122 MODERN PAPER-MAKING
of the silicate, as the silica itself gives a slightly alkaline reaction with litmus.
The contents of the beater must therefore show a definitely acid reaction,
otherwise a great amount of the value of the silicate will be wasted.
The proper order for the furnishing of the beater is to add first the silicate
of soda and, after it has become thoroughly mixed with the stuff, sufficient
alum to precipitate it completely. If resin size is also being used it should be
added later, and, finally, the alum for the resin.
As silicate of soda is manufactured in many different forms of solution,
differing in viscosity, density and alkalinity, great care must be used in selecting
the most suitable kind for sizing paper,
The amount of silicate to be used depends upon the kind of paper being
made, the substance of the paper and the effect that it is desired to produce.
If it is desired to make a paper with a very hard ratde, as much as 3 or 4 per
cent of silicate may be used, but to improve the hardness of a well-sized writing
paper,. f to i per cent will be sufficient.
Silicate may be used with great advantage in conjunction with starch. The
starch-silicate is made by mixing the two substances in a predetermined pro-
portion and heating with water to about 65° C. until the starch has burst. The
mixture is then added to the furnish and sufficient alum added to precipitate
the silicate.
Starch being much more expensive than silicate of soda, the proportion of
silicate to starch should be carefully regulated in order to ensure the best sizing
and hardening results at the lowest cost.
Paperine.—Paperine is a preparation of farina which is treated with caustic
soda and acid to produce a cold water-soluble starch substitute. This material
is much more economical in use than starch, being retained in the paper to a
remarkable degree. Normally £ to i per cent on die dry weight of the beater
is sufficient to give increased strength and handle. It is best to add it dry to
the beater, when furnishing in very small amounts, through a small open sieve,
which prevents the formation of coagulated lumps. These are very 'difficult
to dissolve and cause sticking at the press rolls, but with normal care there should
be no trouble from this source.
Alum.—This important chemical compound finds many uses in the mill,
and it can be truly said that it is an indispensable ingredient for most
papers.
Aluminium, sulphate, Al^SCX)., is made in three grades. The best grade,
which is used in good quality writing papers and other papers which have
to be free from iron, contains 17 to 18 per cent aluminium oxide (Al«Oi)
and is practically free from iron. The second quality contains 14 per cent of
aluminium oxide and a small percentage of iron, usually about 0.12 per cent