For all of you who are new to working with metal clay, there is a
wonderful Yahoo! group dedicated to the use of both brands of metal
clay, very similar to this list. (metalclay) There are experts and
new users and some of the original experimenters and experts who
participate. Over 3000 members. Tim Mc Creight has even been known
to write an answer occasionally.
To answer your question, the dark color you are seeing is just the
silver particles oxidizing. There is no problem firing dried pieces
that have been “hanging out” for a long period of time. If you want
to carve into it to re-work it you can certainly do that. By now all
of the excess water has probably evaporated and the clay has begun
to shrink and get really hard, so it might be a bit difficult, but go
slowly and you’ll be fine. If you hate it and want to re constitute
it, you can do that as well. There’s a great you tube video on re
constituting dry clay.
PMCC Senior Instructor