Why is the heated ring kept slightly above a material's melting point during zone melting?

In the context of zone melting, the heated ring is maintained several degrees above the material's melting point to ensure the creation and maintenance of a stable, narrow molten zone as the ring travels along the rod.1

By keeping the temperature above the melting point, the process effectively creates a liquid region that moves slowly through the solid charge. As this zone advances, liquid forms at the front interface while solid crystallizes at the rear interface. This movement allows for the redistribution of impurities, as they are typically more soluble in the liquid phase than in the solid phase. Consequently, the impurities are carried along with the molten zone and concentrated at the end of the rod, which can then be removed.12