Repair jewelers are frequently called upon to solder close in to a set
gemstone. Besides the normal dangers of heating a gem in these days of
glass infills and other challenges it behooves a jeweler to keep a gemstone
cool during soldering if at all possible. Remember Some 'Old-time' repair goldsmiths pride themselves on their speed using a
mini-torch and they hold the set gem on a ring being sized in their
fingers, going in very rapidly on the shank solder join with a pinpoint
flame, so fast and hot that "if your fingers get hot you were too slow".
While this method works with the right mini-torch and has a certain macho
charm it does not fit every circumstance - and may lead to singed fingers.
To avoid endangering the stone while soldering it is often packed in some
cooling or refractive material, Kool Jool® and similar proprietary
products, wet matted paper towel and so on to protect it. Another material
used to protect gems is paper clay, a product found in many craft shops and
originating in Japan. In my experience wet paper towel wadded up around the
stone is the best of these because there is good control of its degree of
wetness and hence the safety of the stone. I use a plant mister to refresh
the moisture on the wet paper towel during the soldering operation. Gold
objects are reasonably easy to deal with in terms of heat conductivity but
silver (which has the best heat conductivity of all the metals) can be a
real problem to deal with. This is why many jewelers will simply unset a
silver ring before soldering it. Silver conducts heat so fast that it is a
good idea to use the hottest torch flame available (i.e. an oxy-acetylene,
oxy-hydrogen or oxy-propane), have the gem itself immersed in water during
the soldering and to have a flame reflector positioned above the water line
to increase the heat on the join. I place the ring head with the gemstone
under water in a crucible while the ring is held down with a soldering
weight. Using a jewelers saw one cuts off a thin slice of soft refractive
kiln brick which is tucked inside the ring above the water level to
additionally reflect heat upwards and slow the evaporation of the water.
Keep tabs on the water level so as not to have it inadvertently go too low
and endanger the stone.
A method of protecting gemstones from heat comes from the shop of Robert
and Barbara Kaylor in Boise, Idaho. They use a crucible with lead shot in
it to hold the ring steady for soldering. When the ring is positioned
correctly in the shot water is added to the crucible to cover and protect
the gem. It is important to monitor the water level while heating to ensure
it does not go too low, as if it does one has a nasty lead mess on the
ring, not to mention potential stone damage. Stainless steel shot would be
a good alternative to lead shot, no health problems and no danger of
accidentally melting on your piece.
A really excellent way of doing repairs without heat endangering gems is to
use a laser welder; clean, fast, almost any material: and still too
expensive for me to have one of my very own. There are ever more shops have
one though, and for specialty repairs it can be worth sending a piece out
for service repair with a laser welder. I have also used a miniature fusion
welder to 'spot weld' a failed seam on a 24k karat bezel around a piece of
amber. I attached an electrode of 24k gold, dialed up the power to 'high'
and proceeded to fuse the seam closed. A little clean up required, but it
neatly solved my problem without damaging the amber.Heat and Gems In repair shops diamonds are routinely soldered against in re-tipping
operations. Rumor has it that diamonds are subject to heat damage
cumulatively, that after twenty minutes or so cumulative high temperature
time they can begin to suffer from work such as soldering; so that after
numerous repairs on a piece one might have to consider this as a potential
problem (don't know whether this is true or not-just a story I heard).
Glass in-filling, which can be found in diamonds and corundum will not work
well when heated. Corundum is often heated during repairs but it is
susceptible to borax flux damage at high temperatures. Synthetic corundum,
synthetic spinels and garnet are quite good with heat. Every so often a
garnet will turn to a small black magnetic lump. In all cases with heat
avoid rapid temperature changes-very jeweler has at least one bad story
about this. Heat will cause colour changes in certain stones. If liquid or
gas filled inclusions are present (beryl for example) then heat may cause
expansion of the gas or liquid and lead to stone breakage. Many stones have
been treated or enhanced and are thus unsuitable for heating; 'stabilized'
materials, dyed stones etc. It might be noted that in the field of SIP
(stone-in-place casting) diamonds, cubics, and surprizing things like
peridot are being successfully cast in place now.