This is the most commonly asked question we receive from people who have never worked with fluxes. FLUX: A chemical composition critical to the heat joining methods of soldering, brazing, and welding that: Chemically: A substance that aids, induces, or otherwise actively participates in fusing or flowing, as: A substance applied to a surface to be joined by welding, soldering, or brazing to facilitate the flowing of solder and prevent formation of oxides. A mineral added to the metals in a furnace to promote fusing or to prevent the formation of oxides. An additive that improves the flow of plastics during fabrication. A readily fusible glass or enamel used as a base in ceramic work.