Over time air causes the metal to tarnish which is a natural process. You can naturally clean copper with materials usually already found in your home. We'll see a few in this video today. Facebook - https://www.facebook.com/VentunoKitchenTips?fref=ts Twitter - https://twitter.com/kitchentips1 Youtube - https://www.youtube.com/user/VentunoKitchenTips A Ventuno Production http://www.ventunotech.com