How strong are diamonds when red hot?

When sizing a ring that is heavy in construction and covered in
diamonds for most of the circumference, bending the ring to a smaller
size is a problem. There is no place to apply real pressure other
than on a diamond. My solution has been to bend the ring red-hot
using the pressure of a pair of strong tweezers.

When red-hot there is no spring in the metal and the gap is closed
perfectly. Applying pressure to some of the diamonds when red-hot
has not hurt them so far, but does anyone here know for sure if I am
pushing my luck? Are diamonds more fragile when they are red-hot?

Regards, Alastair

Hello Ala,

There is a chance that you can burn the diamond when you heat it up
that high. The surface may become a frosted appearance. There is
also the risk of thermal shock esecially if the diamond comes in
contact with a liquid and if there is a feather within the stone. I
have seen this happen while cutting diamonds when the aid of a borax
solution is dripped onto a redhot diamond (The borax prevents the
surface of the diamond from burning, specifically the large table
facet, while blocking.)


Jeff Nechka (G.G.)
Premier Gems LTD

Are diamonds more fragile when they are red-hot? 

Though they’re pretty tough, if they’ve got serious inclusions, you
could be risking damage when a cold tool chills a hot diamond. That’s
still a slim chance, though. You’re biggest risk is burning the
diamonds. Red hot is usually too hot for diamonds. Be sure they’re
very very clean first, and coated with a substantial layer of boric
acid or other fire coat so no oxygen can reach them, or you’ll frost
them. And remember that metal expands when hot, so the hot settings
may have expanded enough so the diamonds can slip out of place. If
that happens, and isn’t corrected before the ring cools, the cooling
metal can put a LOT of stress on the diamond, sometimes shattering
it. I doubt there’s much actual difference in durability when the
stones are hot, but the operation of heating the ring and bending
while hot does present some unusual risks you need to be prepared


I don’t know the direct answer to your question but…I give you
great applause for your fortitude!

There are dangers in what you describe, total collapse being one. Any
discontinuity in the structure of the metal could concentrate?stress
when its that hot.Burnt stones another. Your tweezers wil; act as a
heat sink and you may wind up pouring too much heat on. You’ve done
OK so far but you may be pushing your luck.

When?I approach a ticklish job I take the view “First, do no harm”.
That’s just self preservation. I’d rather go through extraordinary
lengths to avoid a costly mistake, both in terms of financial loss
to me and potentially devastating loss of face.

But as to sizing a ring like you mention. What I have done is after
cutting out the piece for sizing gently and I mean gently tap the
ring with a leather mallet (with ring??resting on wood with a
mandrel inserted. I position the ring?on the size marking a little
smaller than my goal, this allows for spring but prevents going too
far and collapsing the ring)?on either side of the cut to draw the
ends together. This won’t work on every ring. For example something
thoroughly pave’d will have a bazillion holes drilled and the
mounting is already weak. Channels are not so bad if it has
adequately heavy and consistant?azuring.

Depending on circumstances sometimes you can make an insert to bring
it to size. Or shots. Or springs. Or grafting on a superfit or

But sometimes its wise to politely say no after explaining the risks
to the customer’s property.

Applying pressure to some of the diamonds when red-hot has not hurt
them so far, but does anyone here know for sure if I am pushing my

Alistaire, I’m sure you don’t really mean “red” hot, because if you do
you ARE pushing your luck in terms of burning the stones. Truly “red”
hot is getting right up to the burn zone, even with a flux coating.
And once it’s done, it’s done… Otherwise, I wouldn’t worry too much
about mechanical breakage of the diamonds. What I would worry about is
called “hot short (ness)”, which is brittleness in hot metal. If
you’re not very, very careful the metal can crack and even break
completely when bending it hot - and the hotter it is, the more it’s a
concern. Also the more you bend it. I push hot metal, too, but there
comes a point where it moves and leaves huge cracks where the stress
was, or at the hottest point. Do-able, yes, but be careful with that.

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.

Regards, Alastair