Flux (Lat. fluere, to flow), a substance used to facilitate the fusion of minerals, and frequently their decomposition. A great variety of materials serve this purpose, and one or another is used according to the nature of the body to be treated, and the chemical action desired. Some by their ready fusibility induce the same condition in bodies in contact with them which are difficult to melt; others, though they may be as infusible as the compounds they are brought in contact with, present ingredients which possess affinities for some of those in the body to be acted upon, and fusion then takes place, with mutual decomposition and recombination of elements. Thus in treating the common silicious ores of iron, which are extremely difficult to melt, limestone, still more infusible, is employed, and the lime uniting with the silica enters at once into fusion, while the oxide of iron, freed from its original combination, is at the same time decomposed by the carbon of the fuel combining with its oxygen, and the iron flows free. The carbon itself may be regarded also as a flux, its action being to facilitate this process in the same manner as the limestone does.

Should the iron ores be calcareous, the mineral flux to aid their decomposition must be silicious, that the same fusible silicates may be produced. Borax is a flux of very general application, from the readiness with which it forms fusible compounds with silica and other bases. The subject will be considered, as to the application of particular fluxes, in describing the metallurgic treatment of the ores of the various metals. (See also Black Flux, and Borax.)