20 BULLETIN 1485, U. S. DEPARTMENT OF AGRICULTURE 
In the case of species more or less resinous, it frequently happens 
that by the mechanical or sulphite-processes pulps of satisfactory 
strength and color can be obtained, but that the presence of " pitch" 
gives rise to operating difficulties in the paper mill. The laboratory 
experimental runs in which the pulps were studied were so short that 
the degree to which "pitch." troubles would be encountered was not 
ascertained accurately. It is possible that the difficulties would be 
decreased or eliminated by a judicious use, on the grinders or in the 
beaters, of some highly colloidal material such as bentonite. At the 
suggestion of the laboratory staff bentonite has been used commer- 
cially in both sulphite and mechanical pulping operations, with 
satisfactory results. It may become an important factor in extend- 
ing the use of resinous pulp woods. 
HARDWOODS 
The fibers of hardwoods are short and hence not suited to the 
manufacture of strong, tough papers. They do, however, contribute 
to the formation of a well-closed, smooth-surfaced sheet of the type 
that is essential for a satisfactory print paper. In news and similar 
cheap print papers this characteristic is obtained more economically 
by the addition of ground wood than by the addition of a short- 
fibered chemical pulp; hence bleached hardwood chemical pulps find 
their most extensive use in print papers of the higher grades. The 
alkaline-cooked pulps, by virtue of their softness, bulk, and opacity, 
are most favored. The sulphite pulps are harder and less opaque 
but yield better, bleach whiter, and require less bleach. Their lack 
of opacity may be overcome by the addition of mineral fillers. A 
more extensive use of hardwood sulphite pulps in the manufacture of 
book, envelope, and similar paper may be expected, especially in 
paper mills that are already equipped for the production of sulphite. 
The following hardwoods, with diffuse-porous structure, can be 
reduced readily by the sulphite process and bleached with small 
quantities of chemical: Poplars, willows, maples, birches, gums, mag- 
nolias, sycamore, and butternut. Of these with ring-porous struc- 
ture, kackberry, elm, ash, and hickory can be readily or fairly readily 
pulped and bleached. The pulp usually contains small dark-colored 
specks, which, however, disappear on bleaching. Of diffuse-porous 
woods difficult to pulp and bleach to a satisfactory product there 
were tested basswood, red gum, and Ohio buckeye; of ring-porous 
woods difficult to pulp and bleach, common catalpa, chestnut, sassa- 
fras, slippery elm, red and white oak, and black locust. In both 
cases the difficulty of pulping is probably due to extractive matter 
or to structure which prevents penetration of the wood by the cooking 
liquor. 
The hardwoods, in general, are unsuited to the manufacture of 
mechanical pulp, on account of the high power consumption required 
and the weakness of the pulp. By the adoption of new principles in 
grinding, however, it may become practicable to use the lighter 
colored of the hardwoods, such as the poplars, birches, and gums, for 
the manufacture of mechanical pulp. 
All the hardwoods are capable of reduction by the soda or sulphate 
processes. The poplars and basswood among the diffuse-porous 
species are very easily pulped and bleached, and yellow poplar, 
willows, soft maples, beech, red alder, and balsa fairly easily. The 
