PRODUCING SODA PULP FROM ASPEN. 17 
of screened unbleached pulp was identical with that of crude pulp, 
but for smaller amounts of chemical it rapidly approached zero, 
while under the same conditions the screenings curve naturally 
approaches and becomes coincident with the curve for the total 
crude pulp. In this group of tests the minimum amount of caustic 
soda for successful cooking, so far as yields alone are concerned, is 
somewhere between 15 and 20 per cent. 
DURATION OF COOKING. 
The duration of cooking at maximum pressure influenced the 
yields in very much the same manner as did the amount of chemical. 
The yield of total crude pulp decreased about 1 per cent for each 
additional hour of cooking at maximum pressure. However, the 
curve (fig. 4) seems to approach parallelism with the horizontal axis, 
thus signifying that beyond a certain point cooking would have had 
no further effect. 1 The time allowed for these cooks to reach the 
maximum pressure was one hour, and the extended curve indicates 
a yield of about 60 per cent for zero hours duration at maximum 
pressure. This shows that the greater part of the cooking was 
accomplished during the first hour, or before the maximum pressure 
was attained, since during that hour about 40 per cent of the wood 
substance had been dissolved and the dissolving effect during the 
next 12 hours was only one-fourth as great. 
As determined by the yield curves, the minimum duration for 
successful cooking under the conditions employed was between one 
and three hours at maximum pressure. No tests were made between 
these two points. 
PRESSURE OF COOKING. 
The curve showing the influence of maximum cooking pressure or 
temperature on yields indicates that all of the tests were made at 
pressures above the minimum required for successful cooking, under 
the conditions employed for these tests; hence, no screenings were 
obtained from any of the cooks, and the curve for screened unbleached 
pulp coincides with that for total crude pulp. Increases of pressure 
from 70 to 120 pounds per square inch resulted in decreasing the 
yields of pulp about 1 per cent for each five pounds, which indicates 
that the higher pressures increase the thoroughness of cooking, other 
conditions being constant. 
CONCENTRATION OF CAUSTIC SODA. 
The tests varying the initial concentration of caustic soda in the 
digester liquors were also made within limits that resulted in thorough 
cooking for all of the tests. Increasing the concentration under the 
1 Figures 12 and 13 show that the active cooking chemical was consumed at the end of 7 hours at maxi- 
mum pressure; it is therefore not apparent from these tests what would be the effect of continued cooking 
in the - presence of available «caustic -soda. 
31091°— Bull. '80—14 2 
