PROPERTIES OF GOOD CHARCOAL. 71M 



continued by opening successive vent-holes in the roof (in Steier- 

 mark also in the sides of the kiln), as in the ordinary method. 



Carbonisation proceeds obliquely backwards, the fire being always 

 more advanced towards the roof than at the base of the kiln. Thus 

 the base of the back of the kiln is the last to be carbonised, and 

 the process is completed as soon as flames emerge from vent- 

 holes there. The charcoal is cooled by removing part of the roof 

 iind putting earth on it, the walls not being opened. 



(vi) The charcoal is first removed from the front of the kiln. 

 A portion is removed daily, and the kiln closed again. 



In Steiermark a commencement is made by removing the 

 charcoal whilst the back of the kiln is still burning. As the 

 front part of the kiln burns longest, and the charcoal there 

 hecomes light, attempts are made to prevent this by its early 

 removal. It should, however, be remembered that this frequent 

 opening increases the draught, and must cause a considerable 

 loss of charcoal. 



Section III. — Properties of Good Chahcoal, xVND Yield 

 OF the Different kinds of Kilns. 



1. Properties of Good Chareoal. 



Charcoal is a dry, more or less lustrous, porous and fairly 

 hard substance, of low specific gravity, without taste or odour. 

 Difi'erent kinds of charcoal, however, show some variations in 

 these respects which modify their relative value. 



(a) Specific gravity. — The specific gravity of charcoal is 

 directly proportional to that of the wood from wdiich it is made. 

 Thus, heavy, broad-leaved woods yield heavier charcoal than 

 coniferous softwoods. The amount of moisture in the wood 

 also affects the specific gravity of the resulting charcoal, dry 

 wood yielding charcoal of a higher specific gravity than green 

 wood. The rate of burning is also influential, quickly burned 

 kilns yielding lighter charcoal than kilns slowly burned. This 

 results from the fact that with a quick fire more charcoal is 

 expended in producing tar, kc, than where the chief process is 

 merely carbonisation. Considering the wide range of specific 

 gravity for wood of one species of tree, and the variable amount 



