THE PARABOLOIDAL OVER-GROnND KILN. 213 



already carbonized, smoke is seen to come out, additional earth 

 should be thrown on until it ceases. 



The intense heat of the kiln bakes the moist covering of earth 

 into a hard brick-like mass in which numerous cracks open out. 

 It is impossible to close such cracks, and the only way to render 

 the covering effective again is to quickly pull off the loose pieces 

 of baked earth and replace them with fresh material. Some of the 

 pieces may even be broken up fine on the kiln with a mallet. Not 

 unfrequently it is impossible to stop smoke without moistening the 

 outer covering ; but the necessity for such action occurs only to- 

 wards the end of the entire carbonization, when there is not suffi- 

 cient humidity left inside to keep the covering moist with the 

 vapours given out. 



IV. — Filling up hollows. — However carefully the wood is 

 packed or the regulation of the draught attended to, it is impossi- 

 ble to entirely prevent the formation of hollows owing to the wood 

 at certain points burning so fast as to become partially or wholly 

 consumed. If such hollows are not at once filled up, the further 

 settling of the wood will cause the kiln to fall in at those points. 

 Hollow places may be detected by beating the sides of the kiln 

 with a club ; where there is a hollow, the cover will yield or even 

 fall in, or return the tell-tale sound. 



To fill up a hollow the covering over it should be quickly torn open 

 with a hoe and short billets of wood thrust in one after another as 

 tight as possible, the covering being restored without delay. In 

 doing this work the utmost dispatch should be used, and hence a 

 quantity of filling and covering material, sufficient for all contin- 

 gencies, should always be kept ready at hand. When the covering 

 is cut open, a good deal of flame will issue through the opening. 



Geneeal. — In order to do the needful at the right moment, the 

 kiln should he constantly watched and tended by a number of men 

 sufficient for all contingencies. For a single kiln containing up to 

 800 maunds of wood, or even for two kilns of that size, two men 

 will suffice. During daylight the watching of the kiln and conduc- 

 tion of the burning offers no special difficulties ; but during the 

 darkness of the night accidents are especially to be feared, parti- 

 cularly if high winds blow. On this account, every evening, before 

 nightfall, all hollows should be examined and filled up and all 

 weak places thoroughly overhauled and strengthened. If the 

 night is expected to be stormy, additional covering should be put 

 on everywhere, and only just enough vent holes left open to prevent 

 the fire from going out. Lastly, at short intervals all through the 

 night, the kiln should be carefully examined on every side. Bun- 



