ICO , ALUM.— Roasting.

judge most advantageous. It is then mantled, as theworkmen term it, that is, covered over with a layerof the already calcined and exhausted ore, in order toprotect it from high winds and excessive rains, and alsoto moderate the heat and let it cool gradually, so thatthe sulphur present may not be volatilized or sub-limed. From three to twelve months, according tothe state of the weather, are generally required tocalcine the heap properly; in rough weather verylittle progress is made. This time includes theperiod of cooling the burned ore, which is done

either by leaving the heap to itself, or checking theroasting by applying a thicker mantling.

The several mounds are so arranged that they maybe in various stages of advancement, some being cold,others about to be mantled for cooling, many in pro-gress, while a further number are commencing, sothat at all times a supply of calcined shale may beready for the liquefying vats.

At Whitby the calcining heaps are raised from 80to 100 feet in height. In consequence of the greateramount of carbonaceous matters which the shale at

Fig- 2.

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these works contains, the temperature of the moundssometimes rises too high, and causes the loss ofmuch sulphurous acid; this may be partly pre-vented by mixing the shale with some of the calcinedand exhausted ore, and when the heap has acquireda larger size, if the heat is still deemed too high, eitherplastering up the crevices with small schist, or mant-ling over the whole as at Hurlet.

About 130 tons of the Whitby calcined schist yieldone ton of alum. In this country it is found advis-able to pile up on the top of the ridge of brushwoodor coal and schist, a pyramidal heap of the mineral,

which, having its surface plastered smooth with onlya few air-holes, protects the mass from the rains, andat the same time prevents the combustion from be-coming too vehement. Should heavy rains supervene,a gutter must be scooped out round the pile forreceiving the aluminous mother liquor, and conduct-ing it into the reservoir.

A continuous but very slow heat, with a smotheredfire, is most appropriate for the roasting of alum shale.When the fire is too brisk, the iron sulphide some-times forms with the earthy matters a species of slag,or the sulphur is dissipated in vapour, either of which