14 BULLETIN 72, U. S. DEPARTMENT OF AGRICULTURE. 



The soda papers were inferior to the sulphate papers in resistance 

 to wear; the latter could be rubbed and crumpled for a long time 

 without showing signs of wear, while the former had a tendency to 

 become fuzzy and tear under similar treatment. Even those sul- 

 phate pulps at very high yields had wearing qualities equal to the 

 best soda pulps. There is little doubt that higher yields of good 

 kraft pulp can be obtained with the sulphate process than with the 

 soda process. Sulphate pulps of fairly good quality can be obtained 

 with yields as high as 61 per cent, while the limit for soda pulps is 

 approximately 50 per cent. With higher yields the soda pulps lose 

 strength and toughness and become brittle. A sulphate pulp with a 

 60 per cent yield can be made into a medium grade of kraft wrapping 

 paper, while a soda pulp having the same yield will produce only a 

 very inferior grade. Considering bursting strength alone, equally 

 strong papers can be made by either process. 



The main advantage of the sulphate process over the soda process 

 is that in the former the pulp can be very much undercooked and 

 still produce a fair quality of paper, while a soda pulp must be com- 

 paratively well cooked before a good paper can be made from it. 

 Moreover, the best sulphate kraft pulps were obtained with a total 

 duration of cooking of only 3.5 hours, while in the soda tests 6 hours 

 were required to secure the best results. 



AUTOCLAVE TESTS. 



The autoclave tests, which, as previously explained, preceded the 

 semicommercial tests, were made to determine the effects of varying 

 the cooking conditions in the production of sulphate pulp. The 

 cooking conditions investigated were: 



(1) Amounts of the various cooking chemical employed. 



(2) Cooking pressures or temperatures. 



(3) Durations of cooking. 



(4) Initial concentrations of chemicals in the digester liquors. 



Aside from the chemicals normally present in sulphate cooking 

 liquors that is, caustic soda, sodium sulphide, sodium sulphate, and 

 sodium carbonate, the effects of sodium chloride and sulphur in con- 

 junction with caustic soda were studied. The tests, Tables 4 to 

 10, inclusive, were made in series, in any of which all cooking con- 

 ditions except the one under observation were held as nearly constant 

 as possible. 



The amounts of sodium carbonate and of SO 2 compounds expressed 

 as Na 2 S0 3 in the cooking liquors were in general small and no 

 mention of them is made in the tabulated data. The amounts of 

 sodium sulphate present are indicated only relatively, except in 

 Tables 6 and 10. 



