JACK PINE AND HEMLOCK FOR MECHANICAL PULP. 17 



Throughout the experiments, particularly the commercial tests, it 

 was found that the pulp of same appearance as regards fiber and of 

 the same apparent strength can be obtained by using burrs of widely 

 different design and fineness of cut, provided the grit of the stone is 

 in each case the same. For example, during the commercial tests 

 the stone was burred at different times with different types of burrs, 

 and the grinding in each case was found to require the consumption 

 of the same amount of power. The production per day was the same 

 also, provided the grit was brought to the same condition of sharpness 

 and the other variables were kept constant. 



During some of the preliminary tests the surface of stone was 

 dulled with a fire brick, as is often done in mills. This appears to 

 have been unnecessary; in fact, the result is detrimental rather than 

 beneficial. 



Better pulp was obtained and the production was increased slightly 

 by crushing the tops of the ridges formed in burring by means of a 

 solid, smooth bush roll. This method does not smooth off the indi- 

 vidual particles of sand on the stone as dulling with the brick does, 

 but rather sharpens them. During the tests conducted on mixed woods 

 a surface obtained by the use of a three-to-the-inch straight-cut solid 

 burr and a 12-cut spiral burr was used. The stone was first dressed 

 with a three-to-the-inch burr, forming grooves hi the stone approxi- 

 mately one thirty-second inch deep; then the portion of the stone 

 between these depressions was roughed with a 12-cut spiral burr. 

 This caused the grit to stand out and gave a maximum of useful 

 grinding surface. The pulp obtained with this surface was almost 

 entirely free from shives, and the fibers were long and fine. The sur- 

 face of stone used during these tests is shown in Plate III, figure 2. 



A great deal of experimentation still remains to be done, not only 

 with burrs of different cut and design, but more especially with stones 

 of different grits, since it appears that the grit is more responsible 

 for the quality of pulp obtained than any other variable feature in its 

 production. 



Where the pulp stone is deep burred, however, the grit is not so 

 important a factor of quality. When the power consumed in making 

 a ton of bone-dry pulp is as low as 50 to 60 horsepower, the added 

 production which must be secured to bring the power to this low 

 value is obtained through the action of the ridges on the stone and 

 not through the grit. When it is desired to manufacture a pulp of 

 high quality, however, it is the grit of the stone and the manner of 

 raising it which must be considered. The type of burr used and the 

 depth of dressing both influence production, but it is only the latter 

 that influences the quality. When the pulp stone has been dressed 

 so as to provide just sufficient depression to carry away the ground 

 23688 12 2 



