It used to be that only the poorest grades of paper were made from wood pulp; now however not only newspapers but nearly all books and magazines are printed on a forest product. Formerly only spruce wood was considered suitable for paper manufacture, but the rapid depletion of this raw material led to the employment of many other species of trees and our Government scientists are constantly searching for and experimenting with new woods for this purpose.
Because of the insatiable appetites of our large newspaper printing presses and because a newspaper is not intended to be an imperishable document but a fleeting record of passing events, quality is of little importance in the preparation of pulp for newsprint and everything must be sacrificed for speed. The pulp is made of “ground” wood as distinguished from the chemically prepared wood pulp known as “sulphite.” The wood is first stripped of its bark and then cut into pieces running from two to four feet in length, depending upon the size of the magazine of the grinder. The wood is pressed against a revolving grindstone by hydraulic pressure and at such an angle that the fibers torn off will be as long as possible. The fibers are borne off by a stream of water which carries them through a set of screens. These remove any unground slivers, and eventually the pulp is drained of water and pressed into laps, or, if it is to be used immediately, it goes directly to the beating engine. Here it is mixed with a certain amount of “sulphite pulp,” usually from 20 to 25 per cent. To correct the yellowish color of the pulp, a small amount of blue and red coloring is added. To give body to the paper from 5 to 15 per cent of clay is mixed into the pulp and, to bind the fibers together and give the paper a finish that will be at least partly nonabsorbent, a quantity of rosin and soda ash is added with enough alum to precipitate the rosin upon the fibers. The beater thoroughly mixes the mass of material and converts it into a fluid of the consistency of thin milk, which then passes on to the Fourdrinier machine.
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.
THE FOURDRINIER PAPER-MAKING MACHINE
From the Jordan engine the pulp is passed on into the Fourdrinier machine, which we are now ready to investigate. The principal element of this machine is a traveling belt of copper wire gauze which takes the place of the screen or sieve used in the hand process. To confine the pulp to the belt there are two endless bands of rubber, known as “deckles,” along each edge of the screen, which travel with the screen. To insure proper interlacing of the fibers, as in the hand process, the screen is shaken laterally at the rate of about 300 vibrations per minute.
The fluid pulp enters the machine through a flow box, where a constant hydraulic head is maintained, so as to preserve a uniform discharge to the wire gauze. The milky stream issues through a narrow opening in the flow box, known as a “slicer.” This may be adjusted to alter the width and also the thickness of the stream. No one would suspect from the appearance of the milky fluid that it is filled with fibers, but a microscopic examination of a drop of the thin liquid shows that it is filled with tiny particles floating in the water like logs in a river. It is interesting to watch this stream, as it passes out upon the screen, change magically before one’s eyes, from a milky stream into solid mass of pulp.