Where clean sharp sand cannot be had, burnt clay ballast or coke breeze are very good substitutes. Coal slack is not to be recommended. The coke breeze should be obtained from the nearest gasworks. The burnt ballast may be prepared in the following manner, the object being to burn the clay hard, as in brickmaking. Four or five old drain pipes, 9 in. or 12 in. diameter, are laid in line with open joints. Around one end of the flue so formed is placed a heap of wood, say 3 ft. high and 6ft. across the base. Over this conical shaped heap of wood is spread a good layer of coal, and on the coal a layer of clay 6 in. or S in. thick may be deposited. Before attempting to burn the clay, it should be well turned over, and tempered and dried in the air. When the fire is burning fairly well, more coal or breeze is added, and, when everything is red hot, another layer of clay. More coal and more clay are in this manner added to the heap, until it becomes so large that further additions to it cannot conveniently be made. The fire is then allowed to die down, and the ballast is broken up and taken to the mortar mill. One cubic yard of clay measured in the solid, before digging, will, when burnt and broken up, make 1 1/4 cub. yd. to 1 1/2 cub. yd., and will weigh about l ton.

From fewt. to lewt. of coal is required to burn 1 cub. yd. of clay; or, according to some authorities, about 11 cub. yd. of breeze and 4 tons of coal, including slack, will burn 103 cub. yd. of clay.