What caused the color change in the Statue of Liberty?

The color change in the Statue of Liberty from its original reddish-brown color to the current green patina is due to a process called oxidation. The statue is made primarily out of copper, and when copper is exposed to the elements, it reacts with oxygen in the air. This reaction forms copper oxide, which is a dull brown color. Over time, as the copper oxide continues to react with carbon dioxide and moisture in the air, it forms copper carbonate, which gives the statue its green patina. This process is also known as weathering and is a natural occurrence for copper when it is left outdoors. The green patina actually serves to protect the underlying metal from further corrosion.

Copper is a malleable and ductile metal with very high thermal and electrical conductivity. When new, copper has a bright metallic luster. When left exposed to the atmosphere, it undergoes a series of chemical reactions. With exposure to air and moisture, such as rain or humidity, copper forms a stable and protective layer that is a mix of hydroxide and carbonate as mentioned, called patina.

The Statue of Liberty, officially named Liberty Enlightening the World, was a gift from France to the United States as a symbol of freedom and democracy. It was dedicated on October 28, 1886, and was made with sheets of copper hung over a framework of steel supports. Since copper is a reactive metal, the patina formation is normal and expected to occur in such exposed conditions.

The patina is highly resistant to corrosion, which is why copper, brass (an alloy of copper and zinc), and bronze (an alloy of copper and tin) have been used for centuries in architecture, art, and coinage. While some historical structures and artworks are cleaned or polished to maintain a shiny copper appearance, many are left to develop a patina because of the protective qualities it provides, as well as the aesthetic appeal of the green color.