124? ALUM. 
rock ; after which the different fragments are loosened, 
in the form of slaty leaves or plates, that are of a dark 
grey colour. To obtain the alum, a bed of fagots is 
formed from ten to twelve feet in depth. By the side 
of this a scaffold is erected, which enables the work- 
men to form a pile of mineral about fifty feet long, 
and forty feet high. While this pile is forming, the 
fagots are lighted. By the gradual operation of the 
heat, a calcination takes place, in consequence of 
which the alum is afterwards rendered capable of being 
more easily separated than it otherwise would be from 
the stone in which it was contained, and from other 
extraneous matters that are combined with it. After 
this, the mineral is washed in shallow vessels, so ar- 
ranged that the water may be poured from one into the 
other. . By this process the alum becomes suspended in 
the water, while all the earthy particles subside to 
the bottom. The next operation is to evaporate the 
water saturated with alum. This is done by boiling 
it in large leaden caldrons, fixed, on past iron bars, 
over a furnace. As soon as the contents of the cal- 
drons are brought to a proper state, they are drawn 
off into casks, where the alum concretes into a mass. 
The hoops are then taken off, and the alum is broken 
and left to dry ; after which it is packed in casks for 
sale. 
Alum is an article of indispensable importance to 
dyers, not only on account of its cleansing and opening 
the pores of the substances to be dyed, and thus render- 
ing them fit to receive the colouring particles, but also 
from its more essential property of fixing the colours 
in such manner that they cannot afterwards be washed 
out. By tanners it is in great request for giving firm- 
ness to the skins after they have been rendered flaccid 
in the lime-pits. It is employed in the manufacture of 
paper, and by engravers, and other artists. In the 
making of candles, alum is added to the tallow, to 
render it glossy, and to give it greater firmness and 
consistence ; and, mixed with cream, it aids the sepa- 
