Copper naturally turns green over time as it reacts with Oxygen in the air. This chemical reaction is known as oxidation. The Oxygen and some water in the atmosphere react with the surface of the copper. That causes the copper and Oxygen atoms to bond to create copper oxide. That is the green or blue coating on copper. Over time, the layer gets thicker and thicker! The Statue of Liberty in New York is a great example of copper oxidation! The statue was originally made of copper, but the Oxygen in the air and the water around it caused oxidation to happen, which turned the statue green!