POTTERY OR STONEWARE CLAY 8. 171 



age was 14 per cent, and the bricklet buff in color. At 

 2200 F. the shrinkage was 16 per cent and the bricklet 

 grayish buff, while vitrification occurred at 2200 F. ac- 

 companied by a total shrinkage of 17 per cent. Viscosity 

 took place at 2500 F. The tensile strength was only 

 moderate, being 84 to 85 pounds. 

 The chemical composition is : 



Analysis of Pottery Clay, J. 0. Bean, Tuscaloosa Go. (No. 100). 



Silica ................................ . ..... 60.03 



Alumina ................................... 24.6 



Ferric oxide ................................. 3.69 



Lime ........................................ 13 



Magnesia ................................. .380 



Alkalies .................................... tr. 



Ignition .................................... 11.342 



Total fluxes ............................... 



(No. 32 S.) 

 STONEWARE CLAY. 



ROBERTS' MILL, COAL FIRE CREEK, PICKETS CO. 



A gray, tough, rather fine grained clay, which in water 

 slakes somewhat slowly to a mixture of grain 

 one-sixteenth to one-thirty-second of an inch 

 in size. Taste gritty. Patches of fine sand 

 and ore scattered through the clay, and associated with 

 them are a few small flakes of white mica. 



The clay when ground to 30 mesh and mixed with 21.8 

 per cent, water gave a' workable mass of quite plastic 

 character, which shrunk 4 per cent in drying and 8 per 

 cent in burning, making a total shrinkage of 12 percent. 



Air dried briquettes of the clay had an average tensile 

 strength of 117 pounds per square inch and a maximum 

 strength of 142 pounds. 



Incipient fusion occurred at 2000 F.; vitrification at 

 2200 F. and viscosity at 2400 F. 



