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

burning coal:

__Cu + __S → __CuS (black)

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

three compounds:

__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.