POTTERY OR STONEWARE CLAYS. 165 



In water it slakes in angular fragments, and when worked, 

 requires 32 per cent, of water to develop its plasticity. 

 The clay is rather fine grained, but with a conchidal 

 fracture, and shows iron stains on its joint surfaces. 



The tensile strength does not appear in this case to 

 stand in direct relation to the plasticity, for the maximum 

 is only 56 pounds per square inch, and the average 49 

 pounds. 



The clay burns to a buff color, and a dense body, and 

 is quite refractory. 



The total shrinkage at 2350 F. is 18 per cent. At 

 2700 it is 18.05 percent. 



In the Deville furnace, at cone 30, the clay vitrified 

 and showed no evidence of becoming viscous. 



The second or sandy clay (No. 89) slakes very quickly. 

 It gives a moderately plastic, but though not so tough a 

 mass as the preceding. The tensile strength is however 

 higher, being 74 pounds on the average, and 92 at the 

 maximum. 



The air shrinkage is 8.75 per cent ; at 2200 F. the 

 total shrinkage was 11 per cent.; at 2350 the total shrink- 

 age was 12 per cent. 



The clay fuses at cone 30 in the Deville furnace. 



Associated with these stoneware clays is a bed of ochre 

 which fuses easily to a brownish glass. Its composition 



Analysis of Ochre, Edgewood, Elmore Co. 



Silica 51.14 



Alumina 30.13 



Ferric oxide 8.35 



Lime * tr. 



Magnesia tr. 



Alkalies tr. 



Ignition 10.15 



99.77 



