714 WOOD-CAllBONLSATIUN. 



of moisture it may contain as well as difierent rates of burnings, 

 it is evident that there must be a considerable range in the 

 specific gravity of charcoal. 



Klein states that the specific gravity of charcoal ranges from 

 0*14 to 0*20, also that green wood loses 70% to 75% of its weight 

 during carbonisation, so that the weight of charcoal is from 

 25% to 80% of that of the wood from which it has been made. 



(b) Appearance of charcoal. — Good charcoal should bo black, 

 with a steel-blue, metallic lustre and a conchoidal fracture. 

 "When too long burned charcoal is dark black, without lustre ; 

 if insufficiently carbonised, reddish (foxy), although v. Berg 

 states that occasionally, during dry weather and under other 

 circumstances, perfectly good charcoal may have a reddish colour. 

 "When dark black and dull, charcoal is soft, friable and has been 

 overburned. Good charcoal when struck gives a metallic sound, 

 which is clearly apparent on shaking a basket full of it, whilst 

 overburned eliareoal gives only a dull sound. 



(c) Absorption of moisture.— Charcoal is extremely absorptive 

 of all liquid and gaseous bodies, many economic uses being 

 founded on this property. From a forester's point of view this 

 property of charcoal is highly important if it be sold by weight, 

 as the absorption of water makes it heavier than before. It does 

 not appear to increase in weight by more than 8 — 12% by 

 absorbing moisture from the air, but by direct contact with water 

 its weight may in a few minutes increase 25 — 30%, according to 

 its greater or less porosity, and this may increase to 60 — 1'^0% 

 after 8 hours immersion in water. A large proportion of 

 absorbed water subsequently passes off as vapour. 



(d) Heating-power. — A good charcoal should burn without 

 flame or smoke, and give out a more or less prolonged, intense 

 heat. When it has been under-carbonised, charcoal burns with 

 a flame, whilst overburned charcoal is reduced to ashes more 

 quickly than good, heavy charcoal rich in carbon. 



It is clear that a cubic foot of wood gives out more heat than 

 the charcoal which may be made of it, as much carbon and all 

 its hydrogen are abstracted in the by-products during carbonisa- 

 tion. This loss is about 40%, or the heating-power of the wood 

 is to that of the charcoal made from it, as 100 : 55 or GO. But 

 the volume of the charcoal is hurdlv half that of the wood which 



