Art Clay Copper - staining?

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?!

I found this https://www.cooltools.us/Articles.asp?ID=158, if you scroll down to the bottom it advises on oxidation. I know it is about bronze clay but assume because of the heavy copper percentage in bronze the advice would be the same.

Oh, thank you loobyvee....that's a really interesting article, and am reassured about my stains :-)