5.
[Illustration: FIG. 6.]
[Illustration: FIG. 7.]
[Illustration: FIG. 8.]
There are two kinds of furnaces required, the "wind" and "muffle"
furnaces. These are built of brick, fire-brick, of course, being used
for the lining. They are connected with a chimney that will provide a
good draught. Figure 6 shows a section of the wind furnace, fig. 7 a
section of the muffle furnace, and fig. 8 a general view of a group
comprising a muffle and two wind furnaces suitable for general work.
When in operation, the furnaces are covered with iron-bound tiles. The
opening under the door of the muffle is closed with a loosely fitting
brick. The floor of the muffle is protected with a layer of bone-ash,
which absorbs any oxide of lead that may be accidentally spilt. The fire
bars should be easily removable.
Few tools are wanted; the most important are some cast-iron moulds,
tongs (fig. 9), stirrers for calcining (fig. 10), and light tongs of a
special form for handling scorifiers and cupels (_see_ SILVER).
[Illustration: FIG. 9.]
[Illustration: FIG. 10.]
The coke used should be of good quality; the formation of a fused ash
(clinker), in any quantity, causes ceaseless trouble, and requires
frequent removal. The coke should be broken into lumps of a uniform size
(about 2 in. across) before being brought into the office.
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