Copper Plating Information
Copper will corrode if not sealed from the elements. Sometimes corrosion is aesthetically interesting and many people use chemicals to intentionally patina the copper on their piece. If you want shiny copper, it must be sealed.
For information on the process of copper plating please click on the links below.
General Overview of process
https://en.wikipedia.org/wiki/Electrolysis
Copper specific process
https://en.wikipedia.org/wiki/Copper_plating
Simplified Overview
https://www.youtube.com/watch?v=FnJ0V7B7nKo