The reasons for this are
sufficiently obvious: the sudden evolution of much steam will blow a
cupel to pieces; and, if the whole of the water has not been removed
before the cupel is filled with molten lead, the escaping steam will
bubble through, and scatter about particles of the metal. If some
particles of unburnt carbon remain in the bone ash, a similar result
will be produced by the escape of bubbles of carbonic acid as soon as
the fused litharge comes in contact with them. The cupels having been
prepared are arranged in a definite order in the muffle, and the assay
buttons are arranged in a corresponding order on some suitable tray
(cupel tray, fig. 41); the heat of the muffle being at bright redness.
Then with the help of the tongs (fig. 42) the assay buttons should be
placed each in its proper cupel; a note having been previously made of
the position it is to occupy, and the door of the muffle closed.
[Illustration: FIG. 41.]
This part of the work should be done promptly, so as not to unduly cool
the muffle: the start requires a fairly high temperature, and is a
critical part of the process. A black crust forms at once on the surface
of the lead; but this ought soon to fuse and flow in greasy drops from
off the face of the metal, so as to leave the latter fluid with a
well-defined outline, and much brighter than the cupel.
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