180 GEOLOGY OF OHIO. 



combustion is entirely within the burner's control, for by admitting 

 air in front of the fire the draft in the fire is diminished. 4th. The radia- 

 tion is at a minimum. 5th. The comsumption of coal is at a minimum. 

 6th. The quality of the coal used may be very inferior with no further 

 trouble than increased frequency of clinkering, for the fire acts like a gas 

 producer and will consume anj^ kind of inflammable material with a pro- 

 duction of hot flame. 



The two last fireplaces are the best in use and are easily adapted to 

 any form of kiln. Between the two there is little to choose in point of 

 efficiency; the latter, especially, when provided with the movable front 

 wall is the easiest to clean and manipulate. Comparing these two and 

 any of the other types, there is a distinct economy in coal, in their favor. 

 The saving is slight in the case of the second and third forms described, 

 and considerable in the case of the first. 



The second condition in the arrangement of a successful kiln is the 

 use of means to produce a strong draft and yet one wholly under control. 



In burning material in down draft kilns, the course of the heated 

 gases is directly contrary to nature. They must go downward, through 

 the wares, before they can escape up through the stack. The advantage 

 of the use of this downdraft is in the greater regularity of the distribution 

 of the heat. In updraft kilns, the gases establish certain channels 

 which offer the easiest passage upwards. The burning is thus apt and, in 

 fact, is nearly sure to be irregular or uneven. In the downdraft kiln, the 

 heat progresses downward inch by inch as the draft maintains a constant 

 flow from above to below, and this distribution is nearly even in every 

 part of the kiln. 



To produce this draft, a stack has to be provided which will have 

 sufficient "head" or power to cause this temporary defection of the heated 

 gases from their upward passage. 



The best form of stack for producing this draft is a matter which 

 has furnished ground for much discussion and disagreement among clay 

 workers. There are several plans in use : 



1. Single stacks — or one separate stack to each separate kiln. 



2. Compound stacks — or stacks which have two or more kilns depen- 

 dent on them. The stack may be divided at the bottom into compart- 

 ments looking upwards so as to prevent the different currents of gases 

 from baffling each other, but the stack-pipe is one chamber for the greatest 

 part of its height above ground. 



3. Multiple stacks — or the use of two or more small stacks, all 

 working on one kiln space. 



Single stacks for a round kiln are preferable to any other arrangement, 

 on account of the following reasons: 



1st. The draft on the kiln in question, being always actuated by one 

 and the same stack is always approximately uniform ; or rather it varies 

 through approximately the same cycle, each burn ; the only sources of 



