OPACITY 337
tint is not the true medium of differentiation, the function of the stain being
to exaggerate the characteristic appearance of the fibre. A table of colour
reactions on fibres by the more commonly used stains is given in the Appendix.
Furnish may be approximately estimated by a count of the different fibres
present, although beating adds to the difficulty of such an estimation.
Transparency and Opacity.—Transparency in paper depends on the com-
parative absence of light reflecting or absorbing faces or facets in or on the
fibres, minerals or other items of its composition. Any treatment—parch-
mentizing, waxing, finishing, fibrillation, or the addition of such substances as
size, starch, etc.—which tends to cause the fibres to pack closely together and
eliminate or fill up air space produces transparency by allowing light rays to
pass through the sheet more or less unbroken or unreflected.
For example, vegetable parchment before and after treatment. This paper
is beaten free and consequently is very opaque, but after the manufacture is
completed it is the reverse. Blotting has its opacity from its bulk, which is the
result of quick and free beating with very sharp tackle, and the softness and
want of felting qualities given to the cotton fibres by treatment. Yet the same
furnish may be treated to produce a comparatively transparent sheet, if highly
fibrillated, sized and finished as a writing paper.
Envelopes to match must be opaque. To get this result the fibres must be
beaten long and free and, if possible, china clay and a lower quality of fibre
may be utilized.
The second press should not be used; this omission will give a little more
bulk and leave the under side of the sheet with a rough surface. All fillers
and colours of a mineral or pulpy character produce opacity. So also do all
dyes when used to get a deep shade, but a slight addition of a good blue dye
will give an increased transparency to paper. A poor or cheap colour always
gives a more opaque result than a good colour, owing to the greater quantity
required to produce a similar shade.
The transparency of paper generally is a good indication of the quality;
the better the quality the more transparent will the sheet appear, owing to the
greater care that has been taken in boiling and bleaching the fibres, and the
purity of the colour, size, etc. A finish given on the machine with iron rolls
gives a more transparent paper than the super-calender or plate glaze, owing
to the greater compression and loss of bulk. The colour and quality of water
used at the mill influence the transparency of the paper, so that one mill will
have difficulty in giving the opacity, and another the transparency or purity.
In the first case the stock may be dulled down with a touch of nitrate of
iron, and brought up to the colour with blue and pink; in the second, it may
be necessary to use a higher grade of stock and to touch up the shade with dye.