352 CLAYS AND CLAY INDUSTRY. 



samples are very plastic, sticky clays, with little coarse grit and 

 practically no mica. Owing to their high plasticity, they soaked 

 up considerable water in tempering, and showed an air shrinkage 

 somewhat higher than most of the other clays, as well as greater 

 tensile strength. 



Slaking.: — -The majority of the Alloway clays are quite dense 

 and slake slowly when thrown into : water. 



Water required. — The amount of water necessary to temper the 

 mass ranged from 26.5 per cent, up to 48.7 per cent., with an 

 average of 35 per cent., which is greater than the average water 

 required in any of the other clays. The high water absorption 

 is to be expected, however, in such plastic clays. 



Air shrinkage. — This varied from 7.6 per cent, to 11 per cent., 

 and in a general way stands in direct relation to the amount of 

 water required for mixing. The air shrinkage is somewhat high, 

 however, so that in the manufacture of this clay, some more sandy 

 clay would have to be added to it. This can usually be found in 

 the loamy beds overlying the clay at many localities. Near York- 

 town, where the Alloway clay is used for bricks, enough loam is 

 added to the mixture to decrease its shrinkage the proper amount. 

 The laboratory tests on samples from this locality showed an air 

 shrinkage of 8 per cent, and 9 per cent, for the clay alone, and of 

 7.6 per cent, for the mixture. As the latter was not quite so stiff 

 as the material which is worked on the stiff-mud machine, the 

 shrinkage in actual practice would be a little less. 



Tensile strength. — With very few exceptions, all of the Allo- 

 way clays show a very high tensile strength, one having the high 

 average of 453 pounds per square inch, and a phenomenal max- 

 imum of 506 pounds per square inch. 1 Two samples only had a 

 low tensile strength. 



Burning qualities. — It is difficult to generalize regarding the 

 burning- qualities of this clay, but the majority of samples burn 

 steel-hard at cone 05, and were all red-burning. The fire shrink- 

 age is variable, as can be seen from a comparison of those burned 

 at cone 1 (see table opp. ), but some samples, it will be noticed, 



1 This sample was in part made up of a secondary clay resting directly on 

 the Alloway clay, and derived from it by stream action. 



