The plus-silver is due to the fact that in assaying the base bullion by cupellation, the silver lost by volatilization and cupel-absorption is neglected.
The base bullion (assaying 300 t oz.
The base bullion is imperfectly Pattinsonized, giving lead rich in silver and bismuth, which is cupelled, and lead low in silver, and especially so in bismuth, which is further desilverized by the Parkes process.
Since 1825 the total product of lead refined from domestic ores and domestic base bullion was, up to the close of 1908, 7,091,548 short tons.
It holds its own, however, when base bullion contains bismuth in appreciable amounts, as in the Pattinson process bismuth follows the lead to be cupelled, while in the Parkes process it remains with the desilverized lead which goes to market, and lead of commerce should contain little bismuth.
Considerable quantities of foreign ores and base bullion are also refined in the United States.