120 MODERN PAPER-MAKING
be used for special papers, but has the effect of reducing strength and making
a limp and flabby paper.
After 5 minutes the dispersion is diluted with water at 38° C: first, 70 litres
through the f-inch pipe, and the remainder through the 2-inch pipe. As the
colloid rises up the sides of the pan during dilution the slow speed is started,
and the mill is finally filled and the agitation stopped. When filled, the size
is of 45 per cent concentration.
Skin glue can be used as a colloidalisator in place of casein. This gives
improved sizing, but the colloid is less stable.
When using glue, only 8^ Ib. are used at 50° C. in 60 litres, without caustic
soda. The glue should be soaked for | hour before using. Only 8 Ib. 6 oz.
of caustic soda is used instead of 9 Ib., as when using casein, for the first cooling
and neutralising of the resin.
Starch.—In. very ancient times starch was the only material known for the
sizing of paper. It is now used in addition to other sizing agents to give a
hard rattle and an improved finish to paper. A slight increase in strength may
also be obtained by its use. It increases transparency, especially in highly
glazed papers.
A great deal must be used to get much result, as it readily drains out on the
wire and suction boxes. As it is carried mechanically by the fibres, the size
of starch granules is important. Those of potato starch are largest, maize
smaller and rice smallest of all.
Each granule is a little capsule or container filled with a gelatinous sub-
stance. The heat of the drying cylinders, acting on the wet granule, causes it
to swell and burst, the contents fusing and binding the fibres together. Starch
is better retained by the stock in the presence of sodiunpi silicate in the propor-
tion of i pint of sodium silicate to 5 Ib. of starch. These may be mixed together
with warm water, then heated until the starch is partly 'burst' and cooled
with a little cold water to stop the action. This is called starch silicate; the
precipitate is very white and has more hardening effect on the fibres than starch
or sodium silicate used alone. About 2 pints of silicate and 10 Ib. of starch are
required for 4 cwt. dry weight of stock. Less resin size is used in this case.
The usual amount of alum has to be slightly increased to precipitate the
starch silicate thoroughly. Starch is, however, an expensive material to use,
and in free beaten stock its effect may be entirely lost. In well-fibrillated stuff,
which carries the granules in greater quantity, its results are more apparent.
Silicate of Soda.—The use of silicate of soda ('waterglass') as an auxiliary
sizing agent is gradually becoming more general, and excellent results may be
obtained by using the silicate either alone or, more often, in conjunction with
resin size and starcL