10 
BULLETIN 80, U. S. DEPARTMENT OF AGRICULTURE. 
The tests fall naturally into four groups; in each group all the condi- 
tions were held as nearly constant as possible except the factor under 
investigation, which was varied in successive tests or "cooks" accord- 
ing to a definite plan. The plan of the tests is shown in Table 3. In 
addition to the factors mentioned in this table all other factors under 
control were so far as possible held constant. Those for which values 
were specified are the following: 
Amount of chips for each, charge, 40 pounds bone-dry weight. 
Dryness of chips, air dry. 
Causticity of cooking liquors, 95-98 per cent. 
Temperature of charging cooking liquor, 22° C. (72° F.) 
Temperature of digester room, 22° C. (72° F.) 
Duration of cooking before maximum pressure is reached, 1 hour. 
Duration of cooling and relieving digester before blowing, 5-10 minutes. 
Blowing pressure, 30 pounds per square inch. 
Table 3. — Plan of coohing experiments. 
Cooking conditions under investigation. 
Test 
group. 
Num- 
ber of 
tests. 
Initial concentration 
of caustic soda in 
digester liquors. 1 
Amount of caustic 
soda per 100 
pounds of wood. 1 
Maximum cooking 
temperature 
(equivalent steam 
pressure). 
Duration of cooking 
at maximum pres- 
sure or tempera- 
ture. 
I. 
II. 
III. 
6 
6 
6 
4 
Constant— SO grams 
per liter. 
Same as Group I 
. . .do 
Variable— from 15 
to 40 pounds in 
steps of 5 pounds 
each. 
Constant— 25 pounds 
(value selected 
from Group I tests 
as most satisfac- 
tory for later tests). 
Same as Group II . . . 
do 
Constant— 100 
pounds per square 
inch. 
Same as Group I 
Variable— from 70 to 
120 pounds per 
square inch in 
steps of 10 pounds 
each. 
Con stan t—1 
pounds per square 
inch (value se- 
lected from Group 
III tests as most 
satisfactory for 
later tests). 
Constant— & hours. 
Variable — from 1 to 
11 hours in steps 
of 2 hours each. 
IV. 
Variable — from 110 
to 50 grams per 
liter in steps of 20 
grams each. 
(value selected 
from Group II tests 
as most satisfac- 
tory for later tests). 
Same as Group III. 
*In commercial practice it is customary to vary the amount of chemical used and its initial concentra- 
tion both at the same time when attempting to change the severit3 T of the cooking due to these factors. 
This results in the volume of the cooking liquors being kept approximately the same, which is a desirable 
feature. In these tests, however, it was the intention to find out the effects of each factor separately. 
TESTING PROCEDURE. 
The apparatus employed in cooking is shown in figure 1. Figure 
2 shows diagrammatically the course of the material through the 
various stages of treatment and testing, and in this way the relation 
of one part of the procedure to another is made clear. 
After the amount of moisture in the chips had been ascertained 
by means of sample A, the charge was weighed out and put into the 
digester. Caustic soda solution, of the desired concentration and 
volume, had been prepared by diluting the necessary quantity of 
analyzed stock solution (sample B) with water. It was then heated 
