When copper reacts with oxygen it produces dark tarnish and when it reacts with water it produces green tarnish. This tarnish is actually the copper material breaking down and is evidenced by a green powdery substance on the copper. It is why many outdoor, copper statues have a green hue. Remove tarnish as soon as possible to prevent it from wearing out prematurely. Use common ingredients that you may already have in your home to clean the green copper tarnish. Equal parts of salt, vinegar & flour made into a paste, applied and rinsed away, will do the job. Use this process only on untreated copper as treated copper often has an antique patina that can be ruined by excessive scrubbing. If you would like to retain the original look, please apply a Clear Coat spray (available at most hardware stores). Apply in a very light coat to avoid "drips", allow to dry and apply another light coat.