14 BULLETIN 404, U. S. DEPARTMENT OF AGRICULTURE. 
approximately 3 per cent. It became apparent that a finer screen 
would probably serve as well and effect a saving of small but good 
hurds. 
Cooking. — Cooking is the technical term for the operation by which 
fibrous raw materials are reduced to a residue of cellulose pulp by 
means of chemical treatment. In these tests about 300 pounds of 
hurds were charged into the rotary with the addition of a caustic- 
soda solution, such as is regularly employed in pulp mills and which 
tested an average of 109.5 grams of caustic soda per liter, or 0.916 
pound per gallon, and averaged 85 per cent causticity. Sufficient 
caustic solution was added to furnish 25 or 30 per cent of actual 
caustic soda, calculated on the bone-dry weight of hurds in the charge. 
After closing the rotary head, it was started rotating at the rate of 
one-half revolution per minute, and in about ^.Ye minutes steam at 
120 pounds per square inch was admitted at such a rate that the charge 
was heated in one hour to 170° C, which is the theoretical equivalent 
of 100 pounds of steam pressure per square inch. It was found, 
however, that when the temperature reached 170° C. the pressure 
was usually 115 or 120 pounds instead of 100 pounds, due to air and 
gases inclosed in the rotary. At this point the rotary was stopped 
and steam and air relieved until the pressure dropped to 100 pounds, 
or a solid steam pressure. The temperature was maintained at this 
point for the number of hours required to reduce the hurds, which 
was found to be about five, after which the rotary was stopped 
and steam relieved until the pressure was reduced to zero, when the 
head was removed and the stock was emptied into a tank under- 
neath, measuring 5 \ by 6 by 2 feet deep, where it was drained and 
washed. Samples of waste soda solution or "black liquor," which 
were taken from some of the " cooks" for analysis, were drawn while 
the stock was being thus emptied into the drainer. 
Determination of yield. — For determining the yield of cellulose fiber 
the stock in the drain tank was washed with water until free from 
waste soda solution, when, by means of a vacuum pump communi- 
cating with the space between the bottom and the false perforated 
bottom, the water was sucked from the stock, leaving the fiber with 
a very uniform moisture content throughout its entire mass and in a 
condition suitable for removing, sampling, and weighing for a yield 
determination. Tests have shown that it is possible to sample and 
calculate the yield of bone-dry fiber within 0.05 per cent of the actual 
amount. 
It has been found that stocks from different materials vary greatly 
in their ability to mat in the drain tank, thereby enabling a good 
vacuum to be obtained, some stocks permitting a 25-inch vacuum to 
be obtained, while others will not permit more than 5 inches. For 
this reason the moisture content of the stock will vary from 65 to 85 
per cent. 
