292 MODERN PAPEK-MAKING
For the reason that size of different strengths may be easily obtained in this
way, some mills still continue to make their own. By using a bath of size
made entirely or almost entirely from the first draw, a very hard-sized paper is
obtained, and one the strength of which is very greatly increased; for other
papers, weaker solutions will suffice.
A small percentage of white soap is added to the size and thoroughly mixed
up before adding the alum; about 2\ per cent is the usual amount. If the soap
is not put in before the alum the size will curdle. The soap must be pure tallow
curd, and may be bought, specially made, under the name of 'paper-makers'
soap'. The soap helps to give a high finish to the paper, and it also prevents
the formation of glistening alum specks on the surface of the paper.
Alum is added in the proportion of about 14 or 16 Ib. to 112 Ib. of gelatine,
reckoned on the dry weight. The function of the alum is threefold: it serves
to stabilise the consistency of the sizing solution at various strengths and tem-
peratures, and also acts as a preservative by arresting the formation and growth
of destructive bacteria, w7hich quickly destroy the gelatine and cause it to
putrefy; the third function of the alum is to render the gelatine resistant to ink
penetration.
The effect of the alum on gelatine solution is very curious. If it is slowly
added, it causes the gelatine to thicken until it becomes almost solid; when,
however, more alum is added, the gelatine becomes fluid again, and it is in this
condition that it is used in the size-bath. About 12 or 14 per cent of alum
will be required to produce this solution. The normal way of testing the
density of the solution in the mill is by means of a special hydrometer which
has two scales of figures.
For a good quality paper in substance about i6j-X2i, 21 Ib., 480*8, the
temperature of the bath should be 110° R, and the density 7, as shown on
the hydrometer. This figure is arrived at by allowing for the temperature on
the scale.
Only the best sulphate of alumina must be used, and it must contain no
iron or free acid.
Excellent gelatine may now be obtained in the form of small sheets ready
for dissolving, and this saves the paper-maker the lengthy and rather unpleasant
business of preparing the size from skins.
These skin and bone glues are made in a wide variety of qualities and colour,
from dark brown to almost colourless (bleached). In order to find a grade
suitable for tub-sizing, various samples may be tested. The qualities most
desired are penetration at high temperatures and good stiff gelatinisation at
ordinary atmospheric temperatures.
This simply means that the solution should penetrate the surface of the