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Why Do Pennies Turn Brown?

Have you ever wondered “Why do pennies turn brown?” Pennies turn brown because the copper in them oxidizes, or tarnishes. Read on to learn about the composition of pennies, copper, and copper oxide and why these things make pennies turn brown.

Why Do Pennies Turn Brown?

Here are the compositions of pennies since they were first minted in 1793:

  • From 1793 to 1837, pennies were pure copper.
  • From 1837 to 1857, pennies were bronze with 95 percent copper and 5 percent tin and zinc.
  • From 1857 to 1864, pennies were 88 percent copper and 12 percent nickel.
  • From 1864 to 1962, pennies were again bronze.
  • In the year 1943, pennies were zinc-coated steel.
  • From 1962 to 1982, pennies were 95 percent copper and 5 percent zinc (the tin was removed from the alloy).
  • From 1982 to the present, pennies are 97.5 percent zinc with 2.5 percent copper (in the coating).

So the answer to the question “Why do pennies turn brown?” is that the copper becomes oxidized. This tarnish can look red, orange, or brown. Pure copper is actually more of a peach color and changes as oxidation occurs. Tarnish is not the same as rust, however. When the top layers of a metal, like copper, brass, silver, or aluminum tarnish, the tarnish forms a protective layer and no more oxidation occurs.

What is Copper?

Copper is a soft, malleable metal with the chemical symbol “Cu.” Its name is "cuprum" in Latin. It conducts electricity and heat very well, so it is used as an electrical and thermal conductor and in alloys, building materials, and to make pennies.

Copper is essential in all plants and animals. It helps your body use iron, so a copper deficiency can manifest anemia-like symptoms. A deficiency can cause impaired growth, bone abnormalities, higher incidences of infections, and problems with cholesterol and glucose metabolism. However, too much copper in your body will lead to toxicity, which can lead to liver problems.

Copper has antibacterial, germicidal, and antimicrobial properties. Anything made of brass, like doorknobs, will rid itself of bacteria within eight hours. Therefore, making doorknobs and surfaces with copper alloys in hospitals and public facilities will reduce the transmission of germs.

What is Copper Oxide?

Copper oxide forms when copper is exposed to oxygen. There are two types of copper oxide: copper I oxide, also called cuprous oxide, and copper II oxide, also called cupric oxide. They can both be used as semiconductors and pigments, but they are slightly different.

Copper I oxide, or cuprous oxide, is reddish in color and is made artificially. It is used in marine paint, as a fungicide, and as a pigment. If you inhale it, it can cause shortness of breath, coughing, and serious respiratory tract problems. If eaten, it can irritate the gastrointestinal tract and cause vomiting, pain, and diarrhea.

Copper II oxide, or cupric oxide, is blackish in color and is used as a pigment in clay glazes. You can get it in red, blue, green, gray, pink, or black. It is used to make rayon and is given to copper deficient animals. It is also toxic just like Copper I oxide. It can also cause problems with vision and skin discoloration. Both of these oxides can cause metal fume fever, which has flu-like symptoms.

So, copper is essential in our bodies but can be toxic in large quantities. Copper oxide is very useful, but should not be ingested or inhaled. In summary, the answer to the question, “Why do pennies turn brown?” is that the very reactive metal copper oxidizes when it is exposed to oxygen and changes color from a pinky peach color to reddish brown.