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.

Cooking.—­Cooking is the technical term for the operation by which fibrous raw materials are reduced to a residue of cellulose pulp by means of chemical treatment.  In these tests about 300 pounds of hurds were charged into the rotary with the addition of a caustic-soda solution, such as is regularly employed in pulp mills and which tested an average of 109.5 grams of caustic soda per liter, or 0.916 pound per gallon, and averaged 85 per cent causticity.  Sufficient caustic solution was added to furnish 25 or 30 per cent of actual caustic soda, calculated on the bone-dry weight of hurds in the charge.  After closing the rotary head, it was started rotating at the rate of one-half revolution per minute, and in about five minutes steam at 120 pounds per square inch was admitted at such a rate that the charge was heated in one hour to 170 deg.  C., which is the theoretical equivalent of 100 pounds of steam pressure per square inch.  It was found, however, that when the temperature reached 170 deg.  C. the pressure was usually 115 or 120 pounds instead of 100 pounds, due to air and gases inclosed in the rotary.  At