WOOD PULP AND PAPER 33 



the ice is melted and much of the dirt removed. The soaking they receive 

 also facilitates the grinding process. 



In the case of the largest bolts a splitting machine is provided on one 

 end of the barking room to reduce the largest bolts to a size that can be 

 accommodated in the grinders. 



Cold and Hot Ground Wood Pulp. 



The most important part of making ground wood pulp lies in the 

 grinding and screening methods which are employed. A great many 

 experiments have been made, but each individual manufacturer generally 

 follows his own ideas on the subject. 



There are two distinct kinds of ground wood pulp, namely, cold and 

 hot ground pulp. These vary greatly in degree of coarseness, and in the 

 length and strength of fiber. When wood is ground into fibers in the 

 presence of large amounts of water a fine, even grade of pulp is produced. 

 This is known, commercially, as cold ground pulp. Contrasted to this 

 form the hot ground pulp is produced under conditions of high tempera- 

 ture and comparatively little water. Hot ground pulp is coarse and 

 contains long fibers. 



The operation of reducing wood to pulp is carried on in a separate part 

 of the mill, in the grinding room where from 4 to 24 grinders or more are 

 operated simultaneously. The wood is brought in on trucks and stacked 

 up at some point convenient for the operator. The grinding machine 

 consists of a strong, iron, circular box, inclosing a heavy grindstone 

 mounted on a horizontal shaft. These grindstones are made of gritty 

 sandstone and are largely imported from England for the purpose. Some 

 artificial stones are also in common use. The surface of the stone is 

 grooved and pitted to make it rough. In size these stones are usually 

 from 54 to 60 in. in diameter with a 27-in. face. Around the circumfer- 

 ence of the casing, openings or pockets are located in which 2-ft. or 4-ft. 

 bolts of wood are placed and pressed against the rotating stone by means 

 of hydraulic pressure. The stones revolve at the rate of about 240 revo- 

 lutions per minute and from 200 to 400 horse-power are required to drive 

 each grmder. The texture of the stone, the rapidity with which it turns 

 and the rate of pressure of the wood against the stone determine in a large 

 measure the character of the pulp made. A stone with an exceedingly 

 rough surface will produce very coarse fibers, whereas if the stone is per- 

 mitted to become too smooth or dull, the fibers will be too short and the 

 resultant pulp too fine. Dull stones are sharpened while revolving, by 

 pressing a " burr " or " jigger " made of especially hardened steel, against 



