42 



FOREST PRODUCTS 



acid-proof brick and silicon cement, which prevents the acid from acting 

 on the metal and also keeps the heat in. The Ud or cover and valve parts, 

 especially the " blow-valve " at the bottom, are made of hardened 

 lead as the acid has little effect upon this. About three-quarters of the 

 way up the retort is a small pit cock from which liquor can be drawn from 

 the retort. It is by this method that the man in charge (cook) tells 

 when the wood is cooked. 



The steam, which furnishes the heat for the cook, is admitted at the 

 bottom and drawn off at the top of the retort through a vent. 



Photograph by U. S. Forest Service. 



Fig. 6. — Digester used to "cook" chips in the manufacture of sulphite pulp. 



In carrying out this operation, the blow valve or outlet valve at the 

 bottom of the retort is closed and the retort is filled up to the top with the 

 chipped wood. The wood chips are usually stored in large bins directly 

 above the retorts, to which they were conveyed from the screen in the 

 wood room. When the retort is full of chips, it is pumped almost full of 

 acid from the acid storage tanks. A space of about 6 ft. is left, from the 

 top of the acid to the top of the retort. This is to allow for boiling and 

 overflow. The lid is then put in place and securely bolted down. The 

 steam is then turned on and it is allowed to cook for about eight hours. 



