Someone I know purchased a bracelet from an art gallery - it was silver and copper. She told me that the first time she wore the bracelet it got wet and the parts that were copper turned black. Why would this happen?
Exposure to hydrogen sulphide (sulphur) can cause copper to oxidize and turn black. Healthy human sweat can contain sulphur, and moisture is also a contributor to color change. Copper oxidizes very quickly and the oxidation can usually be removed with heavy polishing.
To prevent oxidation, components can be sealed with a brush-on or spray-on fixative. Both Mona Lisa™ Metal Leaf™ top coat sealant and SpectraFix™ are good options.