Flux
A substance added during smelting to combine with the gangue and form a fusible slag.
Unit
Dimensionless
In depth
An acidic gangue such as silica requires a basic flux like limestone, while a basic gangue requires an acidic flux like silica. The resulting slag melts at a workable temperature, is less dense than the metal and floats on top, where it can be drawn off separately.
Examples in the real world
Limestone added to a blast furnace, giving CaO that reacts with SiO₂ to form calcium silicate slag.