ALUM	123
or less. The third quality, usually called alumino-ferric, contains considerably
more iron than the other two, and is only used in low grades of paper; it is
also used for the purification of water.
For the highest grades of writing paper, potash alum, AlJ^SOiJ^^HsO,
or crystal alum is still used. This is an almost colourless salt of aluminium
and potassium sulphates, but it is very much less soluble in water than the
aluminium sulphates and much more expensive.
Alum or aluminium sulphate is generally made into a solution with
water before being added to the beater, although it is the practice in some
*news* mills to buy it "kibbled* or in powder and put it dry into the
breakers.
The best way to prepare the solution is in wooden or lead-lined tanks or
chests having lead pipes and taps. Iron or brass must not be used, as the free
acid will eat these away. Wooden tanks are usually provided, two for dis-
solving the alum and a third and larger tank as a store, from which the solution
is run by gravity to the beater room or carried in wooden buckets.
A satisfactory method is to have a wicker cage or basket which will con-
veniently hold the contents of one or more bags of alum, and suspend it by
means of a wooden pole across the top of the tank at such a height that the
water in the tank can reach well up the cage. The alum is put into the basket
and the tank filled with water. As soon as the water reaches the alum it readily
dissolves, and the saturated solution sinks down in the chest, allowing fresh
water to reach the alum. The action may be accelerated by rocking the basket
on the pole or by causing the circular cage to revolve.
The saturated solution will stand at about 62° to 64° Tw., and a gallon of
the solution will contain about 3^ Ib. of aluminium sulphate. For convenience
in working in the mill, the solution may be diluted to 20° Tw., in which case
a gallon will contain about I Ib. of alum.
The chief uses to which alum is put in the mill are:
 1.	For the precipitation of resin size.
 2.	For the precipitation of starch.
 3.	For the precipitation of silicate of soda.
 4.	To brighten the colour of the paper.
 5.	To help the resistance of the paper to ink.
 6.	To soften the water if it is hard.
 7.	To prevent or lessen frothing at the machine.
 8.	For the preparation of gelatine size for tub-sizing and to preveat
putrefection.
9.	To assist in the bleaching of rags.
10, As a mordant for certain dyestufls.