Why is molten lead held just above its melting point during drossing?

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Metallurgy — lead processing A zinc-lead blast furnace and lead-splash condenser. This technology is utilized mainly for producing zinc while managing both lead and zinc from raw materials, a technique that was highly influential in 20th-century smelting.

During the drossing process, molten lead (base bullion) is held at a temperature just above its melting point—approximately 330 °C (626 °F)—to facilitate the removal of impurities, particularly copper.1

At this specific temperature, the solubility of copper in lead is extremely low. Consequently, the copper segregates from the molten lead and rises to the surface of the bath, forming a scum known as dross. By maintaining this temperature, metallurgists can effectively skim off the copper-rich dross before proceeding with further refining steps to achieve commercial-quality lead.1