*50 GEOLOGY OF OHIO. 



depends on the dry pan. It is not possible to secure an even supply of 

 clay for feeding the pug mill in this way. Any inattention, any variation 

 of the dryness or hardness of the stock, any choking of the screen, or 

 any one of a thousand conditions is always interfering, and the result is 

 a constant fluctuation, a feast or a famine, at the pug mill. Hence the 

 temperer has to face the difficult task of adding the proper amount of 

 water to a clay supply which does not remain constant two minutes at a 

 time and it is needless to say that no man can maintain uniformity under 

 such conditions. 



When a storage bin is situated above the pug mill and the clay from 

 the screens falls into this bin, then there is nothing to interfere with the 

 maintenance of a steady stream of clay except the constant difficulty, 

 which is inherent in the nature of the case, of getting any supply of 

 ground clay to feed freely or run down grade without choking and bank- 

 ing up. Ground clay is like snow; it will amalgamate on very small 

 excuse, and in order to get a mass of ground clay to "run" at any regular 

 rate it is necessary that not only every natural advantage should be given 

 it, but that the temperer should have some read} 7 means of dislodging and 

 stirring it up. 



The storage bin should be as tall as is feasible, should be larger on 

 the bottom than the top, the supply should strike in the center of the bin 

 and the outlet should leave the center of the bin at the bottom, and the 

 outlet should be smaller when it leaves the bin than at the bottom, in 

 order to prevent choking in the spout. Even with all these precautions 

 observed, the clay will require to be dislodged frequently. 



It is possible to arrange a screw, or other mechanical appliance at 

 the bottom of a bin of clay which shall deliver a constant supply as long 

 as any clay remains in the storage bin. In order to use the pug mill to 

 temper clay advantageously this provision would be worth all it costs. 



3d. Pug mills are a good means of uniformly wetting and mixing 

 clays which by nature are already plastic. In such clays the use of the 

 grinding action is not only unnecessary but is detrimental. 



The use of each of the two forms of tempering apparatus is thus 

 seen to have its own especial field of usefulness. The wet pan in 

 tempering rough, hard and rocky cla3 T s and shales; the pug mill in 

 tempering soft plastic clays; the latter can be used in tempering dry, 

 ground clays of any nature, but the results are not as good as they ought 

 to be, except when the natural plasticity of the claj' is high. Where 

 mixtures of clay are used to effect changes in the chemical character of 

 the material the use of a wet pan is nearly indispensable to good results. 



The use of hot water in tempering has a very beneficial effect in 

 many cases. Aside from the comfort to the men who handle the wet 

 brick in cold weather, and the assistance to the drying of the ware when 

 it comes warm from the machinery, the disintegrating effect of hot 

 water and steam is more pronounced and complete than cold water. 



