Why Is the Statue of Liberty Green?

The iconic blue-green of the Statue of Liberty

The Statue of Liberty was reddish gold when it was new. Over time, the copper oxidized to form green verdigris.
The Statue of Liberty was reddish gold when it was new. Over time, the copper oxidized to form green verdigris. Kathleen Campbell / Getty Images

The Statue of Liberty is a famous landmark with an iconic blue-green color. However, it wasn't always green. When the Statue was unveiled in 1886, it was a shiny brown color, like a penny. By 1906, the color had changed to green. The reason the Statue of Liberty changed colors is because the outer surface is covered with hundreds of thin copper sheets. Copper reacts with the air to form a patina or verdigris.

The verdigris layer protects the underlying metal from corrosion and degradation, which is why copper, brass, and bronze sculptures are so durable.

Chemical Reactions That Make the Statue of Liberty Green

Most people know copper reacts with air to form verdigris, but the Statue of Liberty is its own special color because of its unique environmental conditions. It's not a simple single reaction between copper and oxygen to produce a green oxide, like you might think. The copper oxide continues to react to make copper carbonates, copper sulfide, and copper sulfate.

There are three main compounds that form the blue-green patina: Cu4SO4(OH)6 (green); Cu2CO3(OH)2 (green); and Cu3(CO3)2(OH)2 (blue). Here's what happens:

Initially, copper reacts with oxygen from the air in an oxidation-reduction or redox reaction. Copper donates electrons to oxygen, which oxidizes the copper and reduces the oxygen:

2Cu + O2 → Cu2O (pink or red)

Then the copper(I) oxide continues to react with oxygen to form copper oxide (CuO):

2Cu2O + O2 → 4CuO (black)

At the time the Statue of Liberty was built, the air contained a lot of sulfur from air pollution produced by burning coal:

Cu + S → 4CuS (black)

The CuS reacts with carbon dioxide (CO2) from the air and hydroxide ions (OH-) from water vapor to form three compounds:

2CuO + CO2 + H2O → Cu2CO3(OH)2 (green)

3CuO + 2CO2 + H2O → Cu3(CO3)2(OH)2 (blue)

4CuO + SO3 +3H2O → Cu4SO4(OH)6 (green)

The speed at which the patina develops (20 years, in the case of the Statue of Liberty) and color depends on the humidity and air pollution, not just the presence of oxygen and carbon dioxide. Patina develops and evolves over time. Nearly all the copper in the Statue is still the original metal, so the verdigris has been developing for over 130 years.

Simple Patina Experiment With Pennies

You can simulate the patination of the Statue of Liberty. You don't even need to wait 20 years to see results. You will need:

  • copper pennies (or any copper, brass, or bronze metal)
  • vinegar (dilute acetic acid)
  • salt (sodium chloride)
  1. Mix together about a teaspoon of salt and 50 milliliters of vinegar in a small bowl. The exact measurements aren't important.
  2. Dip half of the coin or other copper-based object into the mixture. Observe the results. If the coin was dull, the half you dipped should now be shiny.
  3. Place the coin in the liquid and let it sit for 5-10 minutes. It should be very shiny. Why? The acetic acid from the vinegar and sodium chloride (salt) reacted to form sodium acetate and hydrogen chloride (hydrochloric acid). The acid removed the existing oxide layer. This is how the Statue may have appeared when it was new.
  1. Yet, chemical reactions are still happening. Don't rinse off the salt and vinegar coin. Let it dry naturally and observe it the next day. Do you see the green patina forming? The oxygen and water vapor in the air are reacting with the copper to form verdigris.

Note: A similar set of chemical reactions causes copper, brass, and bronze jewelry to turn your skin green or black!

Painting the Statue of Liberty?

When the Statue first turned green, people in authority decided it should be painted. The New York newspapers printed stories about the project in 1906, leading to a public outcry. A Times reporter interviewed a copper and bronze manufacturer, asking whether he thought the statue should be repainted. The company's vice president said painting was unnecessary, since the patina protects the metal, and that such an act might be considered vandalism.

Although painting the Statue of Liberty has been suggested several times over the years, it has not been done. However, the torch, which was originally copper, corroded after a renovation to install windows. In the 1980s, the original torch was cut away and replaced with one coated with gold leaf.