MANUFACTURES, ETC. 241 



able is never equal to that produced witli bleacted 

 sulpMte pulp. 



All commercial wood pulps, both, sulphate 

 and sulphite pulps, are produced in various 

 grades, classified broadly as " bleaching " and 

 " strong " pulps. The variations correspond to 

 difEerences in the degree of digestion as deter- 

 mined by the strength of the boiling lye and 

 the temperature and time of treatment. A 

 fully digested " easy bleaching " soda pulp will 

 consume from 15 to 18 per cent of bleaching 

 powder, and will lose from 8 to 10 per cent in 

 the process of bleaching. 



" Strong " soda pulps are darker brown in 

 colour and cannot be bleached economically, 

 whilst the extra strong or " kraft " varieties are 

 suitable only for brown wrapping paper. 



The introduction and development of the 

 sulphite process marks an epoch in paper making. 

 It has added " to the paper makers' supplies 

 celluloses of different characteristics from the 

 ' rag ' celluloses, chemically inferior, but structur- 

 ally offering certain advantages in regard to 

 the production of cheap papers." 



The chemicals employed in this process are 

 sulphate and bisulphite of lime, bisulphite of 

 magnesia and free sulphurous acid. The wood 

 must be so prepared that the lye can quickly 

 and completely penetrate its tissues. This is 



