458 PHILOSOPHICAL TRANSACTIONS. [aNNO 1^78. 



On the English Alum Works, By Daniel Colwall, Esq. N° 142, p. 1052. 



Alum is made of a stone dug out of a mine, of a sea- weed and urine.* — ^The 

 stone is found in most of the hills between Scarborough and the river Tees in 

 the county of York. As also near Preston in Lancashire. It is of a bluish 

 colour, and will cleave like Cornish-slate. That mine which lies deep in the 

 earth, and is indifferently well moistened with springs is the best. The dry 

 mine is not good : and too much moisture cankers and corrupts the stone ; 

 making it nitrous. In this mine are found several veins of stone called doggers ; 

 of the same colour, but not so good. Here are also found those which are com- 

 monly called snake-stones. For the more convenient working of the mine, 

 which sometimes lies 20 yards under a surface, they begin the work on the 

 declivity of a hill, where they may also be well furnished with water. They 

 dig down the mine by stages, to save carriage; and so throw the mineral down 

 near the places where they calcine it ; being exposed to the air before it is cal- 

 cined, it will moulder in pieces, and yield a liquor of which copperas may be 

 made; but being calcined, is fit for alum. As long as it continues in the earth 

 or in water, it remains a hard stone. Sometimes a liquor will issue out of the 

 side of the mine, which by the heat of the sun becomes natural alum. The 

 mineral is calcined with cinders of Newcastle coal, wood, and furzes. The fire 

 is made about 1^ feet thick, 2 yards broad, and 10 yards long. Between the 

 fires are stops made with wet rubbish; so that any one or more of them may be 

 kindled without prejudice to the rest. 



After that ^ or 10 yards thickness of broken mineral are laid on this fuel, 

 and five or six of them are so covered ; they then begin to kindle the fuel ; and 

 as the fires rise towards the top, they lay on fresh mineral. So that, to what 

 height soever the heap be raised, which is oftentimes about 20 yards, the fires 

 without any more fuel will burn to the top, and stronger than at the first 

 kindling, as long as any sulphur remains in the stones. 



* Alum is a triple salt composed of vitriolic acid, alumina and potash or ammonia. The pot- 

 ash obtained by the incineration of the sea weed here mentioned (or from the ashes of other vegeta- 

 bles) serves (like the ammonia contained in urine) to combine with the superabundant acid, a 

 circumstance essential to the crystallization of the aluminous salt, and to its purification from iron or 

 any other metallic or earthy ingredient. The stone or ore of alum above described is termed by 

 mineralogists schistus aluminaris, and contains besides the earth of alum, sulphur and iron, a portion 

 of silica and magnesia, and generally some bituminous matter. By the process of calcination, the 

 sulphur is converted into sulphuric or vitriolic acid, and combines with the aluminous earth. By the 

 processes of lixiviation and boiling this union is further promoted, and the insoluble parts (consisting 

 chiefly of silica and oxyd of iron) are separated ; and on the addition of the ashes of the sea weed 



(kelp) and urine, the crystallization (after sufficient evaporation of the liquor) is effected ^The 



terms " nitre and nitrous" which frequently occur in this paper are improper. 



