Last week I opened a new packet of Art Clay copper, removed a bit to work with, and as quick as I could I wrapped the remaining clay tightly in cellophane, and placed this inside a plastic zip-lock packet. A week later, I opened up the packet, to discover what looks like dark grey oxidised patches on it. I proceeded to work with the clay and shaped it into a pendant. The dried clay still shows this dark staining. Now I am a little worried about firing it, in case there is some unexpected reaction. Any help anyone?!