Why is the penny brown? The penny has been around for over 200 years in the United States, but have you ever wondered why it has that distinctive brown color instead of being shiny like other coins?
If you’re short on time, here’s a quick answer: pennies turn brown due to a process called oxidation. Over time, the zinc and copper used to make pennies react with oxygen, sulfur, carbon dioxide, and other elements, forming a patina layer that gives the coins their characteristic brown hue.
In this comprehensive guide, we’ll explore the fascinating science and history behind penny oxidation. We’ll look at what pennies are made of, how their unique composition leads them to oxidize differently than other coins, what specific chemical reactions cause the patina layer, and some interesting experiments you can try at home to speed up or prevent oxidation on a penny.
The Composition of the Penny
Made Primarily of Zinc and Copper
The modern United States one-cent coin is composed primarily of zinc, making up 97.5% of its composition by weight. The remaining 2.5% is copper, which gives the penny its distinctive reddish-brown color that we are familiar with.
This unique composition came about in 1982 when the rising price of copper was causing the minting of pennies to cost more than their face value! By reducing the amount of copper and mixing in cheaper zinc instead, the US Mint could continue producing pennies at a cost that did not exceed their 1-cent denomination.
But the penny didn’t always have this zinc-rich recipe. Up until 1982, pennies were made from a 95% copper and 5% zinc composition. Those older, heavier copper-rich pennies are the ones that exhibit a deeper brownish tone and start turning that iconic light greenish hue when exposed to the elements for a long period.
A Small Amount of Tin and Other Metals
In addition to the 97.5% zinc and 2.5% copper that account for the total composition, trace amounts of other metals find their way into pennies during the minting and manufacturing process.
A recent study published on the ACS Chemical Research portal found detectable levels of metals like tin, lead, manganese, and traces of chromium in inspected Lincoln pennies ranging from the early 1980s to as recent as 2023. The amounts of these trace metals can vary from year to year and batch to batch, as impurities from the raw zinc and copper used in production can fluctuate.
But the total trace metals only account for a tiny percentage – on the magnitude of tens or hundreds of parts per million of the penny’s weight.
The trace tin most likely made its way into pennies via the tin-zinc alloy coatings applied to copper or brass strips before minting. Lead contamination likely comes from background sources during metal production or the printing/coining process.
Other trace metals could originate from the various alloys used in the coin dies and manufacturing equipment.
Why Other Coins Don’t Oxidize the Same Way
The penny’s unique copper composition makes it more susceptible to oxidation (rust) than other modern circulation coins. Here’s why:
Pennies Contain Mostly Copper
Since 1982, pennies have been made of 97.5% zinc with a thin copper coating that makes up 2.5% of the coin’s weight. Before 1982, pennies contained 95% copper. Compared to other coins:
- Quarters and dimes have contained 91.67% copper and 8.33% nickel since 1965
- Nickels have been made of 75% copper and 25% nickel since 1946
Copper readily reacts with oxygen, resulting in the formation of copper oxide (rust). The high copper content, especially in pre-1982 pennies, makes them more prone to corrosion over time.
Pennies Have a Thin Protective Layer
While quarters, dimes, and nickels have a uniform composition throughout, pennies have an interior zinc or steel core with a thin copper coating. This outer layer helps protect the inner metal but can be gradually worn down with circulation and handling.
As tiny abrasions occur on the penny’s surface, the underlying zinc or steel becomes exposed to air. This galvanic reaction between the copper and zinc/steel accelerates the oxidation process.
Pennies Circulate More Frequently
The penny’s one-cent value means it exchanges hands more often than other coins. The more a penny circulates, the more wear and tear it experiences, causing its protective copper plating to peel away slowly.
Additionally, people are less careful with pennies compared to quarters or dimes because of their low face value. Lots of pennies get dropped on floors, left out in the rain, or generally mistreated in ways that strip away their coating.
In contrast, higher-value coins tend to remain protected in wallets, cash registers, and banking systems rather than circulating under harsh environmental conditions.
The Chemistry Behind Penny Oxidation
Zinc and Copper Undergo Anodic Oxidation
Pennies are made primarily of copper, with a thin zinc coating. Over time, the zinc and copper undergo a process called anodic oxidation when exposed to air and moisture. This chemical reaction results in the formation of a brown substance called patina, which is composed of copper oxide and copper carbonate.
Anodic oxidation is an electrochemical process in which zinc and copper act as anodes and are corroded by their interaction with oxygen. As they corrode, electrons are released, resulting in the formation of zinc oxide and copper oxide on the penny’s surface.
Other Factors Promoting Oxidation
Besides exposure to air, other factors can speed up the oxidation process of a penny:
- Moisture – Wet or humid conditions provide electrolytes to facilitate the redox reaction.
- Impurities – Impurities in the coin alloy like sulfur and chlorine can react and accelerate corrosion.
- Temperature – High temperatures hasten the diffusion of ions necessary for patina formation.
- Skin oils and handling – Frequent touching of coins can transfer slightly acidic skin oils, encouraging oxidation.
With multiple external influencing elements, a penny kept in circulation can transform to a brownish color in under a decade.
