WOOD PULP AND PAPER 51 



processes. It is upon cellulose and a proper knowledge of its nature that 

 the entire paper industry is based. Cellulose is the basis of the vegetable 

 kingdom and makes up the greater part of all woody tissue. Considered 

 chemically, it is one of the most inert substances known and possesses 

 the property of great resistance to the natural destructive agencies. 

 Cellulose never occurs free in nature but always in combination with 

 other members of the fatty series. Cellulose in its pure form is obtained 

 by the removal of other substances during the chemical processes, whereas 

 mechanical ground pulp is merely the physical reduction of the wood 

 fibers to a pulp form. 



In the preparation of the cellulose fibers for the manufacture of paper, 

 vast quantities of water are used but there is no loss of product through 

 its solubility because cellulose is insoluble in water. In many mills from 

 SO to 70 gal. of water are required for washing every pound of paper 

 that is manufactured. Cellulose has little affinity for chlorine and this 

 is of importance because it permits of the use of chloride of lime and other 

 chlorine compounds for bleaching purposes. 



The strength of any paper is due primarily to the strength and cohesion 

 of its constituents. A careful dissection of any paper will show that the 

 fibers are interlacing in all directions. The deposition of the fibers from 

 suspension in water, the interlacing of the fibers, and the isolation of the 

 individual fibers are the basic principles of papermaking. 



In comparing the mechanical and chemical pulps the principal dis- 

 tinction is that the mechanical pulp is not pure cellulose, and, conse- 

 quently, a very inferior grade of paper is secured. The chemical pulp 

 has had the resins, gums and other fatty constituents, as well as the wood 

 cells themselves, removed, leaving only the fibers of cellulose. Mechan- 

 ical pulp, moreover, produces fibers which are short and brittle, whereas 

 the wood fibers in chemical pulp are long, slender and flexible. Paper 

 made from mechanical pulp oxidizes readily and turns yellow on con- 

 tinued exposure to the air, owing to the organic residues contained in it. 

 It is also relatively weak and is used only for newspapers and cheap 

 wrapping papers. Paper made from chemical pulp is manufactured into 

 the finer grades of book and writing papers, etc. 



Bleaching. 



After the manufacture of pulp has been completed it is necessary to 

 bleach it to bring out the proper color. Although considerable pulp is 

 bleached, in comparison with the total amount manufactured the per- 

 centage put through the bleaching process is relatively small. Sulphite 



