362 CLAYS AND CLAY INDUSTRY. 



run high. The lowest one of the series gave 145 pounds per 

 square inch, while the highest of the clay alone was 282 pounds 

 per square inch. The addition of the loam in some cases seems to 

 decrease slightly the strength. Thus the clay from Budd 

 Brothers, City Line station, near Camden (Loc. 143), when 

 tested alone, gave an average of 164 pounds per square inch, but 

 with loam added and mixed for molding on a stiff-mud machine, 

 the tensile strength dropped to 156 pounds per square inch. A 

 brick mixture of Clay Marl I and II, with some loam, used at 

 Maple Shade (Loc. 149) averages 282 pounds per square inch. 



Burning. — All the samples tested burned red, although of dif- 

 ferent degrees of brightness. Where this degree of coloration 

 was obtained at as low a cone as 05, the bricks deepen rapidly in 

 color and lose their brightness when heated above cone 01, at 

 which point even the color was rather deep. Although few of 

 them reached a condition of steel-hardness before cone 1, still it 

 is not necessary to heat them this high to make a good brick. No 

 measurements were made of the common-brick kilns, yet it is 

 probable that they do not exceed the melting point of cone 03, if 

 we may judge from the color and hardness of the brick. 



The fire shrinkage of all is low when burned at cone 05, rang- 

 ing from practically o per cent, to' 8.3 per cent. The last figure 

 indicates a higher fire shrinkage than that of any of the other clay 

 marls. Of the samples tested, those from Windsor (Loc. 192) 

 and Hightstown (Loc. 194) burn the hardest at cone 05, but in 

 the case of the latter the hardness is due probably in part to the 

 fact that the sample came from the boundary of Clay Marl I and 

 Clay Marl II, and, therefore, would naturally possess some of the 

 denser-burning qualities of the clays from the latter formation. 



Uses. — -On account of the high percentage of organic matter in 

 the Clay Marl I, brick made from it must be burned slowly at 

 first, until the carbon is all burned off. Failure to regard this 

 point causes black cores, and sometimes even swelling or bursting 

 of the brick. None of the samples were sufficiently fine-grained, 

 or verifiable at low enough temperature, to be useful for the 

 manufacture of vitrified brick, neither do they as a rule lend 

 themselves to the manufacture of pottery, although it is possible 



