Why is limestone sometimes added as a flux during copper smelting?

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Molten copper pours from a large industrial furnace into rectangular molds at a copper smelting facility, with glowing metal and industrial equipment visible. — copper processing Molten copper being made into plates.

In the context of copper processing, limestone is occasionally added as a flux during the smelting stage to help remove impurities.1

While silica is the primary flux used to react with iron and other oxidized impurities to form a fluid slag, limestone may be used to adjust the chemical composition of the slag. By promoting fluidity and facilitating the separation of gangue minerals and oxidized impurities from the molten matte, fluxes ensure that these unwanted materials can be effectively skimmed off, leaving behind a higher-quality copper product.213