July, 1908.] 



87 



Timbers. 



(rj) The period of burning is compara- 

 tively very short, being only two or three 

 days. 



(7) Control is very easy as the coolies 

 have only to fill up all newly-formed 

 cracks and crevices and nothing else; 

 the chances of anything going wrong 

 are very limited. 



(8) The charcoal is very good in qual- 

 ity, being heavy and metallic and never 

 crumbling to pieces. When the kiln is 

 opened, the fuel billets all retain their 

 full size, even the bark on them being 

 converted into charcoal. 



(9) The unburnt pieces are very few, 

 being chiefly witbin one foot of the 

 ground and confined only to the bigger 

 pieces. These can be subsequently used 

 in another kiln. 



Cost of Working. 



The fuel is felled, billetted and stacked 

 on the spot. Coolies are paid from two 

 to three annas per 56 lb. or Rs. 5 to 

 Rs. Ih per ton according to the den- 

 sity of the forest and the availability 

 of loose earth for covering the kiln. The 

 charcoal sells at an average rate of 

 Rs. 20 per ton, which gives a nett profit 

 of Rs. 7 to Rs. 8 per ton. 



The Improvements to be Aimed at in 

 Making a Charcoal Kiln. 



We know that coal is nothing but 

 vegetable matter deposited under the 

 earth and subjected to the physical con- 

 ditions of heat and pressure. The inte- 

 rior of the earth is naturally very hot, 

 and the weight of the earth above the 

 buried vegetable matter tends to increase 

 the heat. There is in addition little 

 or no air under the earth, especially at 

 great depths. The heat acting on the 

 wet vegetable matter drives all the 

 moisture away in the form of steam, and 

 when the temperature becomes very 

 high chemical changes also set in result- 

 ing in the preparation of marsh gas 

 (CH 4) and other hydrocarbons. In this 

 way vegetable matter becomes converted 

 into coal. From this we can understand 

 that the essential condition required for 

 obtaining good charcoal is heat, and, 

 if possible, pressure and not oxygen. 

 Therefore any improvement in the kiln 

 must always tend to the exclusiou of un- 

 necessary air and maintenance of a high 

 temperature sufficient to draw out the 

 last molecule of moisture, in the shape 

 of steam and leave the carbon behind. 

 For instance, a piece of wood placed in a 



vacuum tube and heated with elctricity 

 will be converted into charcoal without 

 loss of carbon. 



Therefore my modification of the kiln 

 consists only in excluding the air as 

 much as possible and maintaining a high 

 temperature only sufficient to carbonise 

 the wood. 1 do not mean my kiln is 

 perfect, but it seems to be an improve- 

 ment. Leaving no holes above and only 

 a foot opening underneath, gives a. rush 

 of air below, but the thorough covering 

 outside checks the draft and only allows 

 air upwards in a very limited quantity. 

 By doing so, there is very little con- 

 flagration, but the heat is maintained in 

 the following manner:— 



What Takes Place when this 

 Kiln Burns. 



(1) Steaming; (2) Heat moderation or 

 checking of conflagration. 



Steaming. 

 As soon as the kiln is set on jire by 

 lighting the centre, the burning pro- 

 ceeds and the adjoining green pieces of 

 wood first part with their water in the 

 form of steam which is further treated 

 by the flame beneath. This heated 

 steam pervades the whole kiln through- 

 out the operation, escaping only in small 

 quantities through the topmost covering. 

 The heated steam parts with its heat to 

 the green wood in the outer ring making 

 up the loss from the flame beneath. 

 Therefore the wh ole kiln is first subject- 

 ed to a steaming process, which takes 

 away the main portion of the mois- 

 ture. 



Heat Moderation. 



(2) The heated steam thus pervading 

 the kiln, in evaporating the moisture 

 from the green wood, loses its latent 

 heat and recoups it from the flame 

 beneath. As this latent heat is an 

 enormous quantity (537 calories) the 

 flame parting with this heat to the 

 steam is not able to burn away rapidly 

 the dried ring of wood which has given 

 off its moisture. However, the burning 

 continues slowly up, and this process 

 continues to the last. Thus, there' rt s an 

 automatic adjustment of burning which 

 makes the charcoal superior in quality. 

 By this process there is less loss of heat, 

 the greater part of it being used in 

 charring the wood. This is the result of 

 converting the wood while green, and of 

 keeping the outer cover as much as 

 possible airtight.— Indian Forester, Vol, 

 XXXIV, April, 1908, No. 4. 



