220 YIELD OF CHARCOAL. 



open very frequently and requires to be constantly moistened ; on 

 the other hand, in rainy weather the covering remains so moist 

 that the steam and other vapours, which form^ in the interior, do 

 not find sufficient vent, and carbonization is consequently retarded, 

 and, if the rain is heavy enough, the covering may be washed away. 

 When carbonization in wet weather cannot be avoided, a thatch 

 roof should be put over the kiln. 



4. Proper control of the carbonization. — The yield is largest 

 • when the progress of the fire is uniform in every direction. In 



the contrary case, those portions which, having become carbonized 

 earliest, are kept on burning until the carbonizalion of the rest is 

 complete, lose a more or less considerable portion of their carbon. 

 Gradual burning, and especially slow burning at the commence- 

 ment, yields not only a larger outturn, but also heavier charcoal. 

 The number of times the covering breaks open or has to be opened 

 to fill up hollows, and slowness or awkwardness in restoring it or 

 in filling up the hollows, result in a very appreciable loss of carbon. 

 To avoid such loss, the wood should, in the first instance, be 

 packed as carefully and as closely as possible, and once the kiln is 

 in full combustion, the chimney and, if the kiln is lighted from 

 below, also the tunnel along the ground should be filled up tight. 



5. Titne occupied in carhonization. — It has already been said 

 that rapid burning results in unnecessary loss of carbon and that 

 moderately slow burning gives the largest yield. The length of 

 time occupied in carbonization will vary between certain wide 

 limits depending on the style and dimensions of the kiln, the size 

 of the pieces of wood, the moisture they contain, the quality of the 

 site, the nature of the weather, and the care with which the kiln 

 has been built up. Small kilns, containing from 500 to 1,000 

 cubic feet of moderately hard, fairly well-seasoned wood, will re- 

 quire from 6vto 8 days. Large kilns, containing from 3,000 to 

 8,000 cubic feet of similar wood, will require 4 weeks in favour- 

 able, and from 5 to 6 weeks in unfavourable weather. Green wood 

 will, in every case, take half as much time again as dry wood. 



6. The method of carbonization adopted. — In the paraboloidal 

 over-ground kiln the yield is generally increased by firing from 

 below, as the wood in the chimney then takes fire more readily 

 and completely, and the fire cone progresses more regularly and 

 uniformly. When the chimney is lit from above, the fire does not 

 often run down to the bottom quick enough to enable it to be re- 

 filled with wood to be carbonized. The result is that, after a little 

 time, the small wood in the chimney is consumed to ashes, a 

 hollow is formed, and the kiln falls in at the top. When the wood 



