PRODUCTION OF SULPHITE PULP PROM SPRUCE. 13 



is used for every cook. What is ordinarily known as the lime test 

 was also tried. In this test some of the cooking liquor from the diges- 

 ter was put in a test tube containing a slight excess of ammonia, 

 and if the lime came down in a dark noculent condition it was time 

 to blow the cook. While this method gave promise, the results ob- 

 tained with it were very erratic and it was finally abandoned. An 

 attempt to judge the degree to which the chips in the digester had 

 been cooked by blowing some out into a canvas bag and shaking in 

 a cylinder was also unsuccessful because of the great irregularity in 

 the samples. Sometimes a sample would show more uncooked shives 

 than the one taken half an hour before. Judging the condition of 

 the stock by dyeing with a basic color, such as diamond green, also 

 showed no regularity. Nor did the depth to which a 13 per cent 

 solution of nitric acid would color the pulp indicate the end point 

 accurately. 



It is common practice in pulp mills to judge when a cook is finished 

 by the color, smell, and analysis of the liquor; but with the wide 

 variety of conditions under which the cooks in this study were made 

 it was at first thought that these methods would not be applicable. 

 However, it was decided to give the color method a trial. The liquor 

 in the digester near the end of the cook has a fight caramel color 

 which darkens rapidly on standing. 



To finish a series of cooks to the same degree of cooking it is neces- 

 sary to have some fixed color as a standard for comparison. Most 

 solutions of organic dyes change in color rapidly when exposed to 

 the light and no combination of mineral dyes could be made that 

 would match the color wanted close enough. The color desired 

 strongly resembles the color of coffee extract, and it was decided to 

 see if this could not be made permanent enough to run a series of 

 cooks. Two solutions, which had been clarified with the white of 

 an egg, were made, one light and one dark, and the shade desired 

 was matched up as closely as possible by mixing the two. Formal- 

 dehyde solution was then added to stop fermentation and no change 

 in color of the coffee extract was perceptible, even after it had been 

 in use for several months. The results obtained by using this stand- 

 ard were satisfactory from the first. All that is necessary to duplicate 

 a certain cook is to match the color of the liquor at time of finishing 

 with coffee extract to be used as a standard. This ought to find 

 application commercially, as the man in charge will not have to 

 depend on his memory when he wishes to duplicate a certain cook. 

 During the progress of the cook, samples of the liqour were with- 

 drawn from time to time in a test tube and the- color compared with 

 the standard. When the color matched, the steam to the coil was 

 turned off, the pressure in the digester relieved to 70 pounds in five 

 minutes, and the cook blown into the blow pit. 



