The dark film found on copper pennies is primarily composed of copper oxides and sulfides, a result of the coin’s interaction with atmospheric gases and pollutants over time. This process, known as patination, creates a protective layer, although it often appears aesthetically undesirable to those seeking a shiny, pristine coin.
Understanding Patina Formation on Copper
Copper, a relatively reactive metal, readily interacts with its environment. When exposed to air, especially in the presence of moisture and pollutants, it undergoes a series of chemical reactions that lead to the formation of the dark film we see on pennies. This film is not simply dirt; it’s a complex chemical compound.
The Chemistry Behind the Color Change
The initial reaction involves oxidation, where copper atoms combine with oxygen in the air to form cuprous oxide (Cu₂O), which is usually reddish in color. However, this layer is not very stable and can further react with other atmospheric elements. In areas with high levels of sulfur dioxide (SO₂), a common pollutant from industrial activities, the copper oxide can transform into copper sulfide (CuS or Cu₂S), which is significantly darker, often appearing black or brown. The presence of other elements like chlorine or carbon dioxide can also contribute to the formation of different copper compounds, each with its own unique color and texture.
Factors Accelerating Patina Formation
Several factors can accelerate the rate at which patina forms. High humidity provides the necessary moisture for the chemical reactions to occur. Exposure to pollutants like sulfur dioxide, hydrogen sulfide, and ammonia significantly speeds up the process. Direct contact with sweat and oils from human skin can also contribute, as these contain salts and acids that react with the copper. Finally, the composition of the copper alloy itself can influence the rate of patina formation. Pennies made after 1982, which are predominantly zinc with a thin copper coating, tend to develop a different type of patina compared to the older, nearly pure copper pennies.
Identifying the Type of Patina
While most dark films on pennies are a combination of copper oxides and sulfides, recognizing subtle differences can provide clues about the coin’s history and environment. A uniform, dark brown patina often indicates natural aging in a relatively clean environment. A patchy, uneven patina suggests exposure to localized contaminants or uneven handling. A greenish patina might indicate the presence of copper carbonates, often seen on coins that have been buried or exposed to damp soil. Identifying these differences can be challenging, but it can be a fascinating exercise in observing the effects of environmental exposure.
Preservation vs. Cleaning: A Crucial Decision
The decision to clean or preserve a penny with a dark patina depends largely on its value and your personal preferences. Cleaning a coin, especially using abrasive methods, can significantly reduce its numismatic value, as it removes the original surface and can introduce scratches. However, for common, low-value pennies, cleaning might be acceptable if the goal is purely aesthetic. Preservation, on the other hand, focuses on stabilizing the patina and preventing further degradation, without necessarily removing it. This often involves storing the coin in a protective environment, such as an air-tight holder, and avoiding handling.
FAQs: Delving Deeper into Penny Patina
Here are some frequently asked questions regarding the dark film found on copper pennies:
FAQ 1: Is the dark film on a penny harmful to my health?
Generally, no. The copper oxides and sulfides that make up the patina are relatively inert and do not pose a significant health risk through casual handling. However, ingesting the film or breathing in dust from cleaning heavily tarnished coins should be avoided. Always wash your hands after handling old coins.
FAQ 2: Can I remove the dark film without damaging the penny?
Yes, but with caution. The gentlest method involves soaking the penny in distilled water with a small amount of mild detergent, followed by gentle rubbing with a soft cloth. For more stubborn tarnish, a brief soak in a solution of vinegar and salt can be effective, but this can also damage the coin if left for too long. Always test any cleaning method on a low-value coin first.
FAQ 3: Does the dark film indicate the penny is valuable?
Not necessarily. The presence of a dark film is primarily an indicator of age and environmental exposure, not intrinsic value. The value of a penny depends on its rarity, condition (excluding excessive cleaning), and historical significance.
FAQ 4: What is the best way to store pennies to prevent them from developing a dark film?
The best way to prevent patina formation is to store pennies in air-tight, inert holders or albums designed for coin storage. These holders protect the coins from exposure to moisture and pollutants. Avoid storing pennies in PVC holders, as they can damage the coin over time.
FAQ 5: Can I use commercial coin cleaners on pennies with a dark film?
Commercial coin cleaners can be effective at removing tarnish, but they often contain harsh chemicals that can damage the coin’s surface. Use them sparingly and always follow the manufacturer’s instructions carefully. Consider the potential impact on the coin’s value before using any commercial cleaner.
FAQ 6: How does the composition of a penny affect patina formation?
Pennies minted before 1982 are primarily copper, while those minted after 1982 are primarily zinc with a thin copper coating. The zinc-core pennies tend to corrode differently and can develop a less desirable, often brittle, patina.
FAQ 7: Is the dark film on a penny considered corrosion or patina?
The term “patina” is generally used when the dark film is relatively stable and provides some protection to the underlying metal. “Corrosion” is often used when the film is unstable and leads to the degradation of the metal. In the case of pennies, the dark film is typically considered patina, but severe cases can be classified as corrosion.
FAQ 8: Can I artificially create a patina on a copper penny?
Yes, several methods can be used to artificially create a patina, including exposing the penny to ammonia fumes or various chemical solutions. However, artificial patinas are often easily distinguishable from natural patinas and may not enhance the coin’s value.
FAQ 9: How do environmental factors impact the color of the patina on a penny?
Environmental factors such as humidity, temperature, and exposure to pollutants significantly influence the color and texture of the patina. Coins exposed to high levels of sulfur dioxide often develop a darker, almost black, patina, while those exposed to damp soil may develop a greenish patina.
FAQ 10: What is the difference between “toning” and “tarnish” on a penny?
“Toning” is a term often used by coin collectors to describe a visually appealing, even patina, usually exhibiting vibrant colors. “Tarnish,” on the other hand, is generally used to describe an unsightly, uneven, or dark patina. The distinction is often subjective and depends on the aesthetic preferences of the collector.
FAQ 11: Should I use baking soda to clean a dark penny?
Baking soda is a mild abrasive that can be used to clean pennies, but it should be used with caution. Vigorous rubbing with baking soda can scratch the coin’s surface. Use it gently and rinse thoroughly.
FAQ 12: How can I tell if the dark film on a penny is just dirt and grime?
If the dark film can be easily removed with mild soap and water, it is likely just dirt and grime. A true patina is chemically bonded to the copper and will require more aggressive cleaning methods to remove. If unsure, start with the gentlest cleaning methods possible.