I weld to the copper so I have not had to deal with the solder
problem you talk about, but I would try some “Anti-Heat” or some
similar product from a welding store, they protect at higher
temp. than yellow ochre. For larger copper pieces I heat them in a
reducing atmosphere. An atmospheric gas forge works best (no
blower). Some of the Blacksmithing forges are nice, there are
probably plans for one at the Art Metal site. I have used
potters and glass blowers kilns with success. Your piece will
come out black after it air cools. Polish off the black with
bobbin or emery compound and the dark or ruby red will be
revealed.
Douglas Salmon