I do a lot of Silver to gold soldering, usually Fine Silver to 22k
yellow gold. I learned to ALWAYS use gold solder. For the very reason
you found to be a problem. The silver solder melts into the gold if
you overheat ever so slightly. I don't know enough about metallurgy
to give you the scientific explanation. I just know what happens from
years of observation and practice. I trust other Orchid members will
write in to enlighten us!
Yes it seems to cost more to use the gold solder but not in the long
run since you won't ruin your pieces.