PRODUCING SODA PULP FROM ASPEN". 31 



The milder treatments are apparent in the increasing number of 

 ray cells and vessels, the latter being well preserved and showing 

 their markings quite clearly. The fibers are twisted or broken to only 

 a small extent, and yet are so well separated that there are no fiber 

 bundles. 



Undercooked pulps. — Plates VI and VII illustrate the character- 

 istics of undercooked pulps, and show plainly the mildness of the 

 digestion treatments employed in their production. Well-preserved 

 vessels with sharply defined markings are clearly visible, ray cells 

 are numerous, and the walls of the fibers are less dissolved away 

 than in the more thoroughly cooked pulps. Coincident with these 

 characteristics there are also present many fiber bundles or shives> 

 noticeable even when the microscopic slides are examined with the 

 naked eye. Undercooked fibers develop a deep color from the particu- 

 lar stain used in mounting, and on this account appear very distinct. 



Of the several groups of tests, the one varying the amounts of 

 caustic soda per pound of wood resulted in the greatest range of 

 severity of cooking as determined by the microscopic appearance 

 of the pulp fibers. A small amount of chemical resulted in an under- 

 cooked pulp. With increases in the amount the strength of cell 

 walls gradually decreased, the wood vessels suffered gradual destruc- 

 tion, and their markings were dimmed. The ray cells and fiber 

 bundles disappeared soon after the point was reached where the 

 maximum yield was attained. The higher amounts of caustic gave 

 the overcooked effects. 



For varying durations of cooking the effect was practically the 

 same, and undercooked pulps were obtained at the shortest duration 

 used. However, the highest durations employed did not give as 

 severely cooked pulps as were obtained with large amounts of chem- 

 ical. While all of the tests varying the cooking pressures resulted in 

 fairly well cooked pulps, there was a tendency toward undercooking 

 at the lowest pressure used. The tests varying the initial concentra- 

 tions also resulted in well-cooked pulps, except for the highest con- 

 centration, where a slight overcooking effect was observed. 



INFLUENCE OF COOKING CONDITIONS ON COST. 



While it is not feasible from the data at hand to discuss all cost 

 factors affecting the commercial production of pulps, the more direct 

 effects of the cooking conditions employed can be shown. The actual 

 effects on the cost of production, of course, depend upon various 

 other operating conditions at any particular mill, but the general 

 trend of the effects is the same, irrespective of local conditions. 



TIME. 



Shorter durations of cooking result in more efficient use of the 

 digesting apparatus; more cooks can be made per day or per week, 



