18 
BULLETIN 80, U. S. DEPARTMENT OF AGRICULTURE. 
100 
50. 
ft) 
r 
*7 
- — 'i 
r 5 fl 
4 
.10 
.20 .30 .40 
POUNDS NaOH PER POUND OF WOOD 
100 
'-': 
96 
Trf-ir 
■>3»__ 
jw— 
•n 
2 4 8 8 10 12 
DURATION AT MAX. PRESSURE- HOURS 
conditions employed resulted in decreasing the yields of pulp about 
1 per cent for each 13 grams per liter increase in concentration. It 
is thus evident that with a given amount of chemical the greater 
cooking effect is secured by means of the more concentrated solutions. 
A practical limit of course exists at the point where the volume of 
the digester liquor becomes too small to afford favorable cooking 
conditions. 1 
PROPERTIES OF UNBLEACHED PULPS. 
NATURAL COLOR. 
Curves indicating the effects of the conditions of cooking on the 
natural color of the unbleached pulps are shown in figure 5. 
The larger the amount of 
caustic soda used per pound 
of wood the lighter in color 
was the pulp, as indicated by 
the i( parts black" color rat- 
ing, but the curve approaches 
parallelism with the horizontal 
axis as the amounts of caustic 
increase. White pulps or those 
with zero " parts black" would 
not be obtained even if exceed- 
ingly large amounts of chemi- 
cal were used. 
Longer periods of cooking 
produced lighter-colored pulps 
up to the point where the 
maximum yield of screened 
pulp was obtained. Beyond 
this point there was a tendency 
for the pulp to become slightly 
darker as the duration of cook- 
ing was increased. This was probably due to the pulp fibers absorbing 
and retaining coloring matters from the "black liquors." It is gen- 
erally believed that as the cooking becomes more thorough the 
cellulose of the fibers gradually becomes more gelatinous or hydrated, 
and would therefore tend to retain coloring matter during the subse- 
quent leaching and washing treatments. 
The pressure (temperature) of cooking seems to have had compar- 
atively little effect on the color of the pulp within the range investi- 
gated. 
i As the initial concentrations increased, the volumes of digester liquors at the start of cook decreased 
(see fig. 17), since the amount of caustic soda was held constant. Hence, increasing concentrations would 
eventually result in a volume of digester liquor so small that the whole charge of chips would not be covered 
until late in the cooking period after the liquor had been sufficiently diluted by the condensed steam used 
in cooking. In this case part of the chips would receive very severe treatment, while the remainder would 
more or less escape the cooking effect. The resulting pulp would represent a composite of the two con- 
ditions. 
70 80 90 100 110 120 
MAX. PRESSURE- PDS. PER SQ. IN. 
TOO 
50 
t 
|2 
25" 
* 4 ! 
—If 
I 
Fig. 
40 SO 60 70 80 SO 100 110 
CONCENTRATION NaOH -GRAMS PER LITER 
5.— Effects of cooking conditions on the color 
("parts black") of pulp. 
