I’ve not used 12K gold, but I solder 14K onto silver often. Yes
I heat from the bottom, that is the silver. Heating from the
top, that is the gold, first will certainly melt the gold if you
are not careful, actually you have to be careful no matter what,
taking the flame away as soon as the solder starts to flow. Even
if I am soldering gold onto gold, ie., the shot onto the top of
the leaves, I heat from the bottom and then bring the flame to
the top at the very last second so that I don’t over heat the
gold leaf laying on the silver to the “puddling” stage. Believe
me I’ve made lots of puddles of the gold into the silver.
you can see an example of what I am talking about on my page:
Jan McClellan - just over the mountain from you in Oregon.