To remove gangue, certain substances are mixed with it. These are called fluxes. A flux can be basic or acidic. Acidic flux removes basic impurity and basic flux removes acidic impurity.

Flux is also used for increasing conductivity.

SiO₂ (Acidic flux)+CaO (Basic impurity) → CaSiO₃(Slag)

MgO(Basic flux) + SiO₂ (Acidic impurity) → MgSiO₃ (Slag)