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The process that causes some ancient coins to turn blue over time is primarily due to a chemical reaction between the metal of the coin (often copper or bronze) and environmental factors, leading to the formation of a specific type of corrosion known as patina. Patina is a layer of oxidation that forms on the surface of the metal as it interacts with elements such as oxygen, moisture, and acids in the air or soil. In the case of copper-based coins, like those made from bronze or copper-alloy, the formation of patina is a natural and gradual process that helps protect the metal from further deterioration.

The characteristic blue or greenish-blue color seen on some ancient coins is typically caused by the formation of copper salts, such as copper carbonate or copper chloride. These compounds develop when copper reacts with substances in the environment, such as carbon dioxide, chlorides, and sulfides present in the air or soil. For example, when copper is exposed to moisture and carbon dioxide over long periods of time, it can form copper carbonate (CuCO₃), which has a blue or greenish hue. The presence of salts from seawater or salty soil can also lead to the formation of copper chloride (CuCl₂), contributing to the bluish color.

Another factor that can contribute to the blue patina is the presence of copper sulfate (CuSO₄), which can form in environments where sulfur compounds are present, such as in damp or acidic conditions. Sulfur from polluted air or decaying organic matter can react with the copper in the coins, producing blue copper sulfate crystals on the surface of the coin. This blue color is typically seen on coins buried in environments rich in sulfur, such as near marshy areas or saltwater locations.