PRODUCING SODA PULP FROM ASPEN. 29 
Since the completion of these experiments Mr. E. Sutermeister has 
published * the results of some tests in which a small rotary autoclave 
and copper flasks were used as cooking vessels. Yields varying from 
93 to 24 per cent and consumptions of caustic soda varying from 
to 29 pounds per 100 pounds of wood were obtained, 2 giving a relation 
similar to that indicated by the curve in figure 14. However, in his 
experiments a greater reduction of yields was obtained per unit 
decrease in the caustic soda consumed, which is probably due to 
differences in test material, method of experimentation, and appa- 
ratus employed. 
The actual consumption of caustic soda during cooking is a factor 
which is not given sufficient consideration in commercial practice, 
although it is one of considerable importance for an intelligent control 
of the cooking operations. By a careful study of the consumption, 
together with the other effects of the various cooking conditions, it is 
possible to determine the best operating conditions. That pulp 
mills do not ordinarily determine the consumption of caustic soda and 
the efficiency of its use is due to the length of time necessary for the 
analysis of the black liquors. While the method used in these experi- 
ments requires some time for carrying out the analysis, its occasional 
use in commercial operations would be of benefit in determining the 
conditions to be used in future cooks. 3 If there were a rapid and 
accurate method of analysis such as is used in sulphite mill operations, 
it would assist in determining when the cooking had progressed far 
enough, at which time the digester could be blown. Production of 
undercooked or overcooked pulps would thus be avoided. 
SEVERITY OF COOKING AS INDICATED BY MICROSCOPIC APPEARANCE OF FIBERS. 
A good indication of the thoroughness or severity of cooking may 
be obtained by microscopic examinations of the pulp fibers. 4 The 
effects of varying the cooking conditions are shown in figure 15; 
curve A represents the relative abundance of vessels in the pulps; 
curve B, the ray cells; curve C, the fiber bundles or shives; curve D, 
the prominence of the vessel markings; and curve E, the apparent 
strength of fiber walls. Since there are no absolute units for measur- 
ing these effects, the ordinates as shown for each curve represent 
arbitrary units ranging from to 10. The photomicrographs in 
Plates II to VII, inclusive, present some of the more pronounced 
i Paper, p. 15, No. 2, vol. 9, Sept. 25, 1912. 
2 In obtaining yields higher than 75 per cent the test material was treated at atmospheric pressure. Under 
this condition the cooking effect of water alone would have but little influence unless long durations of 
treatment were used. 
3 The boiling of rags with caustic-soda solutions for the production of rag pulps is controlled in this manner. 
* For the normal appearance of fibers in the uncooked Avood see Plates VIII and IX, as well as the discus- 
sion on p. 42. 
