The #6 tip on the Smith Little Torch was just too big so I will
have to try again with a tiny tip (#0) to see if I can direct the
heat solely on the joint. The problem is that these rings are quite
small - I wrapped them around a # 6.5 Pepe dowel. I'm guessing that
it is going to be nearly impossible to get the solder to flow
without overheating the rest of the piece.
The number 0 tip is likely too small. Even with oxy acetylene, a #0
is difficult to light. You don't often need a flame around a
millimeter or two long(pretty much pinhead sized), which is about
what that tip will give you. try a #2 or #3, and adjust for a
suitably small flame, with suitably low regulator pressure settings.
But the problem is likely not really your torch size, but the oxy
acetylene itself. That's a very hot flame when adjusted right. Use
less oxy than you normally would so the flame has a bit of yellow
still to it. very soft and reducing. That will help with getting the
metal too hot and damaging the gold layer. And then the real problem
with things getting too hot, isn't even the torch. It's how hot you
need it to get to melt your solder. What grade of solder are you
using? You should be using a very low temp gold solder. There are
gold solders made for gold fill. They equate to an extra easy low
karat (6K seems common with some of these), or perhaps 10 K grade.
Generally, they'll be melting lower than your usual easy solders for
gold or silver. If you see the metal even glowing much, it's too hot.
The idea with this type of metal, unlike solid gold and silver, is to
pretty much heat just the solder and joint, not the surrounding
metal. when the joint gets hot enough, the already hot solder will
flow in. And for the record, a 6.5 mm mandrel isn't that small.
Small might be winding.4mm wire around a 1 mm or less mandrel. Those
sizes get a little more tricky... :-) But if you're winding that same
small size wire (a quarter to a half millimter diameter wire), then
it's the wire size that would make it "small", not the mandrel. Like
in soldering gold, but unlike silver, with gold fill you only need to
be heating the joint itself. There's no need for anything but the
joint area itself to be hot for the solder to flow correctly. In
fact, you really don't want anything other than the joint itself to
get hot. So as you've already figured out, a smaller torch flame may
be better than a big one.
But what you do with it is more important than the flame size. Try
working with the flame glancing over the joint from a position above
the inside of the ring,(if the ring is flat/horizontal on a soldering
block, or a similar position if the ring is otherwise
held/positioned) with the flame heading then past/glancing over the
joint, and away from the ring. That way, the ring itself isn't in the
flame or it's backwash, at all. Just the joint.
Hope that helps