Since87:
I think it is about 400 C. I took a course at the university about metals. U know hardening and so on, looking on their microstructure (cornsize and stuff). But I'm not sure, and I can't find the book right now. But I think it was 400... Or was it 800??
Anywayz. If someone would want to google this they should searc for a fasediagram for copper. That will show how copper reacts (had or soft, not conductivity though) to heat treatment.
Where did U read about the difference in conductivity for copper and copper?
Edit: I will use my acetylen/air torch. I don't like the idea of putting the copper in charcoal. The copper could take up some of the coal or other stuf and the result would not be as pure copper as before.