They were, more accurately, doré bars with higher contents of silver and other adulterants than the mints would accept.
Mint and private assayers would subsequently refine the doré bars to an acceptable purity, 999 fine, gold bullion, with the silver and base metals removed.
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.
[clarification needed] 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.
[1] According to the United States Census Bureau,[2] the creation of doré bars is conducted at establishments engaged either in "Gold Ore Mining" (NAICS Code 212221),[3] or "Primary Smelting and Refining of Nonferrous Metal (except Copper and Aluminum)" (NAICS Code 331419).