If this clearing
does not take place, the buttons are said to be frozen; in which case
the temperature must be raised, some pieces of charcoal put in the
muffle, and the door closed. If they still do not clear, the heat must
have been much too low, and it is best to reject them and repeat the
assays.
[Illustration: FIG. 42.]
When the buttons have cleared it is well to check the draught of the
furnace, and to partly open the door of the muffle, so as to work at as
low a temperature as is compatible with the continuation of the
process.[11] Too low a temperature is indicated by the freezing of the
buttons and the consequent spoiling of the assays. Experience soon
enables one to judge when the heat is getting too low. A commoner error
is to have the heat too high: it should be remembered that that which
was high enough to clear the buttons at starting is more than sufficient
to keep the process going. At the finish a higher temperature is again
required: therefore the door of the muffle should be closed and the
furnace urged. The finish is easily recognised. The drops of litharge
which in the earlier stages flow steadily from the surface of the alloy,
thin off later to a luminous film. At the end this film appears in
commotion, then presents a brilliant play of colours, and, with a sudden
extinction, the operation is finished.
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