182 GEOLOGY OF OHIO. 



complicated, and control is more difficult in proportion to the number of 

 vents and number of currents to regulate. 



Whatever the method of production of draft and there is certainly 

 room for honest difference of opinion as to what is best, especially in the 

 two main types of down draft kilns, the functions which that draft has to 

 perform are always alike. It must carry the hot products of combustion 

 down through the kiln ; it is the vehicle which transfers the heat from the 

 fireplace to the wares. 



The amount of the draft, or rather the rapidity of the flow of the 

 gases is a matter which each burner must regulate for himself. If the 

 flow is too rapid, the heat will not have time to soak or penetrate the sub- 

 stance of the clay wares, but it is whisked down through them and out at 

 such a speed that only a surface heat is imparted to the wares. Every crack 

 and opening is supplying outside air to nullify the effects of the combus- 

 tion of the fuel in the fireplace, and while the consumption of fuel will 

 be enormous, the progress made in the burning will be very slow. 



The other extreme, that of having too little draft is nearly as bad; the 

 consumption of fuel is also heavy in this case for the burning process is 

 prolonged day after day in a vain attempt to draw the heat downward to 

 the lower portions of the kiln. In this case the upper portions become 

 overburnt and cindered and the lower portions are still soft and unfinished. 

 No skill in manipulation can help a kiln out of trouble from this cause. 

 It is a sine qua non that there must be draft enough; too much is easily 

 regulated by a damper, but too little cannot be remedied in any such 

 way. 



The proper amount cannot be laid down in any fixed rule. Every 

 kiln and every clay has its own character. The best way of determining 

 the correctness of the draft pressure is by a study of the results of the 

 kiln. If the top is always overburnt and the bottom soft, an increase is 

 needed. If the bricks are hard on the surface and soft in the center, de- 

 crease the draft. When it is right, the vitrifying temperature ought to be 

 produced first on top and then downwards, course by course, until the 

 bottom of the kiln is reached. A complete burn in any good round kiln, 

 set with dry bricks of moderate heat-resisting power, ought not to occupy 

 more than seven days, in burning clean to the bottom course, and allowing 

 three da3 r s for the preliminary heating up and raising of fires, and four 

 days to the finish. If it takes more than this, the draft is in need of regu- 

 lation, either more or less is required, except in clays of more than the 

 usual difficulty of vitrification, which demand from one to two more days 

 to attain their best properties. 



3rd. The next essential condition in the burning of vitrified mater- 

 ials, is the uniformity of distribution of the heat over the area of the kiln. 

 The control of this distribution is obtained by the construction of the 

 hollow bottom of the kiln; in all down draft kilns, the floor, or level in 

 which the bricks are set, is merely a false structure designed to hold the 



