38 BULLETIN" 80, TJ. S. DEPARTMENT OP AGEICULTUPE. 



With increasing amounts of caustic soda in the digester charges 

 the cost due to all three factors is decreased until the point of maxi- 

 mum yield of good pulp is attained, after which the total costs 

 increase, due to the increasing amounts of wood and of soda ash 

 consumed. The decreasing cost of bleaching powder only partially 

 offsets the increase due to the other two factors. 



With increasing durations the effect is practically the same, so far 

 as wood alone is concerned, except that the increase in its cost for 

 higher durations is not so pronounced as with increasing the amounts 

 of caustic soda. The soda-ash costs alone are practically constant, 

 and hence increase the wood costs by a constant amount. However, 

 as the durations increase, the bleaching-powder costs decreased suffi- 

 ciently to overcome the effect of increasing wood costs. After the 

 minimum duration for successful cooking (as determined by yields) 

 has been exceeded, the decrease hi total cost is very small, and would 

 not be sufficient to offset increased costs incident to the time element 

 discussed previously. 



For variations in the pressures of cooking, the influence of bleaching- 

 powder costs is especially marked. The minimum costs due to wood 

 and soda ash result from the use of the lower pressures. When 

 bleaching is considered, the minimum cost is obtained by using 

 medium pressures, although the increases for the higher pressures are 

 very small. 



Combined costs for the three factors are practically unaffected by 

 variations in the initial concentrations, but if bleaching is omitted the 

 costs of wood and soda ash are larger with the higher concentrations. 



All of the diagrams show that of the three cost factors considered, 

 wood is of the most importance and that bleaching powder is more 

 influential than soda ash in determining total costs. Increases in 

 costs of wood and soda ash with increasing severity of cooking are, 

 in all cases, offset, to a greater or less extent, by decreases in bleaching- 

 powder costs. If maximum or minimum values 1 had been used for 

 either wood, soda ash, or bleaching powder, instead of the average 

 value, or if a different percentage recovery for the soda ash had been 

 assumed, the general effects would not be changed, although they 

 might become more or less pronounced. 



SUMMARY. 2 



(1) The amount of caustic soda per pound of wood, the duration 

 of cooking, the pressure or temperature of cooking, and the concen- 

 tration of the cooking chemicals employed in the production of soda 



1 See footnote, p. 37. 



2 The more general statements in the summary will be found to coincide in a greater or less degree with 

 previously existing opinions, a fact not surprising when it is remembered that the soda process has been 

 carried on for half a century. On the other hand, satisfactory evidence and data substantiating these 

 opinions have not been available. The present investigation affords such information, as well as a basis 

 for more specific conclusions. 



