THE SCIENCE OF CLAY- WORKING. 46? 



weaker than the rest in order to save a costly break down. Modern 

 machine engineers have found a process much neater in most respects and 

 one not requiring anything to break, in case of an overstrain, viz. a fric- 

 tion clutch or its equivalent. The theory of its action is simply that the 

 parts of the clutch will slip over each other whenever the dangerous limit of 

 strength is approached. I am sorry to say that the friction clutch is not 

 always a safe or reliable fixture; it will often hang when it should let go and 

 will sometimes let go when it should stick. I am not sure that it would be 

 worth anything in the mud and dust that surround the brick machine. 



THE TILE KILN". 



While I have had some experience in burning with certain forms of kilns, 

 I do not feel confidence to advise in regard to detailed methods of burning. 

 I have, however, some well fixed opinions regarding the various classes of 

 kilns and of the economy attending their operation. I entertain the fixed 

 opinion that method can not be described in detail by any one, except 

 for a certain clay in a certain condition. An experienced burner can no doubt 

 handle with success new clays, simply because he can judge by their physical 

 condition about what course to pursue, and then by watching closely he 

 can govern his mode of procedure with the development of the burning 

 process. 



The following essential points must be given consideration: 

 In setting in the kiln, spaces should be left so as to allow an equal draft on 

 all sides of each piece of ware, and so far as possible the pieces should be 

 placed so as to receive an equal amount of heat. The setting should be such 

 that any contraction or expansion due to the burning shall not settle parts of 

 the kiln in such a way as to clog the draft. In setting also no undue weight 

 should be put in any place on the green ware. 



THE PROCESS OF BURNING. 



This process in reality consists of two very different operations; the first is 

 drying, or what is commonly called " watersmoking," in which the ordi- 

 nary moisture is driven off. 



This process must be conducted with much care for this reason : If we exam- 

 ine the condition of the green ware we shall find that even when it is appar- 

 ently dry, it still contains a great deal of water ; yet there is a great difference 

 in this respect as to how the ware is dried. If the ware is dried by steam 

 heat the moisture may be very small in quantity. If we suddenly heat this 

 ware the water is converted into steam; now steam occupies 1,700 times more 

 volume than water, the result is a minute explosion which tends to disinte- 

 grate the particles of the brick or tile, and besides sends a huge volume of 

 steam with much velocity on the parts of the kiln not yet heated. This steam 

 condenses on and tends to disintegrate all the ware in its path. To drive this 

 water from the kiln requires a heat throughout of more than 212° F., and it 

 is an operation which cannot be hurried, except at the expense of quality 

 in the ware. In my opinion it is economy to do this drying largely before 

 the ware is put in the kiln. 



The second operation, or the burning proper, consists in driving off the 

 hygroscopic water, or water chemically combined with the clay. This 



