CHARCOAL-BURNING. 323 



Spoke- shaving by hand of poles to free them from bark 

 causes a loss of about 7-8 per cent, while bark -paring by 

 machinery wastes about 15-16 per cent. 



Cellulose forms a raw material for several industries. Treated 

 with sulphuric acid it makes vegetable parchment, and with 

 nitric acid gun-cotton, which is again used in producing the 

 hard celluloid forming imitation ivory for combs, billiard-balls, 

 &c., when mixed with an equal weight of camphor. 



Charcoal-burning, formerly an important rural industry in 

 Britain before coal was used for iron -smelting, is now only 

 practised extensively in the forest of Dean and in some parts 

 of the Midlands, though elsewhere lop, top, and waste wood not 

 needed for fuel is made into charcoal for estate and smithy 

 purposes. Charcoal can be made from any kind of wood, but 

 the best quality for making gunpowder is yielded by the alder- 

 buckthorn and dogwood shrubs, and by Alder, though Birch 

 and Oak were also largely used for this purpose. Charcoal- 

 burning is the dry distillation of wood by carbonising it under 

 partial exclusion of air in pits, kilns, or stacks. This causes a 

 certain loss of carbon through partial combustion, owing to 

 oxygen being only partially excluded, because, after the watery 

 sap is evaporated at 212 Fahr., the decomposition of the woody 

 substance begins at about 300. 



The oldest British method of charcoal-burning was to dig pits 

 about 4 ft. deep in the ground, with sloping walls, then throw 

 in brushwood, set fire to it, and throw in wood till the pit was 

 full, then cover with turf and earth and allow it to carbonise 

 and cool down for a day or two before reopening and taking out 

 the charcoal. This wasteful process only gives about 30 bushels 

 of charcoal per cord or stack of 120 cubic ft. 



The improved British method consists in burning in dome- 

 shaped kilns built up with billets of wood from 15 in. to 24 in. 

 long, so as to form stacks of 12-15 ft. in diameter on level 

 hearths. A large pointed billet split crossways at the top end 



