The orange coloration that appears on some ancient coins, particularly those made of copper or bronze, is a result of the chemical process of oxidation. When these metals are exposed to the elements, especially oxygen and moisture in the air, they react over time, forming a layer of copper oxide on the surface. This oxidation process typically begins with the formation of a copper(I) oxide (Cu₂O), which gives a reddish-brown color to the metal. However, as the oxidation progresses, it can further develop into copper(II) oxide (CuO) or other copper compounds, such as copper carbonate or copper chloride, which might also contribute to the eventual color change of the coin.

In a humid environment, the copper in the coin can react with carbon dioxide (CO₂) from the air to form copper carbonate (CuCO₃), which is commonly known as verdigris. While verdigris itself is usually green, the orange hue on some coins may be linked to the copper’s exposure to a combination of factors, including moisture, salt, and other environmental pollutants. Over time, this combination of corrosion products can accumulate on the coin, resulting in the distinct orange patina that is sometimes seen.

Interestingly, the orange hue is often most visible in areas of the coin that have been partially shielded from more intense environmental exposure or have been in contact with organic materials that contributed to the oxidation process. In some cases, the orange color may even come from the presence of iron impurities in the coin, as the copper and iron could interact in the oxidation process, creating different oxidation states that result in a reddish or orange tone.

Coins exposed to a particularly salty or acidic environment, such as those found near the sea or buried in acidic soil, can experience rapid and more pronounced oxidation. The presence of salt accelerates the corrosion process by helping to form chloride compounds on the surface, further altering the appearance of the metal.