale is taken out, and then the lime- 

 blowers, as they arc called, proceed, to Aiumore. i 

 remove the limestone by blasting it with 

 gunpowder. 



I find this limestone to have a spc- Coal. 6 



cific gravity of 2-71, and to be com- 

 posed of — 



Carbonate of lime 88-814 



Carbonate of magnesia .... 1*182 



Carbonate of iron 4' 764 



Bisulphide of iron "395 



Silica 3-412 



Carbonaceous matter .... trace 



Water 1-075 



4 

 9 

 



6 



:; 



99-642 



In calcining the ore for alum-making, a flue of brick is con- 

 structed upon the ground, and upon this brushwood and coals 

 are laid which are ignited, and alum shale placed above them. 

 When these have become ignited, another layer of ore is deposited 

 upon it, while at the same time waste alum shale is spread over 

 it to prevent the combustion from being too rapid. The presence 

 of carbonaceous matter and sulphur assists the combustion, and 

 renders a further addition of fuel unnecessary. In from twelve 

 to eighteen months the calcination is ended, and the ridges being 

 broken up, the ore is wheeled to the steeps to undergo the pro- 

 cess of lixiviation. The composition of the calcined shale is as 

 follows : — 



