The ill effects of copper are most
strongly felt when the quantity of lead present is not sufficient to
remove it: the coppery button of silver got under these conditions is
very considerably less than the weight of silver originally taken.
Although the above is a fair statement of the loss attending average
work, it will not do in very important and exact work to place too much
reliance on the figures given, or, indeed, on any other set of figures,
with the object of correcting the result of an assay. Each man must rely
on his own work.
It is easy to determine what allowance must be made for the loss in
cupellation by cupelling side by side with the assay piece an alloy of
similar and _known_ composition. For, if the two pieces are very nearly
alike, we may justly conclude that the loss on each will be the same;
and if, further, we take the average of three or four such
determinations we shall get results accurate within 0.1 per cent. The
method of getting such results may be best explained by one or two
illustrations. This method of working is termed "assaying by checks."
Suppose we have an alloy of silver and lead in unknown proportions and
that by cupelling two lots of 10 grams each there is got from I. 0.1226
gram of silver, and from II. 0.1229 gram. We should know from general
experience that the actual quantity of silver present was from 2 to 4
milligrams more than this.
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