0 grams | 1.3 gram | --
6.0 " | none | 4.0 grams
9.0 " | -- | 9.0 "
12.0 " | -- | 14.0 "
15.0 " | -- | 19.0 "
18.0 " | -- | 24.0 "
21.0 " | -- | 29.0 "
-------------------------------+-------------------+------------------
If litharge is used in the assay instead of red lead 4 grams more nitre,
or 1.5 gram less flour must be used. When more than a few grams of nitre
are added to a charge the proportion of "soda" and borax should be
increased, because one of the products of the reaction of nitre upon
sulphides in the presence of soda is sulphate of soda, and because the
"soda" thus used up no longer serves as a flux; more borax should be
added, as it is the best flux for the metallic oxides which are formed
in the process. If in an assay too large a button of lead is got, even
after this calculation has been made, and the assay is repeated, add 1
gram more nitre for each 4 grams of lead in excess. Sometimes the assay
appears tranquil before the nitre has produced its full effect; in such
cases it is well to seize the crucible with the tongs and mix its fused
contents by rotating them; if this causes an effervescence, the crucible
should be replaced in the fire and the fusion continued.
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