GROUND-WOOD PULP. 13 
In all of the experiments conducted in cooking wood prior to grind- 
ing, the temperature corresponded to the temperature of boiling at the 
steam pressure under which the cook was being made. Attempts 
were made to secure by-products, but the amounts of wood used were 
so small that this could not be done satisfactorily. 
INFLUENCE OF PRESSURE, TEMPERATURE, AND TIME OF COOKING. 
When the length of time of the cook is kept constant and the cook- 
_ ing pressure or temperature is varied, it is found that the color of the 
_ pulp made darkens greatly when these factors are raised, all other 
conditions being the same. This is demonstrated by reference to the 
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_ Fic. 5.—Effect of duration of steaming on color of pulp. (Average values of several runs on spruce.) 
curves in figures 4 and 5, where the amount of black in the color as 
determined by a tintphotometer is plotted against the cooking pres- 
_ sure and temperature. At some steam pressure the wood will char 
_ and the resultant pulp will be black; under this condition a maximum 
_ of 300 parts black would be secured. Itis probable that this steam 
pressure would not be very high, since the values of parts of black 
"increase rapidly after 60 pounds pressure is reached. 
_ The strength of the pulp increases with the length of the steaming 
" period (fig. 6) and seems to reach a maximum in about 8 hours for 
_ both pressures. In figure 7 is shown the variation of the yield of pulp 
per 100 cubic feet of solid rossed wood, with the pressure of steaming 
_and the duration of the cook. The marked effect of the duration of 
_ the cook, especially at high pressure, is evident. The yield decreases 
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