CHAPTER V

REDUCTION TO HALF STUFF-WOOD PULP-
ESPARTO GRASS-STRAW
WOOD pulp is the most important raw material at present known to the paper-
maker. At the present time, the paper-maker in Great Britain need not
necessarily concern himself with the methods employed in the isolation of the
cellulose from wood, for, unlike esparto, it comes, except in one or two cases,
into the country already prepared for making into paper. Wood pulp is divided
into two distinct classes: first, 'mechanical' wood, which is simply ground soft
wood, and is in no sense a pure cellulose; and second, 'chemical' wood, which
is wood cellulose chemically isolated from wood.
It must be recognized that wood pulps cannot be rigidly classified and
graded. The physical properties are usually of chief importance, and the
•fibrous nature of the material does not lend itself to precision testing in terms
of rigid specifications. However, the days of offering pulps merely by samples
are passing. Testing at the pulp mills is usually thorough, and careful control
is always rewarded by better results in the market. Testing by the paper mills,
board mills, and speciality consumers has now reached a high plane in many
laboratories. The test of practical use in the paper mill tells the second, and
decisive, half of the story.
The basic factor is the pulp wood, its species, rate of growth, condition, and
preparation.
The wood-pulp business was built up largely on spruce. The fibre length
(about 3 mm.) is just right for most paper and board products. Spruce is low
in resin content and therefore adapted to mechanical, acid, and alkaline pro-
cesses. The wood is unusually uniform and the colour is bright. The various
species of spruce—'whitewood' (Picea excelsa] in Northern Europe; white
spruce (Picea glauca], black spruce (Picea mariana), and red spruce (Picea rubra)
in North-Eastern America; and Sitka spruce (Picea sitchensis) on the west coast-
have much the same fibre characteristics, but vary somewhat in density and
pulping action.
The closely related species, such as balsam fir (AUes balsmea), of lower
density, in. Eastern Canada and the United States, and white fir (Abies grandis)
on die Pacific coast, have naturally come into use to supplement the demand
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