Of the smelter domestic product 235,559 tons were of desilverized lead and 129,607 of soft lead.
Idaho, Utah and Colorado produce together almost as great a proportion of the desilverized lead, half of which has come in recent years from Idaho.
In Colorado the pyritic ores containing gold and silver in association with copper are smelted in reverberatory furnaces for regulus, which, when desilverized by Ziervogel's method, leaves a residue containing 20 or 30 oz.
In 1842 Karsten discovered that lead could be desilverized by means of zinc. His invention, however, only took practical form in1850-1852through the researches of Parkes, who showed how the zinc-silver-lead alloy formed could be worked and the desilverized lead freed from the zinc it had taken up. In the Parkes process only 5% of the original lead need be cupelled.
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.