Flux

From GAWDAwiki

Flux is a substance applied to the surface of metals to be soldered to inhibit oxidation and aid the flow of filler. Also used to describe substances used in the smelting of metals to assist in the removal of such impurities as slag.^[1]

At the proper welding temperature, the flux will flow over the surface of the billet. The steel is then removed from the forge and the weld is closed with a hammer or press. If the welded surfaces were prepared properly, the flux and dissolved oxides will be forced out of the weld seam as the weld is closed.^[2]