What turns a penny green?

Have you ever noticed that some pennies look green instead of their usual copper color? This green patina, as it’s called, is a natural process that occurs as pennies oxidize and react with the environment over time. But what causes this transformation, and why do some pennies turn green while others retain their original shine? In this article, we’ll explore the chemistry behind green pennies and look at the different factors that contribute to verdigris.

The Composition of Pennies

First, it helps to understand what pennies are made of. Modern United States pennies are made primarily of zinc with a thin copper coating. Here’s a breakdown of the metals:

  • Zinc core – 97.5% of the penny’s weight
  • Copper plating – 2.5% of the penny’s weight

Older pennies used to contain more copper – 95% copper and 5% zinc – but the rising price of copper led the US Mint to change the penny’s composition in 1982. Now, with mostly zinc on the inside, the thin layer of copper coating is what gives pennies their distinctive orange-red hue.

Why Copper Turns Green

The copper in pennies is key to the green patina process. When exposed to moisture and oxygen, copper undergoes a chemical reaction called oxidation, which results in the formation of a greenish layer called copper carbonate.

Here’s the basic reaction:

Copper + Carbon Dioxide + Water -> Copper Carbonate (green patina)

The copper carbonate, known as copper(II) carbonate or cupric carbonate, builds up as a thin film on the surface of the copper. This layer of corrosion is essentially copper rust, and it’s the same process that causes the green patina to form on copper roofs and copper statues.

Factors That Cause Pennies to Turn Green

So what factors speed up the oxidation process and cause pennies to turn green more quickly? Here are some of the main contributing factors:


Moisture provides the water necessary for copper to react and form copper carbonate. Pennies exposed to damp conditions, whether from rain, humidity or spilled liquids, will tend to develop verdigris more quickly than pennies in dry environments.


Access to air causes copper to corrode faster. Pennies stored in sealed containers away from oxygen tend to retain their orange-red color longer. In contrast, pennies left out in the open air oxidize more rapidly.


Impurities in the copper can accelerate the patina process. Trace amounts of sulfur, chlorides and other substances interact with copper to speed up corrosion.

Chemical Pollutants

Environmental pollutants like sulfur dioxide and carbon dioxide mix readily with moisture to produce corrosive acids that eat away at copper. Areas with more automobile exhaust and industrial pollution often see pennies turn green faster.


Certain bacteria feed on copper, producing chemical byproducts that lead to green patina. For example, Pseudomonas aeruginosa secretes gluconic acid that reacts with copper and forms copper gluconate – a blue-green patina.

Saltwater Exposure

Chlorides in ocean water are very corrosive to copper. Seawater accelerates the patination process, so pennies recovered from shipwrecks often have thick green corrosion.


Pennies exposed to heat, whether from sunlight, radiators or other sources, experience increased oxidation rates. The warmth provides energy to power the chemical reactions.


As pennies circulate and get handled, the copper surface abrades, exposing fresh reactive copper. Fingerprints and oils from skin also help trap moisture against the copper.

Length of Circulation

Older pennies have been exposed to the elements longer, giving more time for patina to accumulate. New pennies directly from the Mint may take months or years before signs of green corrosion appear.

The Patination Process Up Close

Now let’s take a microscopic look at the patination process and see how the green corrosion forms on the surface of the copper:

  1. Oxygen and carbon dioxide from the atmosphere start reacting with the exposed copper metal, forming a thin layer of copper carbonate crystals.
  2. The copper carbonate layer is initially very patchy, with some areas thicker than others.
  3. Moisture helps the corrosion spread laterally across the surface, filling in the gaps to produce an even patina layer.
  4. Over time, the layer becomes thicker and denser as more copper carbonate precipitates out of the reactions.
  5. The mature patina may be smooth and uniform, or have a gritty texture depending on the crystal structure.
  6. Once most of the surface is covered, the oxidation rate slows but doesn’t completely stop.

The end result is a dull green film coating the penny, protecting the underlying copper metal from further corrosion. But if the patina layer gets scratched or damaged, the process will start again with the exposed copper.

Does Copper Patina Continue Forever?

Given unlimited time, the patination process would eventually consume all the available copper metal, turning it completely green. However, the corrosion rate slows over time. Chemical reactants have a harder time penetrating the patina layer, and fewer copper atoms get exposed to continue the reactions.

In most real-world situations, the patina layer reaches an equilibrium thickness. Corrosion still occurs, but atoms from the copper core migrate outward at the same rate that surface copper reacts away. This balanced process can maintain a steady green patina layer for decades or centuries.

On a shorter human timescale, a penny’s patina reaches a point where the corrosion is barely perceptible. So while patination doesn’t ever fully stop, it does become negligible after a green coating firmly establishes itself.

What Are the Different Shades of Green?

Not all green patinas have the same coloration. Pennies can develop patinas ranging from yellow-green to deep forest green. What accounts for these differences?

The basic chemical composition is the same (copper carbonate), but subtle variations in the microstructure and thickness of the patina layer can alter the colors. Here are some of the factors that affect the shades of green:

  • Crystal structure – The size, shape and orientation of the copper carbonate crystals affect how light reflects off the surface.
  • Trace elements – Impurities like chlorides influence the patina’s hue.
  • Layer density – More compact patinas appear darker green.
  • Layer thickness – Thin patinas often look lighter green.
  • Viewing angle – The patina can look different depending on the angle it’s viewed from.

