I have played with heating up stones for many years in my kiln. It is
untrue that you need ‘gasses’ for color changing stones. I have
heated, over the years, with some success, tanzanite, aquamarine,
amethyst, and recently tourmaline in my normal casting kiln.
I also have heated many other stones with spectacularly bad results.
Amongst others, larimar, (went white) lavulite (went brown) smoky
quartz (trashed.) Aqua (white), citrine to try and make it lighter
Just a few weeks ago I took some dark olive green tourmaline and in
a fit of lethargy put some in the oven… I think it is Zimbabwean
material and it had already been very badly cut. (faceted) It was
lousy stuff, completely un- sellable. Complete stinkbug stones. The
material is clean though. That’s best to put in an oven. Clean mat=
erial. Although, I have put stuff in the oven that had fractures and
did not get more badly fractured. (even though the color was not
So this is how I got it to a much better color in the tourmaline. So
far I took a stainless steel egg cup and filled it up with ‘satin
cast’ casting powder. Same stuff you cast jewellery with. Then I took
a tourmaline and cleaned it with alcohol and pushed it to the middle
of the egg-cup. Tapped it down firmly and put it into my oven.
300C. Leave it at the temperature for about an hour ( it is only an
egg cup), then I take the egg cup out of the oven and let it cool
outside until room temp. I don’t care for this million hour cooling
down bit, because I am only testing, and the material sucks, so I can
afford to trash it.
400C Same as above, no change
500C Some change, (yay!) a little bit lighter. I use a control stone
of the same original colour to check. Then I go up into 50C
increments At 550C was my best color, noticeably lighter. I went to
650 with this stone, and there was no more color change so I called
it quits. At least it will be sellable after have I recut it. With
disclosure, of course.
Then I took another one and did the same thing some days later. This
time there was no change in colour at all. This is typical. Even from
a similar source, or parcel from the same mine, stones react
differently to heat.
So I decided to increase the temperature in hundred degrees until
something DID happen. This is not something I would normally do, but
just wanted to see how high I could take a heat sensitive stone.
At 700C, nothing had happened to the dark olive green colour. And I
was not even careful to cool it down other that putting the egg
cupout side the oven 750C Nothing. (Most low RI stones don’t go
without major damage…) I certainly did not think a tourmaline would
go so high.
At 800c it died. It became brown and looks a bit like petrified wood
Ok, so that’s extreme. I have never heated tourmaline to that
temperature and I wanted to see how high it would go…
Most stones I have played with are in the 300 to 550 maximum Some
work, most do not. But that does not stop me playing around. About
years ago I heated tanzanite rough to 400-450C. Of the three stones,
two went better and one died. I think I used building sand in a
crucible then. In a controlled situation, I will let the oven cool
down to room temperature before I take the stone out. This cycle can
take many days, so it is good to record what you do. Sometimes, I
put a piece of stone into my casting cycle, just to see what happens.
I take it out before I go too high, like over 550C.
A lot of technical can be found on the web, especially
in related fields like synthetics although specific is
scarce. (Temperatures, time etc)
One future test I want to do is take some clear enamel (frit) and
embed some fractured Tanzanian ruby I got in a melting crucible and
heat the hell out of it, just for fun
A book I would love to own is Kurt Nassau ’ Gemstone Enhancement. History, Science and State of the Art. At $250 it is a bit expensive!
It is available on Amazon, though.
And no, I don’t want to ‘cheat’ anyone. I am just interested in the
Cheers Hans Meevