SILICATE OF SODA 121
The silicate is a compound formed by the fusion of silica with carbonate
of soda, and at ordinary temperature it is a thick, syrupy, sticky liquid which
hardens on exposure to air. It is readily soluble in warm water, and it is often
necessary, especially in cold weather, to add warm water to it before furnishing
to the beater. If this precaution is not taken, the silicate sinks to the bottom
of the trough and lies there, forming a hard, glass-like coating.
Silicate of soda is an alkaline substance, and is precipitated by alum, in
much the same way as resin size. It does not actually resist aqueous inks by
itself, and for this purpose requires the addition of resin size, the retention
of which, along with starch, colour and fillers, it greatly assists, so that it may
ultimately help in the production of a more ink-resisting paper.
Silicate increases the strength of most papers, drying hard and white, and
it also gives a greatly increased rattle and firm 'handle' to all papers. It
may be used with advantage to assist in making the stuff work VetJ on the
machine, as it helps in the retention of water in poorly fibrillated or 'free'
stuff.
As it is very resistant to oils, the silicate may be used to great advantage
in the making of printing papers and posters which have to resist an enormous
amount of coloured and oily inks. It also has the effect of assisting the |>aper
to resist the deteriorating action of sunlight.
While resin-sized papers are often inclined to be sticky at the press rolls,
silicate of soda helps to eliminate this trouble, and it also reduces cockling and
drying defects and helps the paper to lie flat when cut.
The cementing action of the silicate on the fibres keeps down fluff, and
this is very beneficial in papers which have to be clearly punched or perforated
with small holes or cut into narrow strips.
Silicate of soda may be used to repkce part of the resin size, and thus reduce
considerably the cost of sizing, or it may be used as an auxiliary sizing agent,
along with the usual amount of resin size. In this way it will impart additional
qualities to the paper, such as hardness, 'snap' and increased strength.
When the silicate, along with the requisite amount of alum, is added to the
beater, a light and bulky precipitate is formed in contact with the fibres, and
as the precipitate sticks to the fibres themselves, most of it is carried across the
wet end of the machine and is not lost in the back water.
It is also claimed that the precipitate of silicate, which consists of aluminium
oxide, aluminium silicate and silica, retains 25 per cent of its weight of moisture
when passing over the machine under the normal conditions of drying,
and this would assist in the subsequent calendering where a high finish
is desired.
It is most important that sufficient alum should be used in the pr