JACK PINE AND HEMLOCK FOB MECHANICAL PULP. 11 



ing run No. 52 of the hemlock series, with the exception of runs Nos. 

 49, 50, and 51 , the wood for which was soaked in the pond for approxi- 

 mately two months before being used. The only jack pine soaked 

 was that used in the commercial test on seasoned wood of that spe- 

 cies run 14. The jack pine and spruce used in tests on mixed pulps 

 were in all cases dry before grinding. The wood for the tests was 

 prepared approximately 2 cords at a time, sawed into 2-foot lengths, 

 barked, weighed, and piled up for the grinding process. 



To determine accurately the yield, the bone-dry weight per cubic 

 foot of wood, as well as the percentage of moisture present, was 

 determined in each commercial test. All weighings were made in 

 500 or 1,000 pound lots, and the wood was used as soon as ground. 



No attempt was made to remove knots or punky portions of the 

 wood. In fact, all of the tests were carried on in accordance with 

 the usual commercial practice. 



GRINDING. 



Before commencing the grinding tests an impression of the surface 

 of the stone which had been selected was taken by means of a piece 

 of carbon paper and a sheet of coated paper. This impression was 

 later photographed, as shown in Plates II and III. In these the 

 black dots represent projecting points and the white portions 

 between them depressions in the stone. The surface shown in Plate 

 III, figure 2, is particularly interesting, since it is the result of dressing 

 with two different kinds of burrs. 



Before starting the tests the recording thermometer and all of the 

 other recording instruments were placed in operation. The pockets 

 of the grinder were filled, the pressure adjusted to the proper value, 

 and the grinder started. 



For the purpose of check and control, regular readings were taken 

 of the various switchboard instruments, the indicating tachometer, 

 the pressure gauges, and the recording thermometer. On short tests 

 up to 2 hours in length these readings were recorded at 5-minute 

 intervals, but on longer tests the interval was increased to 15 minutes. 

 The speed, pressure, and other variables were maintained as nearly 

 constant as possible. For instance, when one of the grinder pistons 

 was raised the speed was brought back to the desired value by 

 manipulation of the rheostat controlling the motor armature voltage. 



During the qualitative and quantitative tests the pulp stone did not 

 have an opportunity to heat up, and, in consequence, some of the 

 data on power consumption and production may be more or less 

 questionable. In the commercial tests, however, all of which were 

 made under the hot-grinding process, the stone was brought up to a 

 high temperature, which was maintained throughout the run; con- 

 sequently these more nearly approximate commercial conditions. 