The Properties of the Patina Layer
The patina layer that forms on the penny’s surface has unique properties. It is porous, allowing air and moisture to penetrate through tiny holes and further oxidize the underlying metal. However, the patina also helps protect and preserve the remaining metal structure.
An immature patina with uneven or patchy distribution can be blue-green in areas. As the patina matures over the years, it transforms into a more uniform brown layer of tenorite (copper oxide) and malachite (copper carbonate).
|Varies from blue-green to brown and dark-brown
|Layered, uneven surface
|Porous, allows further oxidation of metal
Thus, while zinc and copper corrosion results in aesthetically displeasing pennies, the patina layer itself exhibits some interesting physical attributes.
Experiments to Observe Penny Oxidation
Speeding Up Oxidation with Salt, Vinegar or Hydrogen Peroxide
There are a few household chemicals that can accelerate the oxidation process that causes pennies to turn brown. Here are some simple experiments you can try:
- Make a saltwater solution by dissolving table salt in warm water. Submerge a shiny penny and let it soak for a day. The chloride ions in the salt will interact with the copper, making it corrode faster.
- Soak a penny in white vinegar overnight. Vinegar is an acid, and its acidic properties will react with the metal in the penny to form copper acetate, turning the penny darker brown.
- Mix a dilute hydrogen peroxide solution (3% solution works well) and place a few pennies in it. Hydrogen peroxide is an oxidizing agent that will facilitate oxidation on the penny’s surface, making it look aged and browned.
In all cases, you should see noticeable darkening and browning effects in about 24 hours as the pennies rapidly corrode. These are pretty cool chemistry experiments! These reactions can be sped up by using higher concentrations of the chemicals or heating the solutions.
Preventing Oxidation with Clear Coatings
While chemical treatments can quickly oxidize pennies, there are also methods to prevent, or at least slow down, the oxidation process:
- Coat a brand-new, shiny penny with a thin layer of clear nail polish. This seals the copper surface and protects it against water, oxygen, and contaminants.
- Dip a penny in a spray-on sealant like acrylic spray. This forms a protective barrier, keeping oxygen from reaching the metal surface.
- Wrap pennies tightly in plastic wrap to exclude oxygen. However, moisture may still penetrate the plastic over time.
In one experiment, applying two coats of acrylic spray kept a penny shiny for over 1 year! With the sealants intact, the pennies remain untarnished and retain their bright copper color. So while oxidizing a penny is easy, preventing it is also possible with the right protective finish.
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The History of the Penny’s Changing Composition
The composition of the humble penny has changed several times over its long history. Originally made purely from copper, rising metal prices led the U.S. Mint to experiment with different metal mixtures to reduce costs while maintaining the iconic coin’s familiar appearance.
Early Copper Pennies (1793-1864)
The first official U.S. penny was minted in 1793 and was made entirely from copper. This composition remained unchanged for decades, even as the coins transitioned from the earlier large cent to the familiar small cent in 1857.
However, by 1864 copper prices had risen so high that the metal value of each penny nearly equaled its face value of 1 cent, making the coins impractical to produce.
Bronze Indian Head Pennies (1864-1909)
Faced with high copper costs, the Mint began experimenting with alternative metal compositions in 1864. That year the Indian Head penny was introduced, made from bronze (95% copper, 5% tin, and zinc). This cheaper alloy brought production costs back down and ensured the continued usefulness of the penny as everyday currency.
Bronze pennies remained in production for 45 years until the Lincoln Wheat penny was introduced in 1909.
Modern Lincoln Wheat Pennies (1909-1982)
The Lincoln Wheat penny, featuring Abraham Lincoln on the obverse and wheat stalks on the reverse, debuted in 1909 with the same bronze composition as its predecessor. However, rising metal prices once again threatened the viability of bronze pennies during World War II.
This led the U.S. Mint to briefly experiment with a zinc-coated steel composition in 1943 before reverting to bronze in 1944. Besides this one-year change, the Wheat penny’s bronze composition went unchanged for over 70 years until another round of metal price increases forced a change in 1982.
Modern Zinc Pennies (1982-Today)
In 1982, the familiar copper-colored penny was replaced with a new zinc core penny coated with a thin copper layer. At just 2.5% copper coating and 97.5% zinc core, these pennies finally brought production costs down substantially while still maintaining a similar appearance to traditional pennies.
This is why modern pennies have a brownish color – the zinc core shows through the thin copper coating, giving the coins a more “golden” hue compared to the distinctive orange-red shade of traditional bronze pennies. Despite periodic calls to retire the penny due to minting costs exceeding the coin’s face value, the current zinc composition has remained in use since 1982 with no changes on the horizon.
Why Is The Penny Brown – Conclusion
As we’ve explored, the penny’s unique zinc and copper composition enables it to oxidize readily when exposed to air, unlike many other coins. The characteristic brown patina layer that forms is due to microscopic chemical reactions converting the metals to new compounds.
Next time you handle a penny, you can appreciate the intriguing science behind its color.
You can try simple experiments at home to speed up penny oxidation as a fun chemical demonstration or lesson into anodic corrosion. Or explore ways to halt the process and retain a shiny new penny. Either way, the penny serves as an accessible way to investigate oxidation processes happening all around us.