I’m just curious. This is a good metal alloys issue.
Did you mix your own red alloy? This sounds like an alloying issue
with the red gold and not soldering technique.
I just thought of something. Was the crucible you melted the red
gold in ever used for melting silicon gold (the gold that casts up
without oxidation)? It’s just a thought, as silicon increases the
grain size, and makes for gold that can crack while being worked. I
personally have experienced issues with silicon gold such that I
prefer to keep it out of my shop entirely.
I’ve done a lot of soldering with various colored golds and haven’t
seen this problem, although I certainly have seen some weird results
with cu/au combinations, like a high karat red gold that was so hard
when I dropped the ring on concrete it broke into many pieces! Copper
has problems with oxygen, it absorbs about 20 times it’s own weight
in oxygen when molten IIRC, so some zinc is usually used as a
deoxidizer. Perhaps an alloy with the addition of a small amount of
grain refiner would help as well.