The Statue of Liberty is a landmark every American recognizes. The Statue of Liberty easily identified by its height, stance, and unique blue-green color. When this statue first delivered from France, its appearance was not green. It was brown, the color of its copper “skin.” So how did the Statue of Liberty change colors?

Answer: The change in appearance was a direct result of corrosion. The copper that is the primary component of the statue slowly underwent oxidation from the air. The oxidation-reduction reactions of copper metal in the environment occur in several steps. Copper metal oxidized to copper (I) oxide (Cu2O), which is red and then to copper (II) oxide, which is black

2Cu_(s) + 2O_2(g) ⟶ Cu2O_(s) (red)

Cu₂O_(s) + 2O_2(g) ⟶ 2CuO_(s) (black)

Coal, which was often high in sulfur, was burned extensively in the early part of the last century. As a result, atmospheric sulfur trioxide, carbon dioxide, and water all reacted with the

2CuO_(s) + CO_2(g) + H₂O_(l) ⟶ Cu₂CO₃(OH)_2(s) (green)

3CuO_(s) + 2CO_2(g) + H2O_(l) ⟶ Cu₂(CO3)₂(OH)_2(s) (blue)

4CuO_(s) + SO3_(g) + 3H2O_(l) ⟶ Cu₄SO₄(OH)_6(s) (green)

These three compounds are responsible for the characteristic blue-green patina seen on the Statue of Liberty (and other outdoor copper structures). Fortunately, the formation of patina creates a protective layer on the copper surface, preventing further corrosion of the underlying copper. The formation of the protective layer is called passivation, a phenomenon discussed further in another chapter of this text.