52 MODERN PAPER-MAKING
Hardwoods are fundamentally different because the fibre length is so much
less (i to 1.5 mm*), and the pulp can therefore never take more than a minor
place in the pulp and board industry. For many years the main species was
aspen, or poplar (Populus tremuloides), a soft hardwood, for the manufacture
of bleached soda pulp in America. The scope is steadily widening, and various
species of birch, beech, maple, chestnut, cottonwood, gum, etc., have been
added to the list. Aspen and beech are used in Europe. Eucalyptus and other
hardwoods are being developed in countries not endowed with suitable soft-
woods. The trend seems to be towards sulphite or sulphate cooking, because of
their greater convenience and efficiency compared with the caustic soda process.
The use of hardwoods will probably extend in the field of fine papers and
perhaps considerably for chemical purposes where fibre size is of no importance.
In general it can be said that the limited supply of the spruce type of pulp-
wood in the different northern countries of the world is being, and should be,
reserved mainly for sulphite and mechanical pulps. Other species will play
an increasing part in meeting the world's pulp requirements.
On looking over a collection of wood samples it is interesjting to note that
the brightness, fine texture, and uniformity of the pulpwoods, particularly
spruce, fir, aspen, stand out as raw material giving the appearance of being
best suited to pulp manufacture.
Wood preparation deserves mention as a reminder of seasoning to reduce
pitch troubles and to aid penetration in the cooking of sulphite pulps, knife
barking to ensure best cleanliness and brightness for the highest grades of
unbleached sulphite, and careful chipping to aid uniform cooking of all chemical
pulps.
The chief countries engaged in the manufacture of wood pulp are Scandi-
navia, Canada and the United States, Finland, Germany, 'Czecho-Slovakia and
Russia—in other words, those countries which have extensive forests and
almost unlimited water-power. Water serves "as the chief means of transport
of the logs after they have been cut down in the forests, besides supplying the
power at the mill. The trees are felled and stripped, and rolled, dragged or
carried on light railways to the river, where in the winter they are piled on
the ice to await the spring thaws. They are then floated down the river to
large traps, formed of wooden booms, close to the mill.
Trees immediately required are floated into the mill. Those to be kept
in stock are built into huge piles to be used in rotation when the river is frozen
or when the felling ceases. During the passage downstream a proportion of
the bark is rubbed off, but this is not sufficient. They are cut up into short
lengths of about 2 feet, and are then 'barked' in a machine or by hand, when
they are ready to be made into either mechanical or chemical wood pulp.