8 gram of copper. Finally, the
last gram of lead will carry with it 0.3 gram of copper, and there will
be left a button of copper weighing 8.3 grams. The slag will have
carried off altogether 1.7 gram of copper, which is 17 per cent. of the
metal originally present.
With the more perfect exposure to the air, and quicker removal of the
slag, which results from heating on a cupel, the loss would be heavier.
Karsten got by actual experiment on cupelling copper and lead in equal
proportions, a loss of 21.25 per cent.
Going back to the example: if the slag were collected and fused with a
suitable reducing agent so as to convert, say, half of it into metal,
that half would contain nearly the whole of the copper (such a reduction
is called "cleaning the slag"). On reoxidising this metal, another
button of copper is formed which, added to the first, would reduce the
loss from 17 per cent. to, say, 7 or 8 per cent. And it is conceivable
that by a series of similar operations, almost the whole of the 10 grams
of copper originally taken might be recovered. In practice the problem
is (as far as the copper is concerned) not how to save, but how most
easily to remove it; and since the removal of this metal is quicker from
an alloy containing not too much lead, it is evident that two or three
operations with small quantities of lead will be more effectual than a
single treatment with a larger quantity.
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