6 BULLETIN 1485, IT. S. DEPARTMENT OF AGRICULTURE 
wood for grinding is first cut into bolts of the desired length and the 
bark completely removed. The clean bolts are then ground either hot 
or cold, according to the character of pulp desired. The difference in 
the results obtained is largely in the "freeness," or the rate at which 
water will drain from the pulp on the paper machine. Water will 
drain away much faster from hot-ground pulp than from cold-ground 
pulp. Hot grinding, the American practice, is done by adding only 
sufficient water at the grindstone to keep the temperature of the 
pulp in the neighborhood of 150° F. In cold grinding, which is more 
commonly employed in Europe, much larger quantities of water are 
used, with a resulting temperature very little above the temperature 
of the water. Some ground-wood pulp is manufactured from wood 
that has been steamed or boiled before grinding, but the quantity is 
so small that it is not considered in this publication. The reader is 
referred to Department Bulletin No. 343, "Ground- Wood Pulp," for 
more specific information and reports of tests on various species in the 
manufacture of mechanical pulp. 
In the mechanical process, since very little, if any, chemical action 
takes place, the physical properties of the wood are of first consid- 
eration. One of the most important qualities is the color of the 
wood, which is directly reflected in the color of the pulp obtained. 
Only the lightest colored woods can be considered for use in the manu- 
facture of newsprint, for which almost all mechanical pulp is utilized. 
Although the mechanical process is very destructive to the fibers of 
the wood and their average length is seriously reduced in the opera- 
tion, nevertheless long-fibered woods, such as spruce, are desired, 
since the longer the fibers present in the wood the greater the average 
fiber length of the pulp obtained. 
The hardness and structure of the wood are reflected in the power 
consumed in grinding pulp of standard strength and in the amount of 
disintegration caused by the action of the stone in tearing the fibers 
from the mass. Hardwoods with a dense medullary structure con- 
sume from two to three times as much power as softer, lighter woods, 
and their fibers are to a great extent torn to pieces. As a rule, there- 
fore, their use is barred. Woods which give soft, spongy pulps are 
also undesirable, as are woods which yield pulps composed of mix- 
tures of fine slivers and debris. A measure of control, however, can 
be exerted over pulp properties by the manner of dressing the stone. 
The size and shape of the pieces of wood used in grinding are of 
considerable importance in the mechanical process with the customary 
equipment. The greater the number of pieces of wood fed to the 
grinders per ton of pulp the greater the number of splinters, since 
from each piece there is usually a thin slab which passes from the 
pocket without further reduction after a certain minimum thickness 
is reached. 
Specific gravity is an important consideration, because the higher 
the specific gravity the greater the weight of pulp obtained from 
a cord. 
The pitch content is also important, since, under given conditions, 
more than a certain amount will cause trouble in the subsequent 
paper-making operations. 
Discolorations in the wood caused by fungi, molds, or bacteria 
are usually reflected in the pulp obtained. The durability of the wood 
used in the mechanical process, i. e. ; its resistance to the organisms of 
