Golovko:
Yep.. that sounds right. I remember talking about 400 degrees when we looket at copper in metalurgy class (only basics)
Ok... So U say that thermal conductivity increases with the grainsize. Nice to have that settled out.
It will work to heat copper with a acetylene/oxygen torch. I've done it. The copperplate went from rock hard (some spooky alloy) to soft as a kitten

.
If I remeber correct... As long as U heat the copper more than say the 400 degrees then there is no upper roof for the temperature. As long as U let it cool at a slow rate U will get big corns???
What I'm trying to say is that one dont have to be so precasious about heating it to much???