The iconic copper-plated zinc penny has gone through several color changes over its long history, but today most people are familiar with its distinctive brown hue. But why exactly is the penny brown instead of the shinier copper color it once was? The transition to a brown penny is the result of a natural process that occurs as the penny’s copper surface reacts with oxygen and other elements in the environment.
A Brief History of the Penny’s Changing Colors
Up until the mid-19th century, pennies were made entirely from copper. The pure copper pennies minted from 1793 to 1837 were a shiny orange-red color when new. Over time, as the copper interacted with oxygen and moisture in the air, the pennies would darken to a deeper reddish-brown color known as verdigris.
In 1837, the mint changed the penny’s composition to bronze (an alloy of copper, tin and zinc), giving the coins a yellowish appearance when first struck. The bronze pennies gradually oxidized as well, turning a chocolate brown color over time.
The Lincoln penny was introduced in 1909, made from 95% copper and 5% zinc and tin. This updated bronze alloy initially gave the pennies a lighter golden shade that continued to oxidize to a dark brown with circulation.
By the early 1980s, rising copper prices meant the intrinsic value of a penny was actually higher than its 1-cent face value. So in 1982, the mint changed the penny’s composition to 97.5% zinc with a 2.5% copper plating. These are the copper-plated zinc pennies we use today, identifiable by their lighter coppery shine when newly minted.
Why New Pennies Turn Brown
Brand new pennies have a bright, copper-like color from their thin copper plating. But it doesn’t take long for pennies to start turning darker, eventually becoming the familiar brown shade we expect from pocket change.
This process occurs through a chemical reaction called oxidation, which happens as the metal in the penny’s surface reacts with oxygen and humidity in the air. Oxygen molecules bond with the copper atoms, creating a layer of copper oxide on the surface that shields the rest of the penny from further corrosion.
Copper oxide has a brownish or reddish-brown color different from the metallic orange shade of pure copper. As more and more copper oxide builds up on the surface of a penny over months and years of circulation, the penny takes on the distinctive brown patina we’re accustomed to.
Why Pennies Turn Brown So Fast
Pure copper metal has a slower oxidation rate, so a solid copper penny could stay vibrant orange for years before darkening. The modern zinc and copper penny oxidizes more rapidly for a few key reasons:
- The thin copper plating exposes more surface area to oxygen.
- Zinc acts as an electrolyte, encouraging the copper ions to dissolve and react.
- Impurities in the zinc like iron contribute to galvanic corrosion.
With just 2.5% copper cladding over a zinc core, U.S. pennies have a high ratio of surface area to volume. This allows oxygen to penetrate the thin cladding faster, spreading the copper oxide film more quickly.
The zinc core also behaves as an electrolyte once the plating is compromised, loosening copper ions from the copper cladding and transporting them to the surface where they can oxidize faster.
Finally, the zinc penny alloy isn’t 100% pure. Small amounts of iron in the zinc act as an impurity that sets up galvanic corrosion between the zinc and copper. This electrochemical reaction generates hydrogen peroxide and other oxidizing agents that further accelerate the breakdown of copper.
How Circulation Affects Penny Color
Coins that get minimal use might take years to acquire a dark patina. But a penny that circulates widely will rub against other coins, surfaces, and materials, scraping off some of the outermost layer of copper oxide. This abrasion then exposes more of the copper underneath to oxidation, causing the penny to brown faster.
Pennies also encounter moisture, sweat, and oils from people’s hands that can soak into the surface, increasing the oxidation rate of the exposed copper. So a well-traveled penny ages much more quickly than one tucked away in a coin collection.
Other Factors in Penny Color
The basic copper oxidation process accounts for much of the color change in pennies, but some other factors can also affect the shade:
- Water: Exposure to water causes more rapid oxidation and a darker patina.
- Humidity: Moist, humid environments create thicker copper oxide layers.
- Temperature: Heat accelerates the rate of oxidation and corrosion.
- Air pollution: Sulfur dioxide in the air reacts with copper to form copper sulfate, a dark gray-black coating.
Areas with soft water, heavy rainfall, high humidity, and pollution tend to produce pennies with darker patinas. Dry climates and indoor conditions slow down the development of brown oxidation and patina on pennies.
Cleaning Pennies to Remove Oxidation
Since a brown patina builds up slowly over time, the only way to restore a penny’s shiny copper color is to remove the layer of copper oxide. This can be done through abrasive scrubbing, chemical cleaning, or electrochemical reduction.
Here are some common ways to clean oxidation off pennies:
- Soaking in vinegar – The acidic vinegar dissolves copper oxide.
- Rubbing with ketchup – Contains acids that react with copper oxide.
- Baking soda toothpaste – Gentle abrasive action removes oxidation.
- Distilled white vinegar soak – Removes oxidation and inhibits further darkening.
- Electrolytic cleaning – Applies low voltage current in saltwater to reverse oxidation electrochemically.
However, any cleaned pennies will quickly start to regrow copper oxide within hours or days of polishing. Oxidation can be slowed by coating the pennies in sealants or protective lacquers but not prevented indefinitely.
Historical Snapshot: Penny Colors by Year
Here’s a quick overview of the color progression of U.S. pennies from when they were first minted to now:
| Year | Composition | Initial Color | Oxidized Color |
|---|---|---|---|
| 1793-1837 | 100% copper | Orange-red | Reddish-brown |
| 1837-1864 | Bronze (Cu-Sn-Zn) | Yellowish | Chocolate brown |
| 1864-1962 | 95% copper 5% tin and zinc |
Golden | Dark brown |
| 1962-1982 | 95% copper 5% zinc |
Golden | Dark brown |
| 1982-present | 97.5% zinc 2.5% copper plating |
Coppery | Light brown |
The Future of Penny Colors
With inflation steadily reducing the purchasing power of the penny, the U.S. Mint has considered transitioning to cheaper steel pennies zinc-plated in copper. However, steel pennies were unpopular when tested in 1943 and raised concerns that people would mistake them for dimes.
If pennies do eventually become steel, they would likely start off with a copper-like shine before becoming more gray and rusted as the plating wore off. The longevity of zinc/copper pennies depends on whether rising metal prices make them too expensive to keep producing.
For now, though, the familiar copper-plated zinc penny will continue to circulate. New pennies will enter the system bright and shiny, only to quickly transition to a duller and darker brown as they undergo oxidation and abrasion during everyday use. The pennies acquire a darker patina as years pass, serving as small symbols of persistence and gradual change.
Conclusion
The iconic penny remains an everyday reminder of the inexorable passage of time. While new pennies shine bright coppery red, within months that lustre fades to earthy brown. This ongoing colour shift stems from the natural chemical reaction between the penny’s zinc and copper metals and the oxygen and chemicals that surround it. The penny’s changing hue serves as a mundane example of science at work and the physical evolution of objects over time. For now, the familiar brown penny will continue to circulate as a cultural touchstone, but its composition and colour may change if inflation eventually makes the penny itself obsolete. One thing is certain though: new pennies will always transition from shiny to brown, reflecting the unstoppable forces that shape and transform the world around us.
