34 



FOREST PRODUCTS 



the surface. Some stones wear unevenly and must be ground, so that 

 they will be perfectly symmetrical. The operator must give constant 

 attention to the stones, so that the maximum quantity of the best quality 

 of pulp may be produced. 



In the process of grinding, the door of the pocket is opened, the piston 

 is raised and the pocket filled with blocks, the bolts being placed flatly 

 against the surface of the stone and at right angles to the direction of the 

 revolutions, as shown in the illustrations. When the pocket is filled, the 

 door is closed and the piston lowered. The pressure is exerted at the 

 rate of about 70 lb. per square inch. The temperature of the wood during 

 the process of making cold ground pulp is about 60° F. The stones weigh 



PJiotograph by u. S. Forest Service 



Fig. 3. — A four-pocket grinder used to reduce the wood bolts to fiber by the mechanical 

 process of making wood pulp. The bolts are pressed against a rapidly revolving stone. 



from about 2500 to 3500 lb. each and have an average life of only about 

 six to eight months. Each grinder has a capacity of from 6 to 9 cords 

 of wood per twenty-four hours. All water used in making cold ground 

 pulp is first passed through filters in order that absolute purity may be 

 insured. One man can tend a pair of grinders. 



The water carries the pulp away from the grinders and it is collected 

 and carried off through a large pipe at the base of the grinder. 



In making hot ground pulp the water allowed to flow on the grind- 

 stone during the process is reduced to a minimum. The friction causes 

 the temperature to rise and the resultant pulp is of entirely different 

 quality from that made by the cold ground process. In making hot 



