212 CARBONIZING IN OKDINARY KILNS. 



to take, which may save the necessity of a screen, is to select a 

 sheltered site with a close fringe of trees standing to windward. 



II.— Increasing the deaught. — Whenever unequal subsidence 

 takes place, there is proof positive that in the higher portions car- 

 bonization has been going on more slowly than elsewhere. If the 

 sinking at the lower points is not too rapid, then it is evident that 

 the burning in the higher portions requires to be accelerated, in 

 other words, that more air must be admitted inside them. This is 

 done by making new vent-holes there or enlarging existing ones. 

 The size of the holes and the intervals between them will depend 

 on the degree of acceleration required. The new holes made need 

 not be all at one and the same level. 



Sometimes, it may happen that the rate of carbonization is 

 everywhere too slow. The remedy for this is to make a line of 

 vent-holes all round the kiln immediately below the level at which 

 carbonization is actually going on. The object of these vent holes 

 is not only to increase the inflow of atmospheric oxygen, but 

 also to give free exit to the vapours given out during carboniza- 

 tion, the rapidity of which is impeded by them. The size of the 

 holes and the intervals between them wiU depend on the amount 

 of moisture in the wood and the slowness or rapidity with which 

 the particular wood burns. If a screen has not been erected or 

 the wind is constantly changing, no holes should be made on the 

 windward side, and as the wind shifts about, some of the holes 

 must be closed and new ones opened or enlarged. Every hole 

 must be closed as soon as a clear blue flame or bluish transparent 

 vapours issue forth. 



The vent-holes are thus made to fulfil the double object (i) of 

 securing equal combustion on every side, and (ii) of conducting 

 the carbonization with the requisite speed. The necessity of vent- 

 holes increases with the size of the kiln, and, under favourable 

 circumstances, a small kiln situated in a sheltered spot may hardly 

 require any at all. 



III. — Diminishing the draught. — This is the opposite of the 

 preceding operation,' and consists in increasing the thickness of the 

 outer covering wherever the kiln, by sinking too rapidly, affords a 

 certain indication of over-rapid combustion. Over-rapid combustion 

 can be detected even before sinking actually takes place : at such 

 points an excessive quantity of dense smoke issues continuously. 



As the carbonization progresses from the top downwards, the 

 outer covering over the portions, where the process has just been 

 completed , should be strengthened until no more smoke finds its 

 way through it ; and wherever, from any portion that has been 



