126 MODERN PAPER-MAKING
you to get an opacity equal to a thicker paper pays for the cost of the titanium
oxide which is necessary to achieve this result.
It has a very small particle size, and a closed back-water system or an efficient
form of back-water recovery plant is necessary if the maximum economy in
its use is to be achieved.
Opacity being necessary for papers such as envelope papers and printing
papers, these are usually well loaded, china clay being the filler most in use.
Writing papers may have up to 15 per cent of loading, usually china clay and
terra alba in equal proportions; S/C printings up to 25 per cent of china clay,
and in the case of imitation art, as much as it will carry, which, of course,
depends on the substance and may be as high as 30 per cent. A good heavy
filling of china clay reduces the expansion and contraction of litho papers.
The retention of loading varies considerably. China clay may be retained
to the extent of 80 per cent of the quantity put in the beaters, but is usually
about 50 per cent unless a closed back-water system is used. The retention
is governed by many factors, the chief being the degree of fibrillation of the
stock, the thickness of the paper, the nature of the loading, the characteristics
of the fibres used, the formation of the sheet on the machine, the size or other
binding substances put in the beater, the conservation of -the back water and
the length and flow of the machine shutes.
Well-fibrillated stock, and fibres that are very fine and pack closely in
paper, such as esparto and straw, have a high retention. Free stock for
blottings and harsh fibres like soda pulp from coniferous wood have a low
retention.
Papers that are heavily resin-sized or have starch, silicate of soda or other
binding substances will carry a heavy percentage of loading.
The formation of the sheet on the machine will, of course, depend on the
beating of the stuff, but too much water on the wire will mean a loss of loading
by excessive suction on the boxes. Up till recently it has always been an estab-
lished practice to add the loading—whether it was clay, chalk, or other mineral—
to the beater, it being always maintained that if the loading were well beaten
into the stuff a greater proportion was carried through into the paper, and less
came away in the white water; this theory has been found erroneous. Much
better results are now being obtained by adding the loading with water in a
thin continuous stream to the stock before it goes on to the machine, before
the strainer. In this way there has been found to be a saving in the amount
of loading required to give a predetermined quantity in the paper. Opacity
has been improved, and less loading has had to be dealt with in the back water.
There is the additional advantage that the beaters hold more stuff arid the
beater bars and plates last longer, owing to the absence of the abrasive action