CHAPTER IX
RESIN SIZING-STARCH-SEUCATE OF SODA-ALUM-
LOADINGS
RESIN, rosin or colophony is the solid residue of the gums or juices of coni-
ferous trees after the evaporation of their moisture and volatile substances.
It is a weak add, to which the name abietic add is given. Its colour ranges
from a light pale yellow to a dark brown, according to its source and the tem-
perature used to drive off the volatile substance. It is insoluble in water, and,
when broken, the fracture shows a clear, glassy surface.
When exposed to light and air, resin changes into a crystalline substance,
becomes friable and loses its amorphous quality. This explains why resin-sized
papers gradually deteriorate, losing colour, strength and ink resistance.
Resinate of soda, resin soap or resin size is made by boiling resin with soda
ash or with caustic soda. Resin size is soluble in water, but there is always
a certain amount of free resin present, unless the soda has been used in such
proportion as to balance exactly the acid resin, when the size is called neutral.
Size which has much free rosin (20 to 25 per cent) is of a creamy or 'white
colour, and is called 'white size', to distinguish it from the other, which is
brown in colour.
Resin size is generally made by boiling powdered resin with a solution of
soda ash in an open steam-heated vessel, the proportions being about I Ib. of
soda ash to 6£ or yj Ib. of resin. This would produce a size with about 10 to
15 per cent of free rosin with 7 hours' boiling. Those paper-mills which make
their own size have found by experiment the proportions that are most suitable
for their stock and for the hardness of their water.
Although it is about 130 years since resin was first found to have sizing
qualities, it still remains a matter of controversy as to the exact action which
causes it to be a sizing agent. It was held for a long time that resinate of
alumina (the predpitate of resin size and sulphate of alumina) was absorbed
by, or stuck to, the fibres, and filled up air space in the paper in much the same
way as gelatine size does.
Modem chemistry offers another and more probable explanation. In the
mixing of the relatively small quantity of size in the beater of stuff and water,
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