98 MODERN PAPER-MAKING
in the others in the same time. It is doubtful whether by having less pressure
on the plate and giving the stock longer time he could ever accomplish it.
Besides the condition of the tackle and the beating pressure, the concen-
tration of the stuff makes a very great difference to the result. The strong loan
and the fine writing paper may again be instanced. In a beater nominally of
400 Ib. capacity, the quantity of the former may be as much as 4 cwt., and
of the latter not more than 3 cwt. The writing paper will not have the strength
of the loan, but this is not expected, or required, the main consideration being
a close and clearly water-marked sheet. The lower the concentration of stuff
in the beater the more chance it has of being cut, owing to the fibres passing
through between the bars and plates in a thin stream, instead of a thick layer.
An exacdy similar effect is seen when finishing a thin paper on the calenders.
When putting through a thicker substance with the same weight of rolls,
the finish is not so high. The thinner paper has fewer fibres in its bulk to form
a cushion, and therefore each individual fibre has the weight more directly
applied to it.
Again, by the higher concentration of stuff, the fibres are subjected to
more pressure and rubbing on each other and against the sides and bottom of
the beater, which helps to roughen the surface of the fibres and separate the
fibrillae that have been opened up by the roll. Also, the head of stuff in front
of the roll is subjected to a battering effect by the roll bars, which is absent
when the stuff is rushing through, suspended in water. Between the plate and
the outlet of the backfall, a good deal of pressure and rubbing takes place.
Although we have taken cotton fibres as an example, these remarks are
applicable to all kinds of fibres. Cotton fibres, especially those from new
cotton cuttings or rags that have not been subjected to frequent washing and
bleaching, are very opaque and bulky. Even when well fibrillated, as in a
thin bank, this is apparent. As we have shown previously, the cotton fibre has
a peculiarity which is of the highest value in giving the qualities of strength,
bulk and opacity. This is its twisted form, which even after drastic treat-
ment in the beater is seldom entirely lost. As an illustration of these qualities
combined, let us take a ledger paper of a substance equal to about 27 Ib. Large
Post, with a water-mark. The beating time would be about 4 to 5 hours,
and our beater woujd be filled to about 3^ cwt of stuff. We would then
expect to find that the stuff would be rather wet on the machine, but the fibres
would be fairly long, or at least a good proportion of them. If some of these
long fibres are examined under the microscope most of them would be
readily recognised as cotton by their more or less twisted form. These are
the fibres that have escaped being much cut or fibrillated during their time in
the beater, and they form the backbone or framework of the sheet While