WOOD PULP 61
pulps give maximum bulk, high opacity, good absorbency, comparatively low
burst, low tear, and very low fold.
Extra free groundwood extends through the range from short coarse fibres
of low strength, representing maximum production and economy of manu-
facture, to long flexible fibres of good strength, representing high cost of
production. The board industry is perhaps the chief consumer, the desired
properties being bulk and fold. The small tonnage of dry mechanical pulp
for long-distance transportation is of this grade, as the fibre bonding properties
are low enough to permit easy breaking up of the dry sheets, and there is fibre
length to spare in the refining operation.
Free pulp is the standard grade for newsprint in this country. The strength
should be as high as possible, and the shive content at a minimum. Com-
paratively high freeness is needed because the moist-baled pulp undergoes
slight refining in the breaker beaters and jordans; the use of china clay further
lowers the drainage rate on the Fourdrinier wire; and, furthermore, the sheet
is rather high in basic weight. In self-contained mills converting direct from
slush pulp, the free grade is usually made for boards and coarse papers.
Medium free groundwood is the general utility grade. Strength is higher,
fibre size is more uniform, and the drainage rate is rapid enough for printing
papers of various kinds, as well as for thin high-grade boards. The many
self-contained newsprint mills in forest countries aim towards this grade for
high-speed machines.
Slow pulp is a speciality for coating papers, telephone directory, tissue and
other better-class groundwood papers. Freedom from shives and grit, best
cleanliness, high brightness, and good strength are usually desirable, and the
slow rate of drainage is unavoidable. Opacity is highest by reason of fine
grinding.
Bleached mechanical is a special product which has reached some importance
and may increase in future with die demand for cheap book and magazine
papers. Pulp of the medium free grade is usually chosen for this after-treat-
ment in the case of bale shipments, or slow pulp for the best possible printing
papers made from slush stock. Uniform brightness, high cleanliness, and
good strength are desired. Western hemlock is an example of a wood which
yields mechanical pulp of reddish colour, and bleaching is sometimes practised
to bring up the brightness to the spruce standard.
Semi-chemical pulps are not yet very important or definite as to grades,
but the following comments will serve as examples;
Brown mechanical pulp, based on the steaming of wood blocks before
grinding, is a border-line grade which might be classified with either mechanical
or semi-chemical pulp. Although the pressure treatment is with steam only,