So the green shades provide clues about the pennies’ corrosion history and environment. A bright green patina typically signals a thin, porous layer, while a deep forest green patina usually indicates an older, thicker, denser layer.

What Other Colors Can Pennies Develop?

While green patinas are most common, pennies can also develop other colored corrosion layers, including:

Brown Pennies

Brown patinas result from copper reacting with sulfur compounds in the air. The copper sulfide corrosion is a chocolate brown color.

White/Blue Pennies

These odd colors happen when pennies are exposed to excessive moisture. The copper reacts to form copper hydroxides and copper carbonates.

Pink Pennies

A red or pink discoloration can indicate an early stage of copper corrosion. Thin, patchy areas of copper oxide give the pinkish tone.

Orange Pennies

Orange patches show areas where copper is just starting to oxidize but hasn’t yet formed the green patina.

These unusual penny colors are less common than green patinas but demonstrate the range of chemical reactions copper can undergo.

What Accelerates the Patination Process?

If you have a shiny new penny and want to speed up the patination process, here are a few tips:

  • Use a saltwater solution – Soaking pennies in salt water for hours or days encourages rapid corrosion.
  • Apply heat – Heating pennies with a blowdryer or other heat source accelerates oxidation.
  • Increase surface area – Scuffing or scratching the surface gives more surface for reactions.
  • Coat with acetate – Brushing pennies with acetic acid introduces more carbon dioxide.
  • Bury in soil – The moisture and impurities in soil facilitate patination.
  • Expose to ammonia – Ammonia fumes create copper compounds that appear green.

Intentional patination like this is sometimes done by crafters and artists seeking an aged, weathered green penny appearance quickly. But it’s also interesting for understanding the corrosion process.

Why Are Some Pennies More Prone to Tarnishing?

While all pennies will eventually exhibit patina given enough time, some seem to turn green much faster than others. Why is that?

Here are a few reasons why certain pennies seem more prone to tarnishing:

  • Manufacturing variations – Impurities and surface defects during minting
  • Circulation differences – Some pennies get handled more than others
  • Environmental factors – Local conditions like coastal humidity
  • Storage conditions – Open air versus airtight containers

In general, pennies with more imperfections and greater exposure to moisture, contaminants and oxidation catalysts will tend to patina quickest. Pristine pennies stored in ideal conditions could remain lustrous for decades.

How to Remove Green Patina from Pennies

What if you have a green penny but prefer the original copper color? Here are a few methods to reverse patination and restore pennies’ orange glint:

Salt and Vinegar

Soak pennies for a few hours in a solution of salt and vinegar. The acetic acid in vinegar dissolves copper carbonate while the salt acts as a gentle abrasive.

Lemon Juice or Lime Juice

Similarly, citric acid sources like lemon or lime juice can dissolve patinas. Let pennies soak for 6-12 hours.

Cola or Coke

The phosphoric acid in cola removes copper corrosion. Place pennies in a bowl of cola for several hours.

Rub with Ketchup

The acids from tomato ketchup help wipe away green patinas. Rub a dab of ketchup on the penny using a cotton swab.

Baking Soda and Water Paste

Gently rub a wet paste of baking soda on pennies to mechanically scrub off oxidation.


Using a small electrical current, electrolysis removes copper corrosion and restores the metal beneath. This is the most thorough method but requires specialized equipment.

After cleaning, store your shiny pennies in an airtight container to prevent re-tarnishing.

Frequently Asked Questions

Does a penny’s green color add value?

No, the green patina that forms on pennies is considered damage, not an added value. Collectors generally want copper coins to retain their original red color. However, an unusual coloration like deep blue-green may interest some collectors.

Can you eat green patina off pennies?

No, you should avoid intentionally eating or swallowing patina off pennies. The copper carbonate is not highly toxic, but pennies also contain potentially harmful metals like zinc and nickel that could be ingested from old patinas.

Does patina protect pennies from further corrosion?

To some degree, yes. The patina layer acts as a barrier that shields the copper surface from moisture, oxygen, and pollutants. This passive film slows down the oxidation rate once it forms. But patinas are not impervious to further corrosion over time.

How long does it take for a penny to turn green?

A new shiny penny can start developing light green spots within 6 months to a year. More noticeable dark green patinas often take 2-3 years to form. Thick, even patinas may take 10-20 years to develop. Factors like environment and handling affect timescales.

Can you patinate a penny without oxygen?

No, oxygen is required for the basic copper carbonate patina to form. Without oxygen, pennies are unlikely to corrode and turn green. Other compounds could theoretically form in an oxygen-free environment but likely not a green patina.

Why do some old pennies not turn green?

Some conditions prevent patina formation on old pennies. Storage in airtight containers limits oxygen exposure. Pure water or distilled water washing prevents patinas. And certain chemical coatings can shield the copper surface.


While a green patina may not be desired on rare coins, it provides some visual interest and historical charm on ordinary pocket change. As a natural indicator of copper slowly returning to its oxidized state, green pennies offer a glimpse of chemistry in action.

The next time you get a tarnished penny in your change, consider its corrosion coat an echo of the coin’s journey. The various colors, textures and patternsetched into the copper each have a story to tell if you know what to look for. With a basic understanding of the patination process, you’ll see each green penny as a unique memento, transformed by time and the elements.

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