CARBONIZATION IN OPEN PITS — YIELD OF OHAHCOAL. 219 



obtained from the latter class of kilns when these are built up 

 with horizontally-laid wood. 



Section III. — Caebonization in Open Pits. 



The pit is from 3 to 5 feet deep and 5 feet and upwards in 

 diameter, the sides being made sufSciently sloping to support 

 themselves. First of all, it is filled up with dry twigs and branch- 

 lets, which are fired and allowed to burn down freely. When the" 

 burning has progressed so far that the wood inside is all aglow 

 and has ceased to give out any smoke, the first instalment of the 

 wood to be carbonized is thrown in. This wood is allowed to 

 burn on until, in its turn, it no longer emits any smoke, when a 

 second instalment is thrown in. This process is repeated until the 

 pit is full of glowing coal. The entire glowing mass is then 

 covered up with a layer of moist earth, thick enough to exclude 

 air. After two or three days the pit will be sufficiently cool to be 

 opened and the charcoal taken out. 



This is an extremely wasteful way of making charcoal, but as it 

 requires no skill at all and next to no supervision, it may be 

 adopted where there is plenty of waste wood that has no other use 

 and the demand for charcoal is relatively small. 



Section IV. — Yield of Chaecoal. 



The yield will depend on various circumstances, the principal of 

 which are — 



1. The nature of the wood used. — Dry wood yields more char- 

 coal than moist wood ; resinous and oily woods more than other 

 kinds (since both the resin and the oil contribute a large propor- 

 tion of the heat necessary for the carbonization) ; and soft woods 

 more than hard woods (since the volatile products of distillation 

 are more easily expelled from the looser tissues of the latter). 

 Branch-wood, as it contains more reserve materials and less 

 ligneous matter, yields less charcoal than the wood of the stem. 



2. The nature of the site. — On a site that is uniform throughout 

 and is well-sheltered, and is also one that has been frequently used 

 before, so that its peculiarities are thoroughly known, tlie yield will 

 obviously be largest. 



3. The state of the weather. — Still weather is much more 

 favourable than windy weather, especially if the wind constantly 

 shifts or blows in gusts. Very dry weather is just as unfavourable 

 as steady rainy weather. In dry weather the covering breaks 



