Flux (metallurgy) Wikipedia
In metallurgy, a flux (derived from Latin fluxus meaning "flow") is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time. They are used in both extractive metallurgy and metal joining. Some of the earliest known fluxes were carbonate of soda, potash, charcoal, coke, borax, lime, lead sulfide and certain minerals containing phosphorus.