6 BULLETIN 348, U. S. DEPARTMENT OF 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 pwp has been 
in use fora 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 limes. 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 grmding 
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. 
