22 BULLETIN 1485, U. S. DEPARTMENT OF AGRICULTURE 
the mechanical pulps, the' results published in Department Bulletin 
No. 343, " Ground-Wood Pulp"; (2) the relation of yield and quality 
of pulp to particular conditions of pulping and to the characteristics 
of individual woods; and, (3) limitations affecting the reproduction 
of laboratory results at the mill. These considerations are briefly 
discussed in the next three paragraphs. 
LABORATORY PULPING TESTS 
Although the operation of the tests recorded in Tables 6 to 12 was 
standardized as far as possible, it was necessary to vary cooking 
conditions according to the character of the wood pulped. It is fully 
recognized that, with very little information available beforehand to 
guide the selection of the conditions, the cooks frequently failed to 
produce the best results. In such cases conclusions as to practical 
pulping values must be arrived at largely by a process of deduction 
based upon later and more exact knowledge of the factors involved. 
The figures shown in the tables represent, however, results obtained 
under specific selected conditions of operation. They make up the 
most comprehensive body of data that has been compiled with regard 
to the pulping of American species by the various chemical processes. 
They are presented in this bulletin as a matter of scientific record 
and for the possible guidance of future research. 
CONDITIONS OF PULPING AND CHARACTER OF WOOD 
For a number of years the laboratory has conducted special studies 
of spruce, loblolly pine, jack pine, and aspen in an effort to relate 
yield and quality of pulp to the conditions of pulping and to the 
characteristics of the wood itself. The results of these special studies 
have been published at intervals in Government bulletins or in the 
trade and technical journals. Many of the most important results 
have been obtained comparatively recently. Had the information been 
available at an earlier stage of the general study of American species, 
the conditions of pulping could have been selected so as to yield 
pulps in the laboratory more nearly equal, in both quantity and qual- 
ity, to those now considered possible of realization on a commercial 
scale. In preparing the descriptive summaries of species, however, 
the aim has been to interpret the results of the pulping tests in the 
light of the more recently acquired knowledge and so arrive at rea- 
sonably accurate estimates of the practical pulping characteristics of 
the species studied. 
COMMERCIAL ADAPTATION OF EXPERIMENTAL RESULTS 
Laboratory equipment is designed to provide accurate control over 
a relatively wide variety of operating conditions. If it were possible 
to duplicate laboratory control on a commercial scale, laboratory 
results could, of course, be duplicated with mathematical accuracy. 
Such, however, is rarely the case. In mill practice unavoidable 
variations or irregularities are encountered, among which may be 
mentioned those in acid concentration, moisture content and quality 
of wood, temperature rise, pressure and wetness of steam, circulation 
of digester liquor, and end point of cook. It must be borne in mind 
also that the data recorded in the tables were obtained on a much 
smaller scale than obtains at the mill. It is true that wherever 
