160 DETAILED REPORT ON ALABAMA CLAYS. 



POTTERY OR STONEWARE CLAYS. 



Many clays which are too impure to be used as fire 

 clays are often admirably adapted for pottery pur- 

 poses. In fact stone ware clays are often somewhat 

 intermediate in their nature between fire clays and 

 pipe clays, that is to say they are too impure for the 

 one purpose and too good for the other. 



In the manufacture of stoneware, it is highly es- 

 sential that the clay should burn to a dense imper- 

 vious body without requiring too high a temperature 

 to accomplish this, and furthermore if the ware is to 

 be unglazed or is to be coated with a transparent 

 glaze it is important that the clay should burn to a 

 good uniform color. In order to obtain the desired 

 result it is not uncommonly the rule to use a mixture 

 of two or more clays for this purpose. 



A stoneware clay should be smooth, and free from 

 coarse grit, otherwise it may be necessary to wash the 

 material, and thus increase the cost of manufacture. 

 The clay, in addition, should be highly plastic in or- 

 der to permit its being easily moulded without crack- 

 ing, and the tensile strength should be not less than 

 150 pounds per square inch. As the ware is to be 

 burned to a vitrified body, it is also desirable that 

 there should be a difference of 15)° to 250° F. between 

 the point of vitrification and viscosity. (Earthen- 

 n^are clays are not vitrified. ) Excessive plasitcity is un- 

 desirable as it necessitates very slow drying and burn- 

 ing of the ware and consequently increases the cost 

 of manufacture; while on the other hand low shrink- 

 age diminishes the loss from cracking or warping. 



Iron is a desirable ingredient not only as it tends to 

 give the body a good red color, but in addition serves 

 asafl'ix. Lime if present as a silicate may forma 



