4 BULLETIN 80, U. S. DEPARTMENT OF AGRICULTURE. 
connected with, the bottom of the digester into a "blowpit" or 
"balloon;" whence it is transferred to large washing pans. Here it 
is drained as free as possible from the strong spent cooking liquors, 
called "black liquors," and washed thoroughly, first with hot, weak, 
black liquors from the last washings of previous cooks, and lastly 
with fresh hot water. The first drainings and washings which con- 
tain the greater part of the alkali cooking chemicals are run to evap- 
orators, concentrated, and later calcined in furnaces. The burned ash, 
called "black ash," is leached with water, and the alkali in the form 
of sodium carbonate is dissolved. The resulting solution is treated 
with quicklime (CaO), which changes the carbonate into caustic soda. 
Modern practice recovers from 88 to 92 per cent of the alkali charged 
into the digesters. By properly controlling the strength of the black 
ash solution and mixing various strengths of recausticized solutions, 
a caustic soda liquor of the desired strength for cooking is prepared. 1 
TREATMENTS GIVEN THE SODA PULP. 
After the pulp has been thoroughly washed it is diluted with a large 
amount of water and screened to remove uncooked portions. This 
is accomplished by either flat plate, diaphragm screens, or by such 
screens in conjunction with centrifugal ones. In the case of aspen 
or poplar the greater proportion of the water in the pulp is then 
removed by means of "slushers," "feltless wet machines," or 
"deckers." The pulp is then treated in a suitable vessel with bleach- 
ing-powder solution and afterwards thoroughly washed. Very little 
aspen or poplar pulp is left in the unbleached state, but is usually 
bleached immediately after it is screened. Those mills making both 
pulp and paper generally carry the bleached wet pulp directly 
through the subsequent paper-making operations; but if the pulp 
is to be sold or stored it is simply run over a paper machine into rolls 
of dry pulp (about 10 per cent water). 
PREVIOUS INVESTIGATIONS. 
The treatises by Cross and Bevan 2 and by Schwalbe 3 and the 
recent experiments 4 by Viewig, Miller-Moskan, Miller, Schwalbe, and 
Schwalbe and Kobinoff give much information on the nature of the 
chemical reactions which take place between caustic soda and cellu- 
lose under various conditions, and on the formation of decomposition, 
mercerization, and other similar products from cellulose. 
i A few mills still cling to the older practice of not recovering the alkali from the black liquors. Such mills 
buy the alkali for cooking in the form of caustic soda (XaOH). The cooking solution is produced by dis- 
solving in water a sufficient quantity of the caustic to give a solution of the desired strength. The black 
liquors are run to waste, and, although the consumption of cooking chemicals is very high, the mills seem 
to operate at a profit. 
2 Cellulose, 1903. Also Researches on Cellulose, 1895-1900; 1900-1905; 1905-1910. 
3 Die Chemie der Cellulose. 1910-1912. 
4 For specific literature references see bibliography in appendix. 
