48 BULLETIN" 80, U. S. DEPARTMENT OE AGRICULTURE. 
Color ratings. — These are numerical expressions of the colors of the unbleached 
pulps, and were determined on machine-made sheets by means of a tint photometer, 
according to the method described on page 54. 
Average strength. — The strength tests were conducted on machine-made sheets of 
unbleached pulp in the air-dry condition. 1 The methods of testing are described on 
page 54. 
Bleach required. — This represents the parts of bleaching powder (35 per cent availa- 
ble chlorine^ required to bleach 100 parts bone-dry weight of unbleached pulp to the 
commercial white color. 
Loss on bleaching. — This is based on the calculated bone-dry weight of unbleached 
pulp lost when bleaching it to the commercial white color, with the stated per cent of 
bleach. 
Causticity of black liquor. — This was determined by analysis, and is the ratio of the 
sodium oxide in the caustic soda existing as such in the black liquor to the total sodium 
oxide in the black liquor. The latter represents the total titratable alkali in the ash 
resulting from calcining the black liquor. 
Efficiency in the use of NaOH. — This is the ratio of the amount of caustic soda actu- 
ally consumed during the cooking operations to the amount of caustic soda originally 
in the charge. It is calculated from the causticity of the black liquor, assuming that 
the soda chemicals in the black liquor are all titratable as sodium carbonate, after the 
liquor has been reduced to an ash. 
Wood, soda ash, and bleach employed per ton of pulp. — The volume of wood is based 
on the average bone-dry weight of aspen used in the tests (2,668 pounds per solid cord). 
The soda ash represents the total soda ash of 99.1 per cent purity (58 per cent Na 2 0) 
necessary to furnish the chemicals employed in cooking, assuming that no loss of 
alkali occurs in causticizing. The bleaching powder represents the dry weight of this 
chemical, losses in making the bleaching solutions being disregarded. 
i The failure of the Schopper tests to show even relatively uniform variations in the strength of pulp as 
affected by changes in the cooking conditions can not be explained unless that the pulps were not suitable 
for these tests. 
