CLAY WORKING INDUSTRIES. 185 



travelled through covered radial flues from the circumference to the 

 central well hole. By this forcing of the draft to the circumference, 

 only, is it possible to get the outside portions as hard burnt as the center, 

 for the circumference is constantly cooled off by radiation and cold air 

 leaking into the kiln and perhaps water, etc., while the center is sur- 

 rounded by heat on all sides and is bound to become equally hot with the 

 outsides by conduction alone if it is furnished no draft at all. 



If these two principles are adopted in reference to the arrangement 

 of the bottom, and good economical fireplaces are used and a single stack 

 of sufficient size with proper dampers for controlling it are provided, 

 the round down draft kiln cannot fail to be a good and satisfactory 

 means of producing vitrified clay ware. There is another point of 

 importance, which directly influences the economy of fuel and time, etc., 

 but it is subsidiary to these essentials without which the highest grade 

 of results are impossible. This point, which has great influence in 

 assisting the draft to bring the vitrifying temperature down to the floor 

 of the kiln, is the relative height of the kiln floors and the fireplace 

 floors. The fireplace floor or gratebars ought to be placed 36 inches 

 lower than the kiln floor or as near this as the conformation of the kiln 

 yard and surroundings will admit. The reason is twofold; 1st. The 

 lower part of the fireholes and bags become choked with clinkers dur- 

 ing the progress of a burn and therefore cease to radiate heat to any 

 extent. If the floor of the kiln and the foot of the bag are on 

 the same level, the bricks around the foot of the bag will not be heated 

 by radiation and the draft will pass them by in a more direct course 

 to the escape points, and a constant percentage of soft material around 

 the bags and walls at the floor may be expected. If however the 

 floor be 18 or 24 or 36 inches up above, the bag wall is always hot at 

 that point and by its radiation powerfully assists in drawing the draft 

 away from the center and other favored spots to those which are natur- 

 ally cold. 



2d. The deeper the kiln bottom, with its network of flues is set 

 in the ground, the more trouble from dampness and water from 

 outside and the more time and more fuel are expended in getting 

 the heated currents of gases to penetrate it and the higher will the stack 

 have to be built, in order to obtain the necessary head to overcome this 

 natural reluctance to go down. 



By raising the floor, the bottom can be made very shallow, and not 

 only is the extra expense of a higher stack saved, but the draft of the kilns 

 begins earlier in the burn thus heating up faster and saving in this way 

 both time and fuel. 



In some situations, the establishing of a different level for the kiln 

 floor and the firing floor, is a decided inconvenience — but wherever it 

 can be obtained it is well worth all the inconvenience it costs, in the 

 increased and regular percentage of hard burnt wares. 



