By the time of the California gold rush, mints were moving away from the age-old process of cupellation to "part" bullion and moving toward the acid refining process developed by chemist Joseph Louis Gay-Lussac for the French mint. By the time of the Klondike gold rush, mints were replacing Gay-Lussac's acid process and introducing electrolysis to refine doré bars into 999.9 purity gold bullion.
By the time of the California gold rush, mints were transitioning from their traditional cupellation method to "part" bullion and adopting the acid refining technique developed by Joseph Louis Gay-Lussac for the French mint. By the time of the Klondike gold rush, mints had replaced Gay-Lussac's process with electrolysis to refine doré bars into 999.9 purity gold.