Used in soldering and welding. A substance added to where the metals are to be joined which will dissolve or prevent the formation of oxide films which would prevent the adhesion or joining of the two metals. The most common flux used is borax or a mixture of borax and other chemicals such as iodine, flurine, chlorine, bromide, or astatine. The fumes from flux are usually toxic.
Also, a substance used to increase the fluidity of slags in a blast furnace during smelting.