20 BULLETIN 343, U. S. DEPARTMENT OF AGRICULTURE, 
GRIT OF STONE. 
If a stone of coarse grit is used considerably less dressing with the 
burr is necessary to attain the conditions of a sharp stone. In fact, 
it is difficult to produce a pulp that is not coarse and full of shives. 
Although a somewhat lower consumption of power per ton of pulp 
is easily obtained, on account of its coarseness the pulp is not satis- | 
factory from a news-print standpoint. The yield of screenings is 
much greater, as one would anticipate. As indicated by the Mulien 
tester, the strength of pulp ground on a coarse grit stone is a trifle less _ 
than when made on a finer grit stone and consuming the same amount — 
of power per ton; but when compared as to their respective breaking 
lengths, there is no difference. However, when the wood is steamed | 
prior to grinding, a strong pulp may be obtained at lower horse- — 
power consumption on the coarse grit stone than on a finer one. A 
comparison of the grits of the stones may be obtained by referring to | 
Plate III, figures 1 and 2. | 
In general, a stone of fine grit yields a fine-fibered pulp and a coarse | 
stone a shivy and coarse pulp. 
PRESSURE OF GRINDING. 
In any commercial grinder the pressure at which the wood is — 
forced upon the revolving grindstone—that is, the pressure per square | 
inch of wood surface in contact—varies greatly. The variations 
in pressure are brought about by the bmding of the wood in the © 
pockets; the grinding of wood of different lengths and diameters; the 
variation of pressure on the grinder cylinder, due to the removal of | 
pressure from one or more of the cylinders, and the use of varying — 
amounts of spit wood. For any cylinder pressure, however, it is rea- | 
sonable to assume that the pressure per square inch on the stone varies — 
between certain limits. 4 
Figure 11 shows the reiation of the pressure on the grinder cylinder, | 
the horsepower consumption per ton, power to grinder, and production — 
in 24 hours, the three curves being obtained on surfaces of different — 
degrees of sharpness. It will be noted that with increasing pressure _ 
the horsepower consumption per ton decreases, and that the horse-_ 
power to the grinder and production in 24 hours increases at a fairly _ 
definite rate. 7 
The yield and quality of pulp produced vary to a considerable 
degree with the pressure of grinding. Finer, although shorter-_ 
fibered, pulps are obtained when grinding at higher pressure, the — 
advantage of the use of higher pressure being that it is possible to | 
use a duller stone and obtain finer-fibered pulp with a consumption 
of the same amount of power as would be used at lower pressure and _ 
sharper condition of the stone. 
