FIBROUS MATERIALS 5
The fact that it is thick-walled accounts for its being much stiffer and stronger
than the cotton fibre. It is round, or rather polygonal, in shape, and not flattened.
Its curious shape is due to tight packing of the fibre bundles in the stem during
growth. As cotton may be distinguished under the microscope by its flatness
and corkscrew bends, so the linen fibre may be distinguished by its knots
or nodules, which give it the appearance of a piece of bamboo, by its small
central canal, and, under high magnification, by cross-markings caused by the
bending of the stem during growth. These characteristics may be easily seen
in the accompanying sketches.
Linen fibres were among the first to be used for making paper, thousands
of years ago, by the Chinese. Nowadays linen seldom forms the entire furnish
of a paper, one reason being that it is scarce, and consequently expensive,
and another its extreme 'wetness' in working, which renders it very difficult
to make strong, thick or even medium-weight papers with it on a Fourdrinier
machine.
Linen is used chiefly in such papers as strong loans, thin banks, tissues and
cigarette papers, and in bank-notes and currency papers such as Bank of England
notes. Its great value is in conjunction with cotton, to stiffen up and give
strength to loans, ledger papers and thin banks.
Wood Fibres (Fig. 2, Nos. i, 2, 3 and 4).—The fibres obtained from wood
may be said to consist of two kinds, for although they are ultimately the same,
the fibres from mechanical wood, as they come to the mill, are different, both
in appearance and properties, from those which come in the form of chemical
wood pulp.
Mechanical wood pulp is distinguished from chemical wood pulp in that
it is prepared from the tree or log by purely mechanical means; that is, it is
ground up into a sort of sawdust in water, and receives no chemical treatment
whatsoever to free it from liquid resins, etc. It is very impure, and the fibres
are short and brittle and often united in clumps by medullary rays.
The illustrations show clearly how these mechanical wood fibres differ
from the chemical wood fibres, and a glance at them will make it apparent
how very different are their properties as paper-making materials. The fibres
obtained from wood by one or other of the chemical processes are fairly long—
they vary very much in length—although shorter than those of cotton or linen.
They are wide in comparison to their length and they are flat and sometimes
twisted. The fibre wafls are usually 'pitted' with small pores or holes.
There is also a decided difference in the fibres obtained from the two classes
of trees. The fibres from coniferous trees, such as spruce and pine, are longer
and stronger than those obtained from deciduous or foliage trees, such as
poplar and aspen. The former are more like cotton, and the latter resemBJe