92 CLAYS AND CLAY INDUSTRY. 



corresponding to the amount of water required to mix it, viz. r 

 21.3 per cent, of the weight of dry clay taken. In the same way 

 similar pairs of points were plotted for the other localities, and 

 each series of points was then connected by a line. Since the 

 different clays were arranged according to the amount of their 

 air shrinkage, the line representing this will naturally rise from 

 left to right. If now the amount of water required for mixing 

 stood in direct relation to' the air shrinkage, the upper line should 

 also rise steadily from left to right, which it does not do in all 

 cases. In the Cape May clays there seems to be less irregularity 

 than in the case of the other formations, in all of which the upper 

 line is very irregular, due, no doubt, to' the variation in texture 

 of the clays. 



The air shrinkage of a clay will not only vary with the amount 

 of water added, but also with the texture of the material. Soft- 

 mud bricks may shrink more than stiff-mud ones, because in the 

 latter case less water is added to- the clay, and it is molded under 

 greater pressure. At the same time, the shrinkage of many 

 soft-mud bricks is low, because so 1 much sand is often added to 

 the clay. The effect of the sand on the air shrinkage is well seen 

 by comparison of samples 695 and 695 S, both from locality 218, 

 1 mile south of Herbertsville. In preparing sample 695 S, 50 

 per cent, of sharp sand was added. 



Tabic showing effect of sand on the air shrinkage and tensile strength of a clay. 



Per cent, of water Per cent, of air Tensile strength 



required. shrinkage. lbs. per sq. in. 



Lab. No. 695, Loc. 218.. . 32.6 5.3 108 



" " 695S, " " . . 15.6 3-3 65 



From the above it is seen that the addition of 50 per cent, of 

 sharp sand reduced the amount of water required a little over 

 one-half. The air shrinkage was reduced 37.73 per cent., but it 

 was accompanied by a loss in the tensile strength of very nearly 

 40 per cent. Similar examples can be seen by an inspection of 

 the tabulated tests in Chapter XVIII, where it will be seen that 

 in some cases a gain is made all around by the mixing process, 

 as for instance, in the clays from the Raritan, used at locality 222, 

 near Key port. Here, sample 606 (Lab. No.) represents a fat, 



