46 MODERN PAPER-MAKING
slow. In the first, the bleach is run into the potcher after the stuff has been
broken in and washed. More bleach is required in this method to whiten the
stuff, and its strength suffers accordingly. In fact, an excess of bleach is run
in, and this excess has to be washed out and finally killed with anti-chlor.
With this method the colour of the pulp is said to 'go back' a little. The
second method is more economical. Sufficient and no more bleach is used
than is calculated to bring up the colour of the stock. The stuff is well mixed
with the bleach in the potcher and then run into tanks or steeps, where the
chemical action proceeds to its conclusion more slowly and naturally. This
gives a better colour and does not do so much damage to the fibres. Some
threads in rag stock that cannot be made to give up their colour with a rapid
bleach will be found to have become quite colourless in 24 hours. This system,
of course, requires a great deal of space for draining tanks, and entails more
handling of stock. Steam heating during bleaching is often resorted to, as
with esparto and wood pulp. While this is of great assistance in obtaining
a good white colour quickly, it requires to be used with caution; 90° F. is
the highest temperature that should be allowed. Above that the cellulose
will be attacked by the. bleach (oxidised) and lose strength to a very great
extent. The fibres will also return to a bad colour after cooling down.
Esparto fibres, in particular, are very apt to be destroyed by too high a
temperature, and will cause trouble by being slimy and soft and sticking to
the press rolls of the machine. A very little dilute acid is not to be despised
as an aid to obtaining a good colour, but great care must be taken when using
this. It should be added to the potcher very much diluted and in small quan-
tities, otherwise chlorine gas will be evolved and escape into the atmosphere,
causing great discomfort to the workers besides loss of chlorine.
A very good accelerator of this type is about J pint of alum solution
(20° Tw.) in about 2 pints of water for 200 Ib. stuff. Acetic or sulphuric acids
are sometimes used where the colour is difficult to bring up to a good white,
but none of these should be used except in unusual circumstances, and then
only with great care.
The cold bleaching of esparto and wood pulp is now generally carried
out in a series of bleaching towers. This system was introduced by Messrs.
Masson, Scott and Co., and has been almost universally adopted in esparto
mills. It takes up very little space and is economical and less costly than the
plants necessary for hot bleaching.
The towers are large vertical structures made of cast-iron or concrete,
lined with glazed tiles or glass, the usual size being about 8 feet in diameter
by 16 or 20 feet high. The bottom is sharply tapered, so that it is not possible
for the pulp to lodge anywhere. At the bottom of the taper is a bend, 2 feet