PRODUCING SODA PULP PROM ASPEN. 55 
gauge. For each pulp 20 sheets whose thickness varied between 0.010 and 0.011 inch 
were tested. The average strength in pounds per square inch per 0.001 inch thickness 
is the quotient of the average test value divided by the average thickness in thou- 
sandths of an inch. A quantity one-tenth of this value is sometimes used in express- 
ing results, and is called the ''strength ratio." 1 The Schopper tester measures in 
kilograms weight the tensile stress required to break a strip of pulp 15 mm. wide. At 
the same time the instrument registers the "per cent stretch," which is the strain or 
elongation of the strip just before breaking, and is expressed as a percentage of the 
original length. The "breaking length" is the length of sheet which, if suspended, 
would break of its own weight, and when expressed in meters is determined by multi- 
plying the weight in kilograms required to break the strip by its testing length in milli- 
meters (180 mm.), and dividing the product by the weight in grams of the portion 
of the strip subjected to test. Five strips of pulp were tested in the "machine 
direction" of the sheet and five across the machine direction, and the average 
values for the two directions determined. 
Bleach required. — The bleaching solution was made by mixing bleaching powder 
(calcium oxy-chloride or chloride of lime) with water and allowing the mixture to 
settle so that a clear solution was obtained. The strength of this solution was deter- 
mined by titrating 5.00 cc. against fifth normal arsenious acid solution, using a solu- 
tion of starch paste and potassium iodide as indicator. The number of cubic centi- 
meters of arsenious acid used, multiplied by 4.0514, gave the strength of the bleaching 
solution in grams per liter of "35 per cent bleach," or bleaching powder, in which 35 
per cent of its weight is chlorine available for bleaching purposes. The bone-dry 
weight (about 50 grams) of the pulp sample used for the bleaching determination was 
first calculated by means of its bone-dry factor. The sample was then thoroughly 
broken up in water 2 to form a uniform pulp mixture. A quantity of the bleaching 
solution containing a known weight of "35 per cent bleach" was added and the mix- 
ture diluted with water 2 to approximately 2,500 cc. This mixture was kept at a 
temperature of 40° C. until the bleach was exhausted, as determined by starch-iodide 
indicator. The bleached fiber was then thoroughly washed free from bleach residues 
and made up into sheets on a small hand mold. These sheets, when air-dry, were 
compared with air-dry standard color sheets made in a similar manner from five or six 
commercially bleached soda pulps mixed in equal proportions. If the first determi- 
nation on the experimental pulp did not give as white a color as the standard, the 
process was repeated on other samples until the standard color was attained as nearly 
as possible. 3 The weight of 35 per cent bleach required to produce the standard 
color is expressed as a percentage of the bone-dry weight of the pulp. The bleaching 
operations were performed in enameled jars provided with agitators and placed in a 
tank of water whose temperature could be regulated by an electric heater. It was 
found best to start the bleaching in the late afternoon or evening, so that the bleach 
was exhausted sometime the next morning. The comparisons with the standard 
color sheets should be made at about the same time each day, using light from a north 
window. 
Loss on bleaching. — For determining the loss on bleaching, a sample of about 2 
grams of pulp was thoroughly broken up in water and bleached in a 250 cc. Erlen- 
meyer flask, using as near as possible the conditions which produced the standard 
1 "Strength factor" or "points per pound" is distinguished from "strength ratio" by the former being 
obtained by dividing the "pop test" by the weight in pounds of a ream of paper. The size of a ream varies, 
but for a standard of comparison a ream of 500 sheets, 24 by 36 inches, is usually preferred for determining 
the strength factor. 
2 The water should be neutral so far as its action on pulp and on bleaching powder solution is concerned. 
The use of distilled water is preferable. 
3 Actual tests have shown that this method gives results almost identical to those secured in pulp-mill 
operations. The method of determining the amount of bleach required by adding an excess of bleaching 
powder and titrating the unconsumed excess after the pulp is bleached sufficiently white, gives much 
lower results. 
