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

Explanation

**Step-by-Step Solution:**

1. **Understanding Poling Method**: The poling method is a technique used for the purification of certain metals. It involves the use of a reducing agent to remove impurities from the metal.

2. **Identifying the Impurities**: This method is specifically employed when the impurities present in the metal are metal oxides. For example, copper can have Cu2O as an impurity, while tin can have SnO2.

3. **Application to Specific Metals**: The poling method is primarily used for the purification of copper and tin. This is because these metals often have metal oxides as their impurities, which can be effectively removed using this method.

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