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122                      METALLURGY   OF  CAST   IRON.
with clay. This system makes it almost impossible for any air to find access to fuel in the hearth, where so many openings for tuyeres, etc., would leave crevices for air to enter. The stack portion being practically a solid body enclosed by a tight shell of iron, no attention is given to it; so also with the bell and hopper at the top of the furnace, as some ventilation is desirable at the top to allow any excess of gas to freely escape. For this purpose, the "bleeder" pipe valve can be forced open, as in no case is the "down comer" valve opened. From this "bleeder" the state of the fire in the furnace can also be fairly judged. Nearly all furnacemen differ somewhat in their methods of "banking." At the present day many have abandoned the practice of encircling a furnace with a curbing above described, and after removal of the tuyeres and pipes they simply pack all holes and crevices with clay rammed tightly in place, and then occasionally wash the outside of the lining or brickwork, which is exposed to the air, with a thick coat of clay wash, thus closing up all crevices or pores which might admit air to the fuel. This plan, while costing much less than the curbing system, has been found sufficiently effective to answer all purposes. In preparing the furnace for being "banked," it is essential to free it as much as possible from its regular charges, and any liquid metal which may be in the hearth below the tapping hole.
To liberate the liquid metal all that is possible from the bed of the furnace, a hole is sometimes made from one to two feet below the level of the top of the regular tapping hole, which permits the metal to run out into an excavation in the ground in the form Mr. Edgar S. Cook, president of the Warwick Iron Co. of Pottstown, Pa. is often surprising how rapidly, as about 75 per cent of the heat generated from the solid fuel is utilized. This is attained where one ton of coke will produce one ton of iron; and Sir........................    2,720     "