Hi this is in reference to the disappearing diamonds fired in Metal
Clay…
I have been trying to find a warning that we had as Metal Clay
artists, but to no avail tonight. Used in the metal clay item was a
bar - from an old ring - with three diamonds set in holes - after
firing the holes were empty. At one time we were advised of the fact
that diamonds would not withstand the length of heating in the Kiln.
I have seen jewelers repairing worn tips holding diamonds, and was
amazed at the temp they could take, but I don’t believe it was
prolonged as to 30 minutes.
My only experience with it - couldn’t take the chance again - the
student was very kind and didn’t sue me!!!
Maybe one of the PMC gurus can elaborate on this situation.
When soldering or casting diamonds in place, the high temperatures
are only held for short periods of time in a reduction atmosphere.
When firing diamonds in PMC Standard, the temperature is held at
1650 for 2 hours in an oxygen rich environment. Diamonds do not
survive this–they burn up in the kiln. When using the 3rd generation
of metal clays (the low fire clays) and firing at a 10 minute firing
schedule, reports indicate that diamonds will survive–whether the
people reporting this have examined the diamonds closely for damage
is unclear.
with regards to your post referring to diamonds completely
disappearing after being baked in PMC.
I have limited knowledge of chemistry but have read a little on this
subject.
You may be interested to know that chemists and jewellers have
recorded this happening since 1690.
In 1796 the mystery was solved by a British scientist, Smithson
Tennant. It turns out that diamonds (a rare, transparent form of
carbon) when burned with a free supply of oxygen will be completely
converted into carbon dioxide leaving no trace of the diamond…
Since diamonds are pure carbon, my guess is that because of the high
temperature and prolonged time, they simply burned up since oxygen
was present to combine with the carbon.
Very interesting. I had a batch of faceted cubic zirconia come out
of the kiln in lovely round cabachons. I have never had that happen
with cubic zirconia–before or since and I do a lot of PMC with
dichroic and faceted CZ. I noted from the import location that they
were from China.
I bought them in earring settings, on a closeout sale at a Kreske or
Woolworth store ??, as CZ in Sterling post earrings. Possibly you
didn’t have diamonds, as you thought, as I most certainly didn’t
have CZ as I thought.
When I first began years ago, I was very interested in casting. I
cast everything. I often cast diamonds and sapphires in place. This
was before the industry wide practice… There were times that I
forgot to leave a hole behind the stone in the wax. What this didn’t
allow was the investment to flow behind the stone and support it when
the wax burned away. Without that little island of investment, the
stone fell into the mold chamber and then became imbedded in the body
of the object. I cut apart some of the rings and found the stones
inside. It would actually be a pretty great project…
This is interesting in the context of the concept of “strength of
materials”. The Nobel Prize winning scientist who got the prize for
his work on Bucky C said this was the strongest material in the
universe. But there are varying definitions of strength. Strong at
what temperature? Stength as in scratch-resistance in diamond? Yes,
but where do diamonds rate if shatter-resistance is the measure of
strength? (not a rhetorical question). Is jadeite more
shatter-resistant than diamond?
Diamonds are the hardest known natural substance on Earth but I
think you may have hardness and toughness confused.
No gemstone can withstand the blow of a hammer. It is toughness that
gives a gem material the ability to hold up against flaking,
chipping, cleaving and fracturing.
When talking about hardness (the ability of a stone to be
scratched) is not the same as durability (the ability of a gemstone
to withstand chemicals and chipping. At Mohs hardness of 10, a
diamond is the hardest material on Earth, but it is not the most
durable. Jadeite has exceptional durability, but only fair hardness.
put a diamond in an oven and heat it at 1405 f/h or 763 c the diamond
will evaprate and release small amount of carbon dioxide. from this
finding the the methode of manufacturing synthetic diamond chemical
vapour deposition
put a diamond in an oven and heat it at 1405 f/h or 763 c the diamond
will evaprate and release small amount of carbon dioxide. from this
finding the the methode of manufacturing synthetic diamond chemical
vapour deposition.
CVD is done under high vacuum which changes the temperature that
carbon will evaporate. At room temperature carbon will stay as a
solid well past 1000 deg C so setting them in PMC shouldnt be a
problem. I suspect that there is a quite simple answer to this, the
PMC has either coated the stones or shrunk so they have popped out.
If the diamonds had flaws in them they may have simply shattered at
the tiny gas bubbles expanded in the heat. The gemmological
Association of Great Britain did a couple of articles on heat
treated gemstones a while back that are worth reading.
I have enveloped diamonds and sapphire in molten glass without any
problems, the stones being small enough (2mm) that heat transfer
didint cause any real problems but I would be careful of doing the
same with bigger stones. The ideal gemstone for PMC is synthetic
sapphire.
Hans…ref your experiment in kiln firing and torch firing the
diamond.
These were 3 small diamonds set in a bar, fired in the Precious
Metal Clay at 1650 degrees for 2 hours. They disappeared!!! I really
have enjoyed reading all the posts about diamonds/carbon dioxide/etc.
I just don’t experiment anymore!
Happy Thanksgiving - we are safe, housed, warm, and all the
adjectives that describe our lives in the USA…be thankful
The reports of the diamonds burning up in PMC were fired at 1650F
(900C) for 2 hours. This is the firing schedule for the origninal
(Standard) PMC. The question is not whether the temperature alone is
an issue, but the amount of time spent at temperature. How long did
you leave the diamond at 900C?
The reports of the diamonds burning up in PMC were fired at 1650F
(900C) for 2 hours. This is the firing schedule for the origninal
(Standard) PMC. The question is not whether the temperature alone
is an issue, but the amount of time spent at temperature. How long
did you leave the diamond at 900C?
The time spent at 900C was indeed two hours. I have since corrected
my web for that omission. Thank you for pointing it out. I added two
more pictures of the diamond after being heated up for five minutes
at an elevated temperature. These suggest that temperature does play
a significant role.
Also,for the greater good of all PMC artists trying to create fine
jewellery, I cooked the diamond at 900C under a vacuum for 45
minutes. Good news! Nothing happened.
So I still am of the opinion that, even though it’s not very clever
to heat diamonds to 900C, it is doubtful they would actually
disappear completely. Damaged, yes. Gone? No.
The reports of the diamonds burning up in PMC were fired at 1650F
(900C) for 2 hours. This is the firing schedule for the origninal
(Standard) PMC. The question is not whether the temperature alone
is an issue, but the amount of time spent at temperature. How long
did you leave the diamond at 900C?
At that temperature, if there’s any oxygen around, it would take
very little time at all (seconds, not minutes) for the surface of the
diamond to become damaged, looking frosted and no longer well
polished. Even if it’s not fired long enough for the whole thing to
disappear, it might as well do so, at least with small stones, since
you still have to pull it out and, if large enough to warrant the
cost, have it repolished. Any working “repair” jeweler who’s
neglected to get his diamonds clean enough, or covered with boric
acid firecoat before soldering on new prong tips, which is usually
done at considerably lower temperatures than 1650F, will have had
this happen, and most such people quickly learn that it’s a risky
idea to let the diamonds get hot enough to be incandescent
(glowing). At 1650, a diamond is glowing rather brightly, and burning
up equally quickly, if exposed to oxygen.
Hi Mary Ellen - this was early on in the PMC thing. At the time only
had the Standard PMC. The diamonds were in the kiln for the 2 hours
at 1650.
It’s amazing what we all have learned about PMC in the past 10
years. I wasn’t a pioneer, but took Tim McCreight’s first class at
Catalog in Motion in Tucson - probably 8 years ago! I consistently
use it in my Silver/Copper/Brass fabrication. Great addition. I met
you at the PMC Conference in Albuquerque and took your class in the
enamelling.
So I still am of the opinion that, even though it's not very
clever to heat diamonds to 900C, it is doubtful they would
actually disappear completely. Damaged, yes. Gone? No.
This is a very interesting mystery. First of all, didn’t you say
that the diamond was in fact NOT damaged at all by two hours at
900C? The pictures of damage at 2800C are really interesting–
thanks for sacrificing the diamond!
So, in re the original loss of the stones in the bar in the PMC-- we
seem to have no choice but to conclude that they were in fact not
diamonds, but clever simulants. People have been fooled before, and
simulants have been around a long time (I think I recall that they
were from a vintage piece). No one wants to believe that their
diamonds are not real, but it seems to be the only explanation that
encompasses all the info we have.
Wait, now-- what metal was the bar they were set in? If it was not
gold or plat, that would support the idea that they were not real
diamonds.
I am one of those PMC instructors (and here I state it publicly) who
described for her students how to fire diamonds (sapphires, rubies,
too) along with PMC (as I had been instructed). This was years ago,
when only PMC Standard, the original formula, was available.
Therefore, the required temp was 1650 degrees F for the required
duration, 2 hours.
Although I had great success firing small sapphires with PMC, only
one of my students arrived with a diamond she wanted to fire. I
always added the caveat Re: cracks in stones, inclusions, etc. and
she was willing to take the chance that the stone might be damaged
by the process.
The result of that firing was…the stone was GONE! No pieces, no
melted blob, no apparent dust. I have to say that the student was
absolutely certain that the stone was a diamond and, thanks to my
caveats, she was OK with the diamond being gone. Whew! There, I’ve
cleared my desk of that mess!
Personally, I don’t really care what the diamond ‘became’ in the
process (CO2, fairy dust, etc.). I have never suggested, since that
episode, to any of my classes that diamonds can be fired with
PMC…period.
For an excellent list of stones that survive firing in PMC and it’s
several formulae, check out Kevin Whitmore’s list somewhere on .