And there is also weighed out two (or better,
four) assay pieces each containing half a gram of the alloy wrapped in 5
grams of lead. The whole lot are then cupelled as nearly as possible
under the same conditions. With four assay pieces, the cupels should be
placed close together in two rows of three across the muffle; the two
check pieces are put in the middle cupels. Suppose the buttons of silver
got weighed as follows:--
Check pieces I. 0.3940 II. 0.3945
Assay pieces I. 0.3905 II. 0.3912
III. 0.3910 IV. 0.3909
The average loss on the two check pieces is 5.7 milligrams, and the
average result of the four assay pieces is 0.3909. Add the average loss
to the average result, and there is got the corrected result, 0.3966.
And if 0.5 gram of alloy contain 0.3966 of silver, 1000 will contain
793.2 of silver, and this is the degree of fineness.
A correction for the loss in cupellation is always made in this way
when rich alloys are being assayed; and in the case of rich ores it may
be done after the manner of the first of the above illustrations. There
is another method of working which relies more on experiment. This is to
smelt the cupel as described further on (p. 114), and to again cupel the
resulting button of lead. The button of silver got in this second
cupellation is added to that first obtained.
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