[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. The furnace
should be well packed by stirring, raising the coke and not ramming it,
and it should be uniformly heated, not hot below and cold above. In
lighting a furnace, a start is made with wood and charcoal, this readily
ignites and sets fire to the coke, which of itself does not kindle
easily.

In commencing work, add (if necessary) fresh coke, and mix well; make
hollows, and into these put old crucibles; pack around with coke, so
that the surface shall be concave, sloping upwards from the mouths of
the crucibles to the sides of the furnace; close the furnace, and, when
uniformly heated, substitute for the empty crucibles those which contain
the assays. It is rarely advisable to have a very hot fire at first,
because with a gradual heat the gases and steam quietly escape through
the unfused mass, while with too strong a heat these might make some of
the matter in the crucible overflow. Moreover, if the heat should be too
strong at first, the flux might melt and run to the bottom of the
crucible, leaving the quartz, &c., as a pasty mass above; with a gentler
heat combination is completed, and the subsequent fiercer heat simply
melts the fusible compound into homogeneous slag.

The fused material may be left in the crucible and separated from it by
breaking when cold. It is generally more convenient to pour it into