ALUM 



111 



greater part, however, require the process of ustulation, from -which they derive many 

 advantages. The cohesion of the dense shale is thereby so much impaired that its 

 decomposition becomes more rapid ; the decomposition of the pyrites is quickened by 

 the expulsion of a portion of the sulphur ; and the ready-formed grcon vitriol is partly 

 decomposed by the heat, -with a transference of its sulphuric acid to the clay, and the 

 production of sulphate of alumina. 



Such alum-shales as contain too little bitumen or coal for the roasting process must 

 bo interstratified with layers of small coal or brush-wood over an extensive surface. 

 At Whitby the alum-rock, broken into small pieces, is laid upon a horizontal bod of 

 fuel, composed of brush-wood ; but at Hurlet small coal is chiefly used for the lo-wcr 

 bod. When about four feet of rock is piled on,- tire is set to the bottom in various 

 parts ; and whenever the mass is fairly kindled,' more rock is placed over the top. At 

 Whitby this piling process is continued till the calcining heap is raised to-the height 

 of 90 or 100 feet. The horizontal area is also 1 augmented at the same time till it 

 forms a great bod nearly 200 feet square, having therefore about 100,000 yards of 

 solid measurement. The rapidity of the combustion is tempered by plastering up the 

 crevices with small schist moistened. When such an immense mass is inflamed, the 

 heat is sure to rise too high, and an immense waste of sulphur and sulphuric acid 

 must ensue. This evil has been noticed at the Whitby works. At Hurlet the height 

 to which the heap is piled is only a few feet, while the horizontal area is expanded : 

 which is a much more judicious arrangement.' At Whitby 130 tons of calcined 

 schist produces on an average 1 ton of alum. In this humid climate it would be ad- 

 visable to pile up on the top of the horizontal strata of brushwood or coal and schist, 

 a pyramidal mass of schist, which, having its surface plastered smooth, with only a 

 few air-holes, will protect the mass from the rains, and at the same time prevent the 

 combustion from becoming too vehement. Should heavy rains supervene, a gutter 

 must be scooped out round the pile for receiving the aluminous lixivium, and con- 

 ducting it into the reservoir. 



It may be observed, that certain alum-schists contain abundance of combustible 

 matter, to keep up a suitable calcining heat after the fire is once kindled ; and there- 

 fore nothing is needed but the first layer of brushwood, which, in this case, may be 

 laid over the first bed of the bituminous schist. 



A continual but very slow heat, with a smothered fire, is most beneficial for the 

 ustulation of alum-slate. When the fire is too brisk, the sulphide of iron may run 

 with the earthy matters into a species of slag, or the sulphur will be dissipated in 

 vapour, by both of which accidents the product of alum will be impaired. Those 

 bituminous alum-schists which have been used as fuel under steam boilers have 

 suffered such a violent combustion that their ashes yield almost no alum. Even the 

 best regulated calcining pipes are apt to burn too briskly in high winds, and should 

 have their draught-holes carefully stopped under such circumstances. It may be laid 

 down as a general rule, that the slower the combustion the richer the roasted ore will 

 be in sulphate of alumina. When 'the Calcination is complete, the heap diminishes 

 to one-half its original bulk ; it is dovered with a light reddish ash, and is open and 

 porous in the interior, so' that 'the air can circulate freely throughout the mass. To 

 favour this access of air, the' masses should not be too lofty; and in dry weather a 

 little water should be occasionally sprinkled oh them, which, by dissolving away some 

 of the saline matter, will make the 'interior more open to'tho atmosphere. 



The following analyses of shales 'are by G. Kerstfen : 



Messrs. Richardson and Ronalds have given some very detailed analyses of the 

 Whitby and Campsio shales : 



