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Fig. 54.—The " Thwaites " Cupola

A, Shaft. B, Brick-lined charging door, c, Air-
belt. D. Tuyeres. E, Receiver. F, Slag hole.
G, Tapping spout. H, Hot-air pipe to receiver,
j, Fettling nole. K, Drop bottom. L, Blast pipes.

cupola furnace for melting iron are these
(figs. 54 to 57).   A tall cylindrical shell,
built   of wrought-iron  or  steel  plates,
lined with fire-brick, daubed with fire-
clay for each cast;  a charging door near
the top;   an air-belt encircling the shell
at a height of a few feet from the bottom,
whence blast under pressure is directed
through  tuyere  openings  to   iron  and
coke supported on a deep bed charge of
coke.    The furnace stands on columns,
and has a hinged bottom to permit of
the dropping out of the residuary coke,
metal, and slag at the termination of the
day's cast.   Peep holes with mica win-
dows are fitted opposite the tuyere holes
through which the furnacemen observe
the progress of the melting, and open-
ings are furnished for the removal  of
the slag, and the tapping of the metal.
Many cupolas include a receiver, a cir-
cular vessel into which the iron, passing
down through the bed charge of coke,
trickles and collects, remaining perfectly
liquid until it has to be tapped out for
pouring.     The   internal   diameters   of
cupolas range from about 18 in. to 6 ft.;
the first will melt about f ton per hour,
the last, about 12 tons.   These are ex-
tremes, the first being of value chiefly
for occasional light casts, and for mak-
ing tests of metal, the last being too
large  for  general   service,  for  which
internal diameters of from 3 ft. to 4 ft.
are preferable.
A cupola is worked as follows: after
re-lining the interior with fire-clay each
morning, the bed charge of coke is laid
in, extending to from 18 in. to 20 in.
above the tuyeres. Over this succes-
sive layers of pig or scrap, lime-stone,
and coke are placed, there being three
or four repetitions in this order until
the charging door is reached. The
fire is lit, and the interior warmed
before the blast is put on. In about