The Statue of Liberty Name: ______________
by Anne Marie Helmenstine, Ph.D. Updated November 21, 2016
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:
__Cu + __O2 __Cu2O (pink or red)
Then the copper(I) oxide continues to react with oxygen to form copper oxide (CuO):
__Cu2O + __O2 __CuO (black)
At the time the Statue of Liberty was built, the air contained a lot of sulfur from air pollution produced by
__Cu + __S __CuS (black)
The CuS reacts with carbon dioxide (CO2) from the air and hydroxide ions (OH-) from water vapor to form
__CuO + __CO2 + __H2O __Cu2CO3(OH)2 (green)
__CuO + __CO2 + __H2O __Cu3(CO3)2(OH)2 (blue)
__CuO + __SO3 +__H2O __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. https://www.thoughtco.com/anne-marie-helmenstine-ph-d-601916https://www.thoughtco.com/copper-facts-chemical-and-physical-properties-606521https://www.thoughtco.com/brass-composition-and-properties-603729https://www.thoughtco.com/bronze-composition-and-properties-603730https://www.thoughtco.com/oxidation-reduction-reactions-604037