Why was iron oxide used as a flux in early copper smelting?

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A periodic table element box for copper shows its symbol Cu, atomic number 29, atomic weight 63.546, electron configuration, and name. Icons indicate copper is a solid, weakly basic transition metal with face-centered cubic crystal structure. A key explains the symbols and color coding. — copper Properties of copper.

In early copper smelting, iron oxide was introduced as a flux to facilitate the agglomeration and separation of the molten metal from its associated minerals.1

During the smelting process, ores contain various impurities known as gangue. By adding a flux, these impurities combine to form a fluid substance called slag, which can be easily separated from the metal. The use of iron oxide for this purpose was a significant technological advancement, often made possible by the presence of iron oxide gossan minerals found in the weathered upper zones of copper sulfide deposits. This process allowed early metallurgists to effectively recover copper from metal-bearing minerals.231