On stones with points that are set in certain ways, sometimes the
points will break from the pressure of the cooling metal. If you
have a marquis set into a massive setting and solder on it, it can
happen that it will come out of the pickle broken just from the
contraction of the metal on cooling.
A little clarification on how this is possible may help. Diamond
expands less upon heating than does the gold, so while it’s all
heated to soldering temps, the diamond is loose in the "expanded"
setting. When everything cools, both will regain their original
dimensions. If the diamond remains in the same position as everything
cools, all is well. But if the diamond shifted even a little while
things were hot and the diamond was slightly loose, if it gets
caught there and cannot return to it’s original seated position as
the metal cools, then you can end up with a whole lot of pressure
being applied by the contracting metal because the points on the
stone are no longer in their seats.
The other situation that happens, and this may be even more common,
occurs mostly when retipping prongs on these stones. As noted above,
the head and prongs tend to expand more than the stone upon heating,
so any gaps between stone and metal in or near the seat become
larger. If the solder you are using to solder on new tips flows down
into the seat or that gap, then as the head cools, there is now less
space, and the nicely fitted seat no longer fits the point, being
now filled with solder. That, just as with the shifted stone, is a
recipe for disaster.
The first situation is avoided in part by simply not being too
aggressive and over heating the whole thing. Work where you need to,
not overly heating the whole assembly. Also, use enough boric acid
fire coat on the diamond. In addtion to providing protection against
burning the diamond, it also provides a bit of a fluid layer the
diamond floats in. As the setting expands, this tends to keep the
diamond from shifting too much out of position, and aids it in
regaining it’s proper position as the metal cools. Also, while
working, simply pay attention to the stone. If it shifts, you can see
it do so, and move it back if you need to before the metal cools down
The second situation is avoided also by working “small”, to keep
thermal expansion of the whole setting to a minimum, so gaps don’t
open up. but mostly, it’s avoided by careful torch control and use
of the right amount of solder so you don’t have lots of extra solder
flowing where you don’t want it to. Even if the stone is not
initially damaged from solder flowing down into a seat, it can
increase the pressure of the setting, making it uneven and less
secure. That can increase the chances of the stone being
accidentally chipped at a later date, either by the customer, or some
subsequent unlucky jeweler.
Hope that helps.