n8 MODERN PAPER-MAKING
to this ideal was the making of resin size with the alkali so proportioned to the
resin that a certain percentage of the latter was uncombined. This can be
carried only to a very limited extent (the product being called 'white size'),
as, in practice, the less alkali used the greater is the liability for the Tree'
resin to coagulate and cause specks to appear in the sheets on its being glazed.
As the alkali is increased to prevent this happening, so also must the quantity
of alum required to neutralise it be increased. The greater the quantity of
alum required, the greater will be the amount of sulphuric acid liberated in
the beater. Besides being a very active agent in the deterioration of the paper,
this free acid is very destructive to the beater plates and bars, and the paper
machine's wires and equipment generally.
The ideal size, therefore, is one which requires no more alum than is neces-
. sary to neutralise the alkaline substances in hard water, and to attract the particles
of resin to the fibres.
The Bewoid process, which was the result of years of research by Dr. Bruno
Wieger, of Brunswick, Germany, goes far towards fulfilling these conditions.
Bewoid colloid is practically a mechanical mixture of resin and water. It
contains 45 per cent resin, all of which is free—that is to say, in its natural
unhydrated state. In manufacture the resin is first melted, then mechanically
dispersed in the special Bewoid mill. From i to i^ per cent of caustic soda
is ruii in, and a small quantity of casein or gelatine is added, which covers the
particles of resin with a very thin protective coating and maintains them in
suspension. The finished size is a pure white colour and only slightly alkaline.
The alum required is therefore very much reduced, in some cases by 50 per
cent, and frothing is eliminated. As this size does not require to be 'beaten
in' it may be added to the beater as it is being emptied, or run into the stuff
chest with the alum, this avoiding the corrosive action of the alum in the
beater.
The Bewoid mill consists of the following parts: A steam-jacketed cylin-
drical pan of 125 gallons capacity, with a manometer, safety valve, blow-off
cock, and steam trap. The vertical spindle jextends through the bottom of
the pan to a ball-bearing, and continues through the top, with bevel wheels,
to a horizontal shaft fitted with two specially designed impellers. This is
arranged to run at 70 to 200 revolutions per minute by means of a two-speed
motor. There is also a measuring vessel of 130 litres capacity graduated in
20-litre divisions, with a 2-inch and ^-inch run-off pipe to run into the Bewoid
mill. The measuring vessel is supplied with a steam- and water-pipe. There
is also a small copper measuring vessel of 7 litres capacity on a tripod stand.
The method of preparation is as follows: The resin, 560 lb., is melted without
water in the Bewoid mill. Melting is assisted by circulating at the slow speed.