210 CADBONIZING IN ORDINARY KILNS. 



6. — The process of carbonization. 



Whether the kiln is lit from below or from above, the whole of 

 the -wood in the chimney must be on fire before the burning is 

 allowed to extend into the wood beyond. 



Assuming that the wood in the chimney is fully ablaze first, the 

 fire spreads thence outwards in the form of an inverted cone with 

 an ever-widenmg base, until the whole of the kiln is on fire. This 

 mode of progression of the fire is explained by the principle of the 

 parallelogram of forces. The heated air and other gases tend to 

 rise vertically, while lateral contact of the wood to be carbonized 

 Creates a tendency for the fire to extend horizontally. The result- 

 ant of these two tendencies is at first an oblique line not far re- 

 moved from the vertical, and since the height to which the fire can 

 extend is limited, and the temperature inside the kiln is constantly 

 rising, the horizontal spread of the fire becomes more and more 

 conspicuous until the whole of the wood at the bottom is carboniz- 

 ed. Thus the carbonization proceeds progressively from the top 

 downwards. 



During the process of carbonization large quantities of various 

 gases are given off. The whole of these gases being unable to 

 leave the kiln, what remains behind condenses inside and trickles 

 down through the lower tiers of wood to the ground, where it is 

 absorbed or from which it flows away through the foot of the kiln. 



While any piece of wood is being carbonized, first of all steam, 

 the characteristic colour of which is a bluish-grey, issues forth. 

 This is followed by russet-coloured vapours, which would, if con- 

 densed, yield pyroligneous acid, tar, wood-spirit, &c. When the 

 carbonization is complete, if the burning is still continued, a clear 

 blue flame proceeds from the carbonized wood, proving that only 

 charcoal is left and is being burnt away. 



Some of the gases given out by the carbonizing wood form ex- 

 plosive mixtures with the oxygen of the air ; if they are not given 

 a free vent, explosions will take place inside the kiln, disarranging 

 the wood and causing the covering to burst. 



7. — Conduct of the carbonizing operations. 



If the formation and expansion of the fire-eone took place uni- 

 formly in every direction, all that would be required would be to 

 keep the covering sufiiciently pervious to air along the edge of the 

 expanding cone (that is to say, at the level at which carbonization 

 was going on) and to maintain it air-tight elsewhere, especially 



