10 BULLETIN 343, U. S. DEPARTMENT OF AGRICULTURE. 



condensation being blown off as soon as the temperature fell below 

 that corresponding to the steam pressure in use. In some cases, 

 after cooking the required length of time, the charge was allowed to 

 stand until it had absorbed a large amount of water. After unload- 

 ing, the wood was weighed and then ground as soon as possible. In 

 several instances samples were taken after cooking and the bone-dry 

 weight and the moisture were determined. 



GRINDING. 



Before starting the test the desired surface was placed on the stone 

 by means of a mechanically controlled burr or bush roll. The stone 

 was then washed until free from loose sand and an impression of the 

 stone was taken by means of carbon and coated paper. (See Forest 

 Service Bulletin 127.) The grinder pockets were filled, the pressure 

 adjusted to the correct value, the recording instruments were placed 

 in operation, and the test was then started. 



The speed and pressure were maintained as constant as possible 

 throughout each test. Pocket binding was eliminated by the con- 

 stant observation of power applied to the grinder, a f ailing off in power 

 consumption with pressure on indicating binding. When a piston 

 was raised, instead of allowing the speed to increase, it was held con- 

 stant by regulation of the voltage on the motor armature. 



In series tests in which the surface of the stone was not altered, 

 but the speed, pressure, or temperature were, the data secured may be 

 more or less open to criticism, since the surface undoubtedly changed 

 slightly from one test to another. However, this change was very 

 slight. 1 Likewise, in short tests it was impossible to heat up the 

 stone thoroughly and here also a very slight error was introduced. 1 



During the tests readings were taken of power, speed, pressure, 

 temperature, and the like, the intervals of reading being 15 minutes 

 in most cases, although 5-minute intervals were sometimes employed. 



LOSSES IN CONVERSION. 



While it is highly desirable that the losses occurring in the barking 

 of different woods be determined, it was impossible to investigate 

 this point satisfactorily. Only small amounts of wood were used, in 

 some cases not more than 8 cords, and determinations on such small 

 amounts would not yield reliable results. 



The amount of wood used during any test and the amount of wet 

 pulp manufactured was accurately determined by weighing. Moist- 

 ure samples were taken of both the pulp and the screenings and the 

 bone-dry weight of each determined. In some cases the amount of 

 wood fiber in the white water was secured by measuring the amount 



1 This point is discussed in more detail in Forest Service Bulletin 127, "The Grinding of Spruce for 

 Mechanical Pulp." 



