There has been a lot of experimentation with this, and the concensus
is that you need a two-stage firing system to fully sinter the
CopprClay from Rio.
I suggest the following schedule: 1st phase - open shelf, 480F to
555F with a 1 hour hold. 2nd phase - in carbon, full to 1650F with a
3.5 hour hold. I also allow my clay to cool in the kiln overnight
and remove it in the morning. That keeps any oxidation problems away.
You may also want to look into the new Art Clay product, Art Clay
Copper. ACC can be kiln fired, but can also be torch fired, and needs
less time to sinter completely.
Sandra Graves, Isis Rising