THE PAKABOLOIDAL OVEE-GROUND KILN. 



203 



freely. Tlie soil should then be allowed to settle for two or three 

 months until it becomes close enough. If it is damp, it should, 

 just before it is used, be warmed up and dried .by burning over it 

 a thick layer of dry twigs and leaves. 



An old site is preferable to one that is perfectly new ; in the for- 

 mer the soil has already undergone the necessary preliminary pre- 

 paration, and it is a matter of experience that in a fresh-made site 

 the yield of charcoal is from 10 to 17, and sometimes even 25 per 

 cent, smaller. But of course a site on which a kiln has just been 

 burnt cannot be used again until the moisture that it has absorbed 

 from the kiln has completely dried up. 



Even before using an old site, the surface must be' carefully 

 re-dressed and the numerous pieces of charcoal, left in it from the 

 previous burning, broken up small and mixed up intimately with 

 the soil. 



3. — Building up of the Mln. 



In building up a kiln, the pieces of wood may (a) be all laid 

 horizontally, or [h) horizontally only in the topmost tier with the 

 rest set up more or less vertically. In the first case the system of 

 piling up the wood is the same as that followed in constructing the 

 paraboloidal pit-kiln, in the Article on which it will be found de- 

 scribed. For this reason, and also because a kiln so formed is much 

 more difficult to build and is more liable to unequal subsidence and 

 breakages (these drawbacks increasing with its size), nothing fur- 

 ther will be said regarding them in this Article. 



First of all, the chimney cr flue through which the kiln is fired 

 has to be formed. For this purpose three straight upright posts, 

 of the same height as the future kiln, should be firmly fixed in the 

 centre of the site, about a foot apart from each other, and bound 

 round with wattling or strands of twisted grass. As the kiln rises, 

 readily ignitible chips of wood or half-burnt fragments obtained 

 from a previous burning are loosely thrown into the flue until it is 

 nearly full. According as the wood in the chimney is to be fired 

 from above or from below, the largest fragments are placed at the 

 bottom and the smallest and most combustible at the top, and vice 

 versa. In either contingency, if the soil is damp, a small board 

 must be placed over the ground under the chips, to prevent the fire 

 from being smothered by the steam rising up from the ground. 



The next step is to pile up the wood to be carbonized. To ensure 

 a circular section to the kiln, the base should be accurately pegged 

 out. The wood is arranged in three or more tiers, according to the 



