GELATINE SIZE 291
sheets may be hung by clips or laid on hessian, in which case they will be dried
flat, or they may be hung over cow-hair ropes and dried with a 'back' or ridge
down the centre, caused by the rope; such papers are called 'backed' and the
ridge has to be cut out by means of a guillotine, the 'deckle edge' thus being
absent on one side of each half of the original sheet.
Tub-Sizing.—In the 'tub-sizing' of machine-made papers in a continuous
web the paper is run off a reel, or straight from the drying cylinders, through
a tub of gelatine size and again reeled up to 'soak', or run straight over an
air-dryer, which will be explained in detail in a subsequent paragraph.
Preparation of Gelatine Size.—Most mills do not now prepare their own
size, but find it more economical and clean to buy the particular grade of gelatine
which is best suited to their need, ready prepared in sheet form.
This method does away with the necessity of buying skins and wet pieces
from tanyards, boiling them down and running off several 'draws' to make
the sizing solution.
For those who wish to make their own size the following details may be
of interest:
The wet hide pieces are soaked in cold water and washed in revolving
drums to free them from the lime which has been used to preserve them.
The size is extracted from the skins by boiling them in copper-lined, jacketed
heaters, into the bottom of which is fitted a wooden frame covered with openly
woven cloth. The solution of size passes through this sieve and the slime and
other objectionable matter are kept back.
The heat is brought up to about 170° F. and the charge stands for about
16 hours before being drawn off. The heat must be put on again before drawing
off, in order that the first draw may run off easily to the store tanks.
The first 'draw' contains the strongest jelly, but one or two more 'draws'
are made in order to extract the whole of the gelatine. The second heating,
with a fresh supply of water, is conducted at a temperature of about 190° F.,
and is allowed to stand as in the first treatment.
A third and fourth infusion may be made, the third being heated up to
about 200° F. and the fourth boiled for an hour or two.
The amount of water added to the heaters for each treatment must be
regulated according to the strength of the size required, and will, of course,
depend upon the quality of the skins. The size should be strained through
flannel cloth over a wire sieve.
The strength of the size for the store tank is regulated according to require-
ments by mixing together various amounts of the different infusions, such
as the first and fourth, second and third, or any other which may be found
suitable.