264 CLAYS AND CLAY INDUSTRY. 



slightly laminated, with black centres at times. The brick 

 showing the lowest transverse test broke off the centre and con- 

 tained numerous sandstone fragments. Nearly all were dotted 

 with pebbles on the broken surface. 



1 6. Front bricks, made from Raritan clay of only moderate 

 tensile strength, but dense burning and of semirefractory char- 

 acter. They are dried in tunnels and burned at about cone 9 

 in down-draft kilns. The grain of the brick is fine, and some- 

 what brittle, with no* laminations. 



17. Made in same manner as 16, but from different clay. 

 Strength is similar. 



18. Common bricks made from a black, dense-burning clay, 

 of high tensile strength and shrinkage, to> which is added a sur- 

 face loam of opposite physical characters from the clay. The 

 bricks are dried on pallets and burned in Dutch kilns. The cubic 

 air shrinkage of the bricks is 5.9 per cent., and the cubic fire 

 shrinkage is 9.2 per cent. 



19. A dense, nearly vitrified brick made from a plastic, dense- 

 burning surface clay. The cubic air shrinkage is 17.3 per cent., 

 and the cubic fire shrinkage 19.4 per cent. The bricks have a 

 smooth, dense fracture, and fine grain, with scattered angular 

 quartz fragments. The surface shows some cracks. 



SOFT-MUD BRICKS. 



20. Common bricks made from a mixture of sandy clay and 

 clean sand. The clay was tempered in ring pits and molded in 

 a horsepower soft-mud machine. It was dried on an open yard 

 and burned in a scove kiln, at a cone number probably not higher 

 than 05. The mixture used has a low tensile strength and air 

 shrinkage. The bricks are moderately hard, have a slight ring, 

 and on the fracture show numerous pebbles and ash fragments up 

 to one-half inch diameter. 



21. Common brick made from a mixture of sandy clay and 

 loam ; tempered in a soak pit, molded in a soit-mud horsepower 

 machine, dried on open yards, and burned in a scove kiln prob- 

 ably at a low temperature. The product is rather porous, lacks a 



