Exterior coating of Statue of Liberty is made up of copper metal which protects the statue. Copper can easily react with other elements. The Statue of Liberty is green in color because a process called oxidation has happened.
In this process copper reacts with oxygen in the air and forms a pinkish oxide compound or we can say cuprites which oxidizes more and form a blackish oxide compound called tenorite or sometimes it forms copper sulphide, a blackish compound. This black compound again reacts in the moisture with CO2 and sulfur dioxide present in the air and finally makes â€œpatinaâ€ which is green colored coating. Furthermore this layer protects the underlying copper and prevents it from more oxidation. The patina that is formed contains three things i.e. brochantite, malachite, azurite. The process of oxidation
or formation of patina is based on the amount of humidity and pollution in the air.
If we analyze the statue more carefully then we will come to know that only .005 inch of oxidation has taken place in a century by the regular action of rain, wind, sun etc. that is why copper is the material in the statue which did not require renovation for centennial. Only the torch of statue which is also made up of copper was built again and was patinated so that it can match with the color of copper in the statue.
Another example of copper oxidation is to look at the pennies that had remained in the water for a particular time. They will also turn to green color. These pennies are not pure copper still they turn green but Statue of Liberty is of copper so it definitely oxidizes.