GETTING DOWN TO THE “BONES” OF THE WOOD
Paper made from ground wood pulp soon discolors and becomes brittle owing to the presence of the lignin and resin of the tree. The superiority of sulphite pulp is due to the fact that chemical processes are used to remove these disturbing ingredients, leaving only the “bones” of the tree, that is, the pure cellulose fiber. Spruce wood is usually used and after being freed of its bark and thoroughly cleaned, all knots are cut out and it is sawed into short lengths which are then split and cut up into little chips from ¼ to ½ inch long. These chips are boiled under pressure in a solution of sulphur dioxide and slaked lime. The “digesters” in which the chips are boiled are large steel cylinders lined with brick set in cement and coated with litharge and glycerin, so as to prevent the sulphite liquor from coming into contact with the metal. A steam pressure of seventy to eighty pounds is maintained for eight to ten hours and then the digesters are discharged into vats. The sulphite liquor in which the resinous matter is dissolved is drained away and the pulp is washed.
Soft woods, such as poplar, basswood, etc., are boiled in caustic soda, producing what is known as “soda pulp.” “Sulphate pulp” is made by boiling in a solution of sodium sulphate, to which a certain amount of sodium carbonate and caustic soda are added. This process is used for producing pulp for so-called “Kraft” paper and brown wrapping paper.
Formerly pulp was always made in the paper mill and hence the mills had to be located near the source of wood supply, but to-day the making of pulp is carried on as an independent industry in remote forest regions where water power is plentiful. The pulp is pressed into laps and compressed into bales weighing about 400 pounds. These are sent over long distances by rail or steamship to paper mills located in or near the big cities, so that the paper may be made near the points of consumption.
Baled paper pulp has to be reconverted into a smooth milklike fluid. A shredding machine is used for this purpose, which tears the pulp apart and mixes it with water. Then it goes to the beater where it receives its quota of sulphite pulp, coloring matter, clay, and sizing, as explained before. After an hour of treatment in the beater, it passes on to a tank in which it is further diluted with water and is then put through a Jordan engine. This consists of a conical cylinder in which is placed a conical roll. Both cylinder and roll are formed with knives that extend longitudinally, but the revolving knives are set at a slight angle to the fixed knives, somewhat after the fashion of a lawn mower. The conical form permits of careful adjustment of the roller in the cylinder. The pulp mixture enters at the smaller end and discharges at the larger end. This machine gives the pulp its final beating and reduces it to a very smooth consistency.