294 MODERN PAPER-MAKING
tub in order that the excess size may be removed from the surface of the paper
before it passes to the air-dryer, or is wound up.
A satisfactory arrangement of tub-sizing plant is illustrated in Fig. 129, and
a much more elaborate tub, with a steam jacket and other improvements, is
shown in Fig. 127.
The paper is led, either direct from the steam-drying cylinders of the machine
or from a reel, over a dancing roll and round a large wooden roll fixed in the
front of the tub. This roll is half submerged in the size, and the web of paper
passes under it and along the bottom of the tub to another large wooden roll,
also half submerged in the size. The web is led round this roll, and out of
the size, over a guide roll, and into the nip of the squeezing rolls.
The squeezing rolls are important, and various combinations are in use.
They may be either brass on brass, rubber and brass, or granite and rubber, the
latter making an excellent combination. These rolls, no matter of what they
are made, must always be kept in good condition, free from ridges and blemishes
of any kind, and they must fit evenly the whole way across, otherwise damp
patches will occur in the paper, and these will show in the finished sheet. Rubber
rolls must be most carefully handled, as they are easily scratched and damaged,
and the least mark in the roll will leave a corresponding mark on the paper,
which may be visible only in certain lights.
The rolls must always be kept clean, and the top roll should be provided
with a doctor and spray of water in order to wash off any froth or scum.
The top roll should be fitted with levers and weights, so that the pressure
may be regulated.
Mention has already been made of the thermometer which is fitted in the
tub; this should be in such a position that it may be easily seen all the time,
as endless trouble may be caused by the temperature falling below its proper
height.
During the working of the tub a great deal of froth is formed by the agita-
tion of the size; this must be removed regularly in order to prevent it becoming
hard and getting on to the web, and being pressed in by the squeezing rolls,
thus causing hard size lumps or spots in the finished paper.
The level of the size in the tub should always be carefully regulated, as
trouble is often caused by the level being allowed to get too low, and the scum
which floats on top settling on the web, causing .blemishes in the finished
sheet.
The best sizing results are obtained when the paper enters the tub in air-dry
condition and absolutely fiat and free from cockles, creases and wrinkles of all
kinds. It will be found advantageous to pass the paper over a scraper before
it goes into the tub, in order to remove loose particles of rubber, which often