Thank you all for your replies. I take it that diamonds are not
particularly fragile to pressure when red hot.
Noted from your replies:- extreme heat will etch diamonds, some
alloys are hot-short, soldered seams will present a problem, and cold
tweezers applied to a hot diamond can produce thermal shock. The
precaution of “no harm” is excellent advice.
With the above in mind I weigh the risk of replacing one or two
small diamonds if they crack under pressure when hot, against the
risk of applying much more pressure to the diamonds when cold; and
having to bend the metal some way past the yield point when cold.
The most common type of ring that I encounter needing hot bending is
made from well cast 18ct white gold with diamonds close-set around
the circumference. This particular alloy is very tough and springy
having a bronze tint which needs rhodium plating after any working.
There is no hot-shortness but the springy property means that bending
the ring smaller is (in my experience) best done while hot. I heat
the ring while in the tweezers so they heat up together; being
carefull to judge the points of pressure and the hottest zone where
bending will happen first so that the gap closes accurately and
roundness of the ring is maintained as much as possible. Some of the
diamonds need tightening in their settings afterwards but that is
normal when sizing down.
The most recent ring that I bent while hot was an 18ct yellow gold
band 10mm wide and 2mm thick, three sizes down to ‘K’. It had white
gold bars along the top providing channels for large diamonds, and
pave set diamonds covering the rest of the yellow gold. There was no
possibility of hammering the ring for minor adjustment or of
stretching it to roundness. I closed the gap by hot bending keeping a
close watch for any sign of easy solder on the channels. The ring was
well made and the gap closed without problems. If there had been easy
solder the job would have been more expensive, but it was going to
get done one way or another.