Why is a transparent container helpful for vertical zone refining?

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zone melting Figure 1: Schematic representation of (A) normal freezing, (B) zone refining

In vertical zone refining, a transparent container is helpful because it allows the operator to visually monitor the process and distinguish the molten zone from the solid material.1

While opaque containers like stainless steel can be used for vertical setups, transparency provides a clear view of the liquid-solid interface, which is essential for controlling the movement and stability of the zone during the purification process.1