YIELD OF CHARCOAL. 715 



produced it, so that charcoal, volume for volume, is more heating 

 than wood. Besides, the heat given out by charcoal is more 

 lasting than by wood, and it radiates heat more intensely. These 

 facts explain sufficiently the higher economic value of charcoal 

 than wood as a heating agent. 



(e) Summary. — A good charcoal may therefore be recognised by 

 the following properties : it must be thoroughly burned without 

 being brittle and show the woody texture distinctly; its fracture 

 should be couchoidal and lustrous, quite black and yet it may be 

 touched without blackening the hands ; it should have few cracks, 

 and give out a clear sound when struck. As regards the 

 inherent qualities of good charcoal, it should have a high 

 specilic gravity, burn slowly without flame or smoke and radiate 

 a strong, enduring heat. 



Bertier and Winkler state that the heating-power of charcoal 

 made from different woods does not vary much if equal weights 

 are used. As regards equal volumes, heavy charcoal gives out 

 more heat than light charcoal. The amount of ash contained in 

 charcoal is usually small, and according to Yiolette, varies between 

 0'6% and 3%, according as the wood is taken from young or old 

 parts of a tree, and is in fact the same as that of wood from 

 Avhich the charcoal has been made. 



2. Yield of Chain xd. 



In discussing the quantity of charcoal which a certain volume 

 of wood may be made to yield, the following points should be 

 considered : — the kind of wood, situation of kiln, state of weather, 

 process and duration of burning, and different methods of car- 

 bonisation. 



(a) Kind of wood. — All wood on being converted into charcoal 

 naturally shrinks. Dry wood shrinks less than green wood, and 

 consequently gives a larger return. Large pieces of wood also 

 yield in volume more charcoal than small pieces, as more of the 

 former can be piled in a kiln. 



Klein gives 21"6% charcoal for coniferous wood, and '25'4% for 

 broad-leaved wood as the shrinkage in girth after carbonisation. 

 V. Berg found a shrinkage of only 12% in length for billets 

 2 meters long. 



