Such impurities can only be removed during the reduction of the metal. In such a process, an acidic substance must be added for basic impurities and a basic substance must be added to remove acidic impurities.
Such substances, which are lighter than the metal in density and accumulate on the melting metal and can be easily removed by flowing, by the combination of acidic and basic substances, are called slag (feces). Although it is not possible to put forward a chemical formula for slags, they are generally phosphates and silicates of calcium and magnesium.