DISTILLATION OF STUMPWOOD. 43 



show the same tendency to follow the weight a cord and field classi- 

 fication of the wood. This tendency is shown also by the yield 

 of heavy crude oil and of pitch. The acid liquor and charcoal, how- 

 ever, are not subject to any such general deductions, although the 

 highest yields of acid liquor are generally given by the green woods, 

 followed by the richer stumpwood. In all probability this is due 

 to the fact that acetic acid is one of the decomposition products of 

 rosin. 



An experienced person can classify stumps in the field into several 

 grades from which the average yields of valuable products differ to 

 such an extent as to necessitate a proper selection of the material 

 before collection. 



COMMERCIAL DISTILLATION PROCESSES. 



There are four general processes for the recovery of products from 

 resinous wood. Two of these are destructive distillation processes 

 and two are nondestructive extraction processes. They are : (a) The 

 common or ordinary destructive distillation process; (b) the con- 

 trolled temperature destructive process ; (c) the steam distillation or 

 extraction process; and (d) the solvent extraction process. Of these 

 the ordinary destructive distillation process is the only one which 

 seems to be well adapted to the stump-disposal project in the North- 

 west. 



ORDINARY DISTILLATION PROCESS. 



The wood-distilling oven now in general use for the destructive 

 distillation of wood is an outgrowth of the old charcoal heap. By- 

 product charcoal kilns, round iron retorts, and rectangular iron or 

 concrete ovens are in use, the rectangular oven being preferred in 

 the best practice. Experience with these different forms has taught 

 that there is a mean temperature which gives the most satisfactory 

 yields. This temperature is necessarily more difficult to maintain 

 in direct-heated retorts, the smaller of which have the further dis- 

 advantage that the charcoal must be removed by hand, necessitating 

 a loss in time required for cooling as well as a fuel loss in reheating 

 the retort for the next charge. 



The uneconomical working of the round retort has led to the de- 

 velopment of the rectangular oven. Such ovens are of steel or con- 

 crete construction and are heated either directly by fires under them, 

 in the case of the steel ovens, or by means of internal-heating flues 

 in the concrete ovens. The second method is said to be better 

 adapted to softwood distillation. The height and width of the ovens 

 are uniform, being in general 8 feet 4 inches and 6 feet 3 inches, re- 

 spectively, and the length ranges from 26 to 54 feet or more, accord- 



