6 BULLETIN 343, U. S. DEPARTMENT OP AGRICULTURE. 



had risen to $8.32 per cord, and for imported to $11.34 per cord. 

 During the same 10-year period the average cost of manufacturing 

 ground-wood pulp increased from $10.84 per ton to $16.58, by far 

 the greater part of this increase being due to the greater cost of the 

 wood used. Thus the manufacturer of mechanical pulp is faced with 

 a steady increase in the cost of his raw material, and one which 

 promises to continue if he must rely almost wholly upon a single 

 wood. The great bulk of mechanical pulp produced is used in the 

 manufacture of news-print paper, and the problem which presents 

 itself is whether woods other than spruce can be used for the purpose. 

 It was to answer this question, at least in part, that the experiments 

 described in this bulletin were undertaken. 



PRESENT METHODS OF MANUFACTURE. 



The present method of manufacturing ground-wood puip has been 

 in use for a long time. Since 1867, when the mechanical process was 

 first introduced into this country, principles and methods have 

 changed but slightly. The size and capacity of the grinders have 

 been increased and the raw material changed from poplar to spruce, 

 but otherwise the industry is following its first lines. As in any other 

 industry, however, details of manufacture differ in different mills. 

 Of two mills producing news-print paper, for example, one uses 

 135 horsepower to the grinder, the other 625. One employs a grinder 

 cylinder pressure of 17.5 pounds per square inch; the other, with a 

 cylinder of the same size, a pressure of 72 pounds. One runs at a 

 peripheral speed of 2,660 feet per minute, the other at a speed of 

 3,540 feet. Such details of operation may differ among mills pro- 

 ducing the same kind of paper even more widely than those just cited. 

 Reports of power consumption show a range of from 31 to 135 horse- 

 power per ton in 24 hours. The first value is undoubtedly wrong, 

 since it has been demonstrated experimentally that such a low power 

 consumption is impossible with the present grinding equipment. 



Upon its receipt at the mill the wood is either stored for a con- 

 siderable time in the yard or else is ponded. Before storage the wood 

 is sometimes cut into 2-foot lengths, or in other cases is rossed. 



The grinders ordinarily take a pulp-stone approximately 54 inches 

 in diameter by 27 inches face. Some stones recently installed are as 

 large as 60 inches in diameter and 48 inches face. Up to a few years 

 ago natural quarried stones were the only kind used in the grinding 

 process, but many mills are now experimenting with artificial stones. 

 The grinding is usually carried on under conditions of high tempera- 

 ture brought about by admitting only small quantities of water to 

 the pit of the grinder. In the cold-grinding process, which is some- 

 times used in the United States and very largely adhered to in Europe, 

 an excess of water prevents any heating. 



