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The Healthy Green Hue of Liberty

Why is the Statue of Liberty green? Learn more on this Moment of Science.

Statue of Liberty on Liberty Island

Photo: wwarby (flickr)

Lady Liberty has only eroded five percent due to her "patina," or green outer shell

“Give me your tired, your poor, your huddled masses yearning to breathe free.”

Lady Liberty stands over three hundred feet above the waters outside New York harbor with her tablet, her torch, and her unlikely green complexion. The tablet and torch seem appropriate enough, but why is the Statue of Liberty green?

The Statue of Liberty is green because its outer shell is made of copper. While Lady Liberty looks solid enough, she is actually hollow inside. When she was constructed over a century ago, a sheet of copper about a tenth of an inch thick was hammered out by hand and attached to a framework of steel supports. This thin sheet of copper is all that keeps Liberty looking like a lady, not a skeleton. The green covering on this copper skin helps to protect it, preventing the copper from eroding away.

The green stuff is called a “patina.” You can think of this as a kind of rust that forms when copper is exposed to the air for a long time. Copper atoms combine with oxygen and carbon or sulfur from the air to form a thin, green protective coating that is not soluble in water. Thanks to her healthy, green patina, Lady Liberty’s skin has eroded only five percent in her first century.

This green patina is so important to the Statue of Liberty’s health that when the statue was restored in 1986, workers sprayed parts of it with corroded copper particles from a dismantled copper roof. They did this to keep the statue as green as possible, to prolong its life for centuries to come.

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