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If the whole space of the furnace covered by the muffle is not filled with
scorifiers, cupels are put in the empty space, in order that they may become
warmed in the meantime. Sometimes, however, it is filled with scorifiers,
when we are assaying many different ores, or many portions of one ore at the
same time. Although the cupels are usually dried in one hour, yet smaller
ones are done more quickly, and the larger ones more slowly. Unless the
cupels are heated before the metal mixed with lead is placed in them, they
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frequently break, and the lead always sputters and sometimes leaps out of them ;
if the cupel is broken or the lead leaps out of it, it is necessary to assay
another portion of ore; but if the lead only sputters, then the cupels should
be covered with broad thin pieces of glowing charcoal, and when the lead
strikes these, it falls back again, and thus the mixture is slowly exhaled.
Further, if in the cupellation the lead which is in the mixture is not consumed, but remains fixed and set, and is covered by a kind of skin, this is a
sign that it has not been heated by a sufficiently hot fire ; put into the
mixture, therefore, a dry pine stick, or a twig of a similar tree, and hold it
in the hand in order that it can be drawn away when it has been heated.
Then take care that the heat is sufficient and equal ; if the heat has not
passed all round the charge, as it should when everything is done rightly,
but causes it to have a lengthened shape, so that it appears to have a tail,
this is a sign that the heat is deficient where the tail lies. Then in order
that the cupel may be equally heated by the fire, turn it around with a small
iron hook, whose handle is likewise made of iron and is a foot and a half long.
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Next, if the mixture has not enough lead, add as much of it as is required
with the iron tongs, or with the brass ladle to which is fastened a very long
handle. In order that the charge may not be cooled, warm the lead beforehand.
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