You’ve made sure the joints have a good fit, are clean, and are well
fluxed. I’m assuming you’re using silver solder that you’ve used on
other silver projects, so there should be no questions about the
solder’s quality. All standard silver solders will solder gold with
no problems (I’m disregarding the karating issue, since we’re talking
about a bimetallic piece).
After those issues, the most common mistake is in how you heat the
joint. In an assembly made of all one type of metal (ie. silver),
when you heat the part, everything should come up to soldering
temperature evenly. (I’m intentionally overlooking those situations
where there are large differences in mass on the parts being
joined…that’s for another post). This is because the metal on each
side of the joint will absorb heat at the same rate. The part will be
very forgiving of your torch technique.
When you work with dissimilar metals, each metal will absorb heat at
different rates. Pure silver has the highest thermal conductivity of
any metal, and sterling silver is not that far behind. There is
quite a big difference in the thermal conductivity of sterling sliver
and 14karat gold (anyone have numbers on this???). What this means
for you is that when you are heating your part, the silver is
absorbing heat at a faster rate than the gold bezel will absorb it.
When this happens, it is common for the silver to reach soldering
temperature before the gold will. Solder will melt and flow to the
hottest area. Therefore if the silver portion comes up to temperature
first, the solder will melt and adhere to the silver, and not to the
gold. If this is the case, when you examine a failed solder joint,
you should see the solder on the silver portion, and not on the gold.
The remedy to this is careful torch control. You should focus more
heat on the gold portion of the joint. The ‘splash’ from the flame
will also heat the silver at the same time. Additionally, the silver
will absorb a great deal of heat from contact with the gold at the
solder joint. By focusing the heat on the gold, you will ensure that
the gold comes up to soldering temperature. When you observe the
solder beginning to melt, you can move the torch to give additional
heat to the silver where needed to ensure that the solder flows
completely on both sides of the joint.
It is possible to heat the silver first and then move the heat to
the gold, but you risk running the solder over the surface of the
silver (and essentially using it all up) before you bring the gold to
Hopefully this helps a bit!