Coins oxidize due to a chemical reaction between the metal in the coin and substances in the surrounding environment, primarily oxygen, moisture, and pollutants. This process, a form of corrosion, results in the formation of metal oxides on the coin's surface.
The Oxidation Process Explained
The oxidation of coins isn't a simple process and involves a few key elements:
- The Metal: Most coins are made of base metals like copper, nickel, zinc, or alloys containing these metals. These metals readily react with oxygen.
- Oxygen: Oxygen is a key ingredient in the oxidation process. The metal atoms on the coin's surface lose electrons to oxygen atoms, forming metal oxides.
- Moisture: Water acts as an electrolyte, facilitating the movement of ions and accelerating the oxidation reaction. Humidity in the air is sufficient to trigger this.
- Pollutants: Pollutants like chlorides (e.g., salt) and sulfur dioxide in the air further accelerate the corrosion process. These compounds react with the metal surface, creating additional corrosion products.
Steps in the Oxidation Process
- Initial Reaction: The metal on the coin's surface reacts with oxygen in the presence of moisture to form a thin layer of metal oxide.
- Oxide Layer Formation: Over time, this oxide layer builds up, causing visible tarnish or corrosion.
- Reaction with Pollutants: Chlorides and sulfur dioxide can react with the metal and the oxide layer, forming other corrosion products like chlorides and sulfides. These can be more aggressive and damaging than simple oxides.
Examples of Coin Oxidation
- Copper Coins: Copper coins develop a green or blue-green patina (verdigris) due to the formation of copper oxides, hydroxides, and carbonates.
- Silver Coins: Silver coins tarnish, developing a black or grey surface layer due to the formation of silver sulfide.
- Nickel Coins: Nickel coins can develop a dull grey or greenish layer of nickel oxide.
Factors Affecting Oxidation
- Humidity: Higher humidity levels accelerate the oxidation process.
- Temperature: Increased temperatures also tend to speed up the reaction.
- Pollution: Exposure to pollutants such as sulfur dioxide and chlorides increases the rate of corrosion.
- Handling: Oils and acids from skin can contribute to corrosion.
Prevention of Oxidation
- Proper Storage: Store coins in dry, airtight containers or albums made of inert materials.
- Avoid Handling: Handle coins with cotton gloves to prevent the transfer of oils and acids.
- Use Desiccants: Place desiccants (drying agents) in storage containers to absorb moisture.
- Protective Coatings: Applying a thin layer of wax or lacquer can help protect the coin's surface from exposure to air and moisture.
