WOOD PULP

for the production of paper wood is broken down, either mechanically or chemically to a soft ma consisting of more or le separate fibres. This: ma is known as wood pulp. Mechanical pulp is made by grinding the wood to a fibrous condition and contains practically all the constituents of the wood. Chemical pulp is made by boiling the wood, under pre ure, with various chemicals which di olve out the intercellular matter, leaving fairly pure cellulose, consisting of the fibres of which the wood was built up. Chemical pulps give stronger paper than mechanical pulp, but the yield of chemical pulp from wood is very much le , the amount obtained varying with different woods, but seldom rising above 50%. Many woods give such small yields of chemical pulp that they cannot be economically used for paper manufacture, but even among those in common use, yields as low as 35% are frequently obtained. The woods most commonly used for mechanical pulp are spruce, hemlock, balsam, fir, aspen, poplar and willow. The woods used for chemical pulp vary according to the nature of the chemical treatment. Sulphite pulp, made by boiling the wood in a mixture of bisulphites of calcium and magnesium, is manufactured from the same woods as mechanical pulp. Soda pulp, made by boiling the wood in a solution of caustic soda, is manufactured from poplar, jack pine, aspen and some others. Generally speaking, woods from conifers are used for sulphite pulp, and woods from deciduous trees for soda pulp; but there is no definite division between the two cla es.