I3o MODERN PAPER-MAKING
chest, every beater that is let down is liable to upset the shade. Where two
chests are used alternately, there is less risk of shades varying except on changing
from one to another.
As for the dyes, most of them the products of intricate chemical
processes, the paper-maker has to take them on trust and find by experiment,
often costly enough, their suitability for his stock and the matching of his
samples. On long runs, apart from the start, when alterations to match the
sample are being made, there is a fair chance of producing a lot with few
variations in shade. But when making small lots, which most fine mills are
compelled to undertake, the matching of samples quickly and accurately with-
out making many shades is a serious and difficult task, which calls for long
experience and knowledge of paper-making, and an artist's eye for colour
or shades of colour. In a paper which may have half a dozen different dyes
combined, the paper-maker has to judge quickly and accurately which one is
out of proportion, and by how much or how little, and immediately put his
judgment to the test by making the alteration.
Unlike the artist, who can alter the tints on his canvas as often as he pleases and
presents only the finished picture as the result of his work, the paper colourman's
alterations, correct or otherwise, are all shown by the shades in the paper.
The quantity of colour used to obtain the deeper shades of azures, blues
and tints depends on several other factors besides quality and beating. Some
of these factors are very obscure; for instance, the electrical charges in the
dyes, and their relation to the electrical charge in fibres and in the chemical
used to fix them. Certain dyes will dye or be absorbed by certain fibres very
readily; others will require the aid of alum or other mordants before they will
be of any use.
Colours that are not held by the fibres either mechanically or chemically
are lost to a great extent in the back water of the machine. In addition to the
actual value wasted, there is the difficulty of dealing with the effluent, which
is so very obvious when allowed to run into a river. Inorganic dyes such as
Venetian red, the ochres, umbers and paste pigments act mostly as fillers or
loading, and are retained best by well-fibrillated" and heavily-sized stock with
an excess of alum. Ultramarine blue is of this class.
Aniline or coal-tar dyes are more readily absorbed by the fibres, and are
retained so much better that the *two-sideness' of deep tints is very much
reduced. Unfortunately, this type of dyestuff is seldom fast to light and is
therefore not often used for fine papers.
'Two-sideness' in paper is for the most part a beating question. Free stuff
allows the drainage of water on the machine wire, and litde suction is required
on the first box to allow of a good water-mark.