PKODUCING SODA PULP FKOM ASPEN. 3 
barked or peeled wood is then cut diagonally with the grain into 
slices or chips about one-half to three-fourths inch thick by means 
of a machine called a ''chipper." These pieces are then further 
broken up by means of a disintegrator, or "shredder," and the 
resulting chips are conveyed to storage bins, usually above the 
digesters. An intermediate screening operation to remove dust 
and dirt and to secure uniform chips is sometimes given them. 
On account of the strong solvent power of the cooking liquors used 
in the soda process it is not necessary to remove completely the knots 
or decayed portions of the wood. At some mills, however, the 
chips before being stored are sorted into different grades from which 
different qualities of pulp are produced. In the case of peeled wood, 
delivered as such to the mill, the outer shavings, if the wood is re- 
cleaned, are kept by themselves and converted into a lower-grade 
product. In some of the older mills the knots were removed from the 
peeled logs with a boring machine; and later the chips were picked 
over by hand. Such procedures, however, have now been practically 
abandoned in America. 
THE COOKING PROCEDURE. 
The digesters used in soda-pulp making are either rotary or sta- 
tionary, and may be either cylindrical or spherical in shape. The 
present tendency in new installations is towards stationary, vertical, 
cylindrical digesters heated by live steam which enters at the bottom 
of the digester in such a manner as to carry the cooking liquor through 
a pipe to the top of the vessel and spray it over the chips. This 
insures good circulation. The chips and cooking liquors are charged 
through a manhole at the top of the digester, the bottom of which is 
provided with a "blow-off" pipe and valve for discharging the pulp 
after the cooking is complete. Such digesters are from 15 to 50 feet 
high by from 4 to 9 feet in diameter. The larger sizes have been 
lately introduced; in the past the most common size held about one 
cord of wood and was 16 feet high by 5 feet in diameter. At the time 
of the 1905 census the average American digester produced about 
1 ton of pulp per cook, and the total combined capacity of the 208 
soda digesters in operation then was 222 tons of pulp per cook. 
As soon as the charging of chips and caustic soda cooking liquors 
is complete, steam is turned into the digester until a certain cooking 
pressure or temperature is reached. This temperature varies at 
different mills, but one corresponding to 110 pounds steam pres- 
sure per square inch is probably the most common at present. The 
pressure is continued from three to eight hours or more. 1 
WASHING OF PULPS AND RECOVERY OF COOKING CHEMICALS. 
After the digestion process is completed the pulp in the digester 
is generally forced out under pressure or "blown" through a pipe 
1 The detailed cooking conditions employed at various mills are shown in the appendix, Tables 16 and 17. 
