WOOD AND ITS USES 31 9 



There are four principal methods of converting wood into 

 pulp, three of which are chemical processes, and one a mechan- 

 ical process. These include the sulphite, sulphate, ground wood 

 and soda processes. 



The sulphite process. — ^The principal woods used in this process 

 are spruce, hemlock and balsam fir, which are brought to the 

 mill in the form of four foot bolts or logs. If these have not been 

 barked or peeled in the woods, this is the first step of the mill 

 procedure. They are then reduced to small chips by a mechanical 

 chipper, after which they are cooked, under high pressure and 

 temperature, in an acid solution consisting of sulphurous acid 

 and calcium bisulphite. This solution removes all non-cellulose 

 materials, which make up about 50% of the weight of the wood, 

 from the pulp, so that the final product is pure cellulose. The 

 non-cellulose fraction is at present a large source of waste, which 

 may some day be used. Following an eight to fifteen hour cooking 

 period, the pulp is washed free of chemicals, screened, and com- 

 pressed into sheets for transport to the paper mill. 



The sulphate process, — This process produces pulp which is 

 manufactured into the Kraft papers, so called because of their 

 great strength. The principal woods used in the manufacture of 

 sulphate pulp are southern yellow pine, jack pine, and hemlock. 

 They are prepared as in the preceding process, being reduced to 

 chips which are cooked under pressure in an alkaline solution of 

 caustic soda and sodium sulphide for one to four hours after 

 which the pulp is washed, screened and dried in sheets. 



The groundwood process. — This is non-chemical in nature and 

 consists of grinding the logs or bolts into a pulp by holding them 

 against the surface of a rapidly revolving grindstone. Large 

 amounts of water are sprayed on the stone to keep the pulp from 

 overheating during the grinding. This is the cheapest form of 

 pulp conversion, and results in a low grade pulp, which alone 

 would make a very weak, inferior type of paper. It is used, with 

 the addition of sulphite pulp, for newsprint paper, also for wall 

 papers, wall boards, and wrapping papers. The original chemical 

 constituents of the wood remain in the final pulp product, with 

 the result that yellowing and discoloration occur with exposure to 

 air and light, being due to oxidation of substances which in 



