Mr. J. Wilson's Researches into the Alum Manufacture. 415 



about seventeen thousand tons are made annually, of which 

 twelve thousand tons are produced in England, and the rest in 

 Scotland. The processes by which alum is produced from the 

 shale or other material are different, as the composition of these 

 varies. 



In the alum-works which were carried on formerly at Smyrna 

 and Solfatara, and in other places where the ore contains a suf- 

 ficient amount of alkali to form alum without an additional 

 quantity, the process is simple. The rock containing the ready- 

 formed alum is dissolved in a lead vessel and the solution cry- 

 stallized. 



At Tolfa, where the alum-stone contains an excess of alumina, 

 the ore is calcined in order to render insoluble the excess of 

 hydrate of alumina; to effect this, the stone is broken up into 

 small pieces, which are carefully calcined till sulphurous fumes 

 begin to be given off. Whenever this happens, the calcined 

 stone is removed to stone cisterns, where it is moistened with 

 water ; in a few days the calcined stones crack and swell, beco- 

 ming like slacked quicklime. After being thus moistened and 

 exposed for some months, they are thrown into leaden boilers 

 and boiled with water for about twenty-four hours, the water as 

 it evaporates being replaced by the mother-liquor of a former 

 crystallization. When the ore has been thus boiled for a suffi- 

 cient length of time the fire is extinguished, and the sediment 

 having subsided, the clear liquor is run into wooden reservoirs, 

 where it crystallizes ; in these it remains about fourteen days, 

 the mother-liquor being used as described for dissolving the cal- 

 cined ore. 



The alum obtained by this process is what is termed Roman 

 alum, and is considered the purest to be found in commerce. 

 It frequently occurs in cubical crystals, and generally is coated 

 with a reddish-brown powder resembling ochre, but which is 

 said to contain no iron, being put on by the manufacturers to 

 give their alum the peculiar appearance which characterizes the 

 Italian salt. 



The alum made in this country is not obtained in this way, 

 as neither potash nor sulphuric acid exist ready-formed in suffi- 

 cient quantity in any of our shales, so that the processes adopted 

 are necessarily more complicated. 



The alum ore is an aluminous shale containing carbonaceous 

 matter, and diffused iron pyrites. By the calcination of this 

 shale the sulphur is oxidized, and unites with the alumina, iron, 

 and other bases, forming sulphates. By digesting the calcined 

 ore in water, and adding a salt of potash or ammonia to the 

 liquor, alum is obtained. 



At Hurlet the method formerly pursued was to leave the alum 



