This metal is more dense than gold. The method adopted for its separation (at the St. Petersburg Mint) from Russia gold is to alloy it with three parts of silver, melt the metal in large black lead crucibles, and keep them at rest for some time, during which the iridium granules sink to the bottom. The upper portion of the gold is then dipped out to within an inch of the bottom of the crucible, and run into ingots. The small portion of the metal left at the bottom contains the greater portion of the iridium, which is separated in the " wet way," by nitro-hydrochloric acid, which dissolves the gold, but does not act on the iridium granules. Some of our California gold contains iridium.
This article was originally published with the title "Separating Osmiam-Iridiam from Gold" in Scientific American 13, 20, 156 (January 1858)