204 FOREST PRODUCTS 



In the modern oven retort operation the process requires from twenty- 

 three to twenty-six hours for completion. When the wood is rolled in 

 trucks into the ovens, the doors are hermetically sealed and the fires are 

 started underneath. In from one to two hours the wood is sufficiently 

 heated up so that water distillation takes place. This distillate contains 

 about 2 per cent acid. Then the green gas comes free for about five to 

 six hours. 



It is considered desirable to heat up the wood gradually and also to 

 let it cool off gradually at the end of the process. The exothermic process, 

 that is, that part of the process in which the wood fibers break down 

 under the intense heat, does not take place until the temperature is run 

 up to about 300° F. In about six hours after closing the doors the tem- 

 perature attains an average of about 450° F. It is then maintained 

 between 450 and 600° F. Temperatures of over 600° F. are considered 

 undesirable. After about six hours of heating the pyroHgneous acid 

 begins to flow, and the best average is maintained up to about the 

 eighteenth hour. An operator can determine from the color of the pyro- 

 ligneous acid whether there is too much heat maintained, and if the wood 

 fibers have broken down sufficiently. At the end of the heating proc- 

 ess, the distillate forms tar to a large extent. After the eighteenth 

 hour the latent heat in the oven settings is sufficient to complete the 

 process to the end, but the heat is gradually decreased until the charcoal 

 is withdrawn. 



As the gases and vapors pass out through the nozzle of the oven, they 

 are condensed into a yellowish green, ill-smelling liquor called pyroHg- 

 neous acid. A copper run takes this condensate to the raw liquor 

 " sump," a tank in the ground and so placed that the liquor will run into 

 it by gravity. Meanwhile, the " fixed " or non-condensible gas is 

 trapped and taken off at the outlet of the condenser and used for fuel 

 underneath the boilers or ovens or perhaps both. A simple gooseneck 

 is used to trap off the gas. 



The pyroligneous acid is next pumped from the " sump " in the 

 ground to a series of wooden settling tubs, of which there should be at 

 least five in number. These tubs are usually from 5 to 8 ft. in diameter 

 and 6 to 8 ft. in height. The purpose of these tubs is to settle the tar 

 and heavy oils. The heavy tar is taken to a wood tar still equipped with 

 a copper condenser. This tar still is of wooden construction because the 

 tar would "eat up" the copper in about a year. The residue remaining 

 in the tar still is utilized together with residue from primary stills as boiler 

 fuel. 



