Molten brazing alloys require a clean surface on which to flow if they are to bond properly. Heating metal during the brazing process causes oxides to form on its surface. These oxides, if not removed, will prevent the filler metal from flowing entirely. A coating of flux applied before heating the joint will protect the area from exposure to the air and absorb oxides, which may form on the surface.
Because different metals produce different oxides, several flux formulations have been developed to handle these variations. Specialized fluxes are available for both low- and high-temperature brazing of silver, aluminum, nickel-silver, and bronze, as well as many soft-solder applications.