The yellowing of ancient coins, particularly those made from metals like bronze or copper, is primarily due to the process of oxidation and the formation of various corrosion products over time. When these coins are exposed to air, moisture, and environmental pollutants, the metal reacts with oxygen and other elements, leading to the formation of a patina—a thin layer of corrosion on the surface of the coin.

For copper and bronze coins, the primary cause of yellowing is the formation of copper oxides and copper salts, especially cupric oxide (CuO), which has a characteristic dark color, or copper carbonate (CuCO₃), which can appear greenish. However, over time, copper also reacts with sulfur compounds in the environment, such as those found in the air or from soil, creating a yellowish copper sulfate (CuSO₄) layer. This process results in a yellowish, sometimes slightly greenish, discoloration. The exact hue of the yellow patina depends on the specific environmental conditions the coin was subjected to, including the humidity, temperature, and the composition of the surrounding atmosphere.

For coins made from gold or silver, yellowing may not occur in the same way, but tarnishing or discoloration can happen due to reactions with sulfur or chlorine compounds. In gold coins, though they are relatively resistant to corrosion, impurities or slight amounts of copper or silver mixed with the gold can result in tarnishing that might present as a yellowish or reddish tint, depending on the exact alloy. Silver, on the other hand, forms a tarnish that may appear as a dull yellowish or dark color when it reacts with sulfur compounds in the environment to form silver sulfide (Ag₂S), which can give the coin a yellowish tint in certain circumstances.