8 BULLETIN 1485, U. S. DEPARTMENT OF AGRICULTURE 
them is considered. Although some of these species may give as 
high as 6 per cent screenings on account of knots or small local 
portions more resinous than the rest of the wood, the uncooked 
portions can usually be successfully removed with standard equip- 
ment, leaving relatively clean pulps of satisfactory color. The pines 
of the white pine series that were tested (northern white pine, sugar 
pine, limber pine, and pinon) can not be satisfactorily pulped, even 
though on inspection they show less resin than the other pines 
mentioned above. Although the amounts of screenings are small, 
the pulps contain particles of partially cooked wood which pass 
through the screen plate slots with the fiber and cause the pulps to be 
specky and of poor color. Not only do the extractive substances 
affect penetrability, but often the structure of the wood itself has 
considerable influence, and many species show differences in cooking 
which can be attributed only to this cause. 
Although the fibers are separated by the sulphite process without 
reduction in ultimate length, fiber length is of such importance that, 
as a rule, only long-fibered woods are pulped in this way. The color 
is usually of minor significance, except in the case of deeply colored 
woods, whose coloring substances are often not dissolved in the* 
cooking operation, so that the color of the pulps is affected. 
Specific gravity of the wood has the same importance in affecting 
the yield per cord as in the case of ground-wood pulp, and it also 
directly affects the quantity of wood per digester charge and thereby 
the output of the mill. 
The bleachability of the pulp and the ease or difficulty of hydration 
are important considerations. They both depend to a considerable 
extent on the cooking conditions. 
Although the soda process is generally used on hardwoods on 
account of the greater opacity of the pulp, the sulphite process 
produces from many species (such as the birches, maples, and gums) 
greater yields of pulp that can be bleached with much lower con- 
sumption of bleach. 
SODA PROCESS 
The soda process is almost entirely devoted to the pulping of hard- 
woods. When the Forest Service pulping experiments were started 
the species in commercial use were the true poplars, yellow poplar, 
the soft maples, basswood, and possibly a few other of the most 
easily pulped species. At present, by the use of improved methods, 
birches, hard maples, gums, beech, chestnut, sycamore, and numerous 
others are also being pulped. The pulp is usually bleached. It 
holds loading materials and takes coatings well. It also possesses soft, 
bulky, and absorbent qualities which, when it is properly mixed 
with bleached or unbleached sulphite, give an opaque, well felted, and 
well formed sheet of paper highly esteemed by the printer. Soda 
pulp is consequently used largely in the manufacture of book, maga- 
zine, lithograph, and other papers for printing purposes, and consider- 
able quantities are employed in the manufacture of blotting paper. 
The soda process can be applied without difficulty to coniferous 
woods and to a limited extent is so applied. Usually, however, the 
greater strength and toughness of pulp produced by the sulphate 
process from the same wood has limited the use of the soda process 
for producing strong pulps. Soda pulp from conifers is hard to 
bleach, and the process is, therefore, little used for bleached pulp 
