Flux = substance added to combine with gangue to form a fusible product (slag). Acidic flux (SiO2): used when gangue is basic (FeO, CaO). FeO + SiO2 → FeSiO3 (slag). Basic flux (CaO, MgO): used when gangue is acidic (SiO2). CaO + SiO2 → CaSiO3 (slag). Slag is lighter than metal → floats on top → easily removed. In the blast furnace, CaCO3 is added as flux: CaCO3 → CaO + CO2; CaO + SiO2 → CaSiO3 (slag). Slag has commercial uses: cement additive, road construction, insulation material.