Cupellation is the process used for refining silver. During this process, impure silver is heated in a cupel (a small, shallow, porous ceramic or bone ash container) along with lead oxide (PbO) as the oxidizing agent. The impurities present in the silver form certain compounds that are either volatile or soluble in the molten lead, while the pure silver remains behind as a residue.

Here is a detailed explanation of the cupellation process:

1. Preparation: The impure silver is first melted and cast into small bars or pellets. These pellets are then placed in the cupel.

2. Cupellation: The cupel, along with the pellets of impure silver, is heated in a furnace at a temperature of around 900-1000°C. As the temperature rises, the lead oxide (PbO) in the cupel reacts with the impurities in the silver, forming various compounds.

3. Oxidation of Impurities: The lead oxide acts as an oxidizing agent, converting the impurities into compounds that are either volatile or soluble in the molten lead. This includes impurities like copper, iron, and other base metals present in the silver.

4. Volatilization: Some impurities form volatile compounds, such as copper forming copper oxide (CuO) and iron forming iron oxide (FeO). These compounds vaporize and escape from the cupel, leaving behind the pure silver.

5. Formation of Lead Button: The impurities that are not volatile or soluble in the molten lead form a molten lead-silver alloy. This alloy collects at the bottom of the cupel and solidifies into a button-like mass.

6. Removal of Lead: The lead button is then separated from the cupel. It can be further processed to recover the lead or recycled for future cupellation processes.

7. Collection of Pure Silver: The residue left in the cupel after the removal of the lead button is pure silver. It may still contain trace amounts of other impurities, but it is significantly refined compared to the initial impure silver.

8. Further Refinement: The pure silver obtained from cupellation can undergo additional refining processes, such as electrolysis or chemical precipitation, to achieve even higher levels of purity.

Overall, cupellation is a crucial step in the refining of silver, allowing for the removal of impurities and the production of high-quality, pure silver.