tIELD OF CHARCOAL. 221 



is laid horizontally, it does not matter -whether the chimney is fired 

 from above or below, as the position of the pieces prevents them 

 from falling in. 



The yield is largest if the carbonization is effected with special 

 apparatus, and least in an open pit. 



7. Skill and zeal of charcoal-burners. — This is self-evident. 



General. — We may now enter into a few general considerations. 

 Assuming that we have used fairly well-seasoned, non-resinous, 

 and non-oily wood, we would still have roughly 20 per cent, of 

 moisture, 5(Xper cent, of the balance being carbon ; so that if no 

 carbon were lost in carbonization, the yield (a purely hypothetical 

 one) would be 40 per cent. But actually, according to Boppe, 

 the following losses occur : — 



1. To raise the kiln to red heat, ... 1 per cent.' 



2. To expel the moisture, ... ... 5| „ 



3. By loss of heat radiated, ... ... 1-2 „ 



4. Carbon carried off in combination in the 



various products of distillation, ... 11 ,, 



Total loss, ... 18I-19A „ 



Thus the highest theoretical yield in carbon can never exceed 

 about 21 per cent, of the weight of the. wood. Adding up for 

 mineral matter and the small quantity of oxygen and hydrogen 

 contained in charcoal, the highest yield in charcoal we may expect 

 is 23 per cent. This figure is completely justified by facts, for 

 resinous or highly oily woods burnt in open kilns yield 25 per 

 cent, by weight of charcoal, and other species only from 20 to 23 

 per cent. 



The total shrinkage in volume may be put down at from 55 to 

 60 per cent, for resinous and oily woods and from 40 to 50 per 

 cent, for other kinds. 



The shrinkage in girth varies between 16 and 25 per cent., that 

 in length amounting to only about 12 per cent. Hence the kiln 

 will sink most when the wood is laid horizontally. The sinking 

 will always be in excess of the figures given above, as a good deal 

 of the charcoal breaks up, the broken pieces sliding in between 

 those lower down. 



Weighing the charcoal soon after it has been taken out of the 

 kiln or oven always gives the outturn more accurately than mea- 

 suring it, as the density of the charcoal will be difPerent according 

 to the wood used and the quality of the charcoal. Nevertheless, as 

 the weighing of a light bulky article is always a slow and tedious 



