Copper is a naturally occurring uncompounded pinkish/peachy colored metal. Because it is ductile and malleable, it is an ideal element for decorative art like coins and sculpture. That Lincoln penny in your pocket is made of copper as is the Statue of Liberty.
Indeed the Statue of Liberty was first dedicated October 28, 1886, its structure was reddish, orangish, brownish. Over the years tarnish and oxides changed the color of that 179,200 pounds of copper to its green verdigris layer.
What happened is that copper skin reacted with atmospheric oxygen and formed a layer of brown-black copper oxide. That in time was replaced by cuprous and cupric sulfide. Finally it turned to copper carbonate, the green patina now covering it.
Many wonder why when the Statue of Liberty was being restored for America’s bicentennial, why the vertigris was not removed from the old construction through thermal heating services.
Could it have been done? Certainly since thermal oxidizer service does exist. However, other factors must be considered. The Statue of Liberty is a national treasure, a national monument. Vertigris provides good corrosion resistance. Attempting to remove it is costly and can weaken the copper skin. Best not to challenge something only to mess with a good thing….