When is 'polling' used as the method of purification of metals ?

Explanation

**Step-by-Step Solution:**

1. **Understanding Polling**: Polling is a method used for the purification of certain metals. It is a process that helps to remove impurities from the metal.

2. **Identifying the Condition for Polling**: Polling is specifically used when the metal contains oxides as impurities. This means that the impurities present in the metal are in the form of metal oxides.

3. **Examples of Metals**: Common examples where polling is applied include:
- Copper, which may contain copper oxide (CuO) as an impurity.
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