19 BULLETIN 343, U. S. DEPARTMENT OF AGRICULTURE. 
to ascertain the influence on the quality and quantity of pulp pro- 
duced by the variation of the grinding variables, such as surface of 
the stone, pressure on the cylinders, speed, temperature, etc. 
The cooking of wood prior to grinding is attended with a number of 
changes in the physical characteristics of the wood which greatly 
influence the quality of the pulp manufactured from it. The cooking 
condition must be chosen according to the use to which the pulp is to 
be put. For the manufacture of news-print paper from pitchy woods 
it is essential that the treatment be a very mild one, and that the 
duration of boiling or steaming and the temperature at which it is 
carried on be such as to allow of the fiber being as light in color as 
possible. Either the pressure, or corresponding temperature, must 
be low and the cook of long duration, or the pressure higher and the 
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STEAMING PRESSURE — POUNDS 
Fic. 4.—Effect of steaming pressure on color of pulp (average values of several rums on spruce). 
cook of much shorter duration. It is generally claimed that cooking 
the wood under pressure while immersed in water will yieid a lighter 
pulp than steaming at the same pressure and for the same length of 
time, but this result has not been noted. 
Boiling wood has certain disadvantages which are not found when 
the steaming process is used. There is a considerable loss of heat, 
and it is necessary to draw off condensed liquors to maintain the 
temperature at any desired value. This makes it necessary to pay 
greater attention to the process. If the temperature and duration 
of the cook are the same, the steamed or boiled wood should be prac- 
tically the same in color and other physical characteristics. One 
decided advantage of steaming is the possibility of draining off the 
condensed liquors in a concentrated form. This is of great benefit 
when by-products are to be recovered. 
