pment
involves the use of machines. Accuracy in degree of control and in
results will vary materially with the size of the test. As the size of
the test increases, certain factors will vary in a beneficial manner,
while others will vary in a detrimental manner, so it is a question for
each investigator to decide, after taking all factors into
consideration, as to the size of test which will give the most
satisfactory results. In work of this nature it is found, on the whole,
that better results are obtained in large tests, although the control of
the factors and the determination of the yield of fiber are more
difficult than in smaller tests.

In the tests described in this bulletin, the Department of Agriculture
employed a rotary digester of its own design,[2] comprising a shell 5
feet 5 inches in length by 4 feet in diameter, capable of holding about
300 pounds of air-dry hurds. It is believed that a test of this size is
large enough to give satisfactory results and that the results are
susceptible of commercial interpretation, while at the same time they
are sufficiently small for complete control and to afford fiber-yield
figures which are both accurate and reliable. Two such rotary charges
gave enough fiber for one complete paper-making test.

[Footnote 2: For a description of this rotary digester, see Brand, C.
J., and Merrill, J. L., Zacaton as a paper-making material, U. S. Dept.
Agr. Bul. 309, p. 28, 1915.]

=OPERATIONS INVOLVED IN A TEST.=

A complete test on hurds comprises seven distinct operations, and the
method will be described, operation by operation, in the order in which
they were conducted.

_Sieving._--The hurds for the first test were not sieved to remove sand
and dirt, but the resulting paper was so dirty that sieving was
practiced in all subsequent tests. The hurds were raked along a
horizontal galvanized-iron screen, 15 feet long and 3 feet wide, with
11-1/2 meshes per linear inch, the screen being agitated by hand from
below. Various amounts of dirt and chaff could be removed, depending on
the degree of action, but it was found that if much more than 3 per cent
of the material was removed it consisted chiefly of fine pieces of wood
with practically no additional sand or dirt; in most of the tests,
therefore, the material was screened so as to remove approximately 3 per
cent. It became apparent that a finer screen would probably serve as
well and effect a saving of small but good hurds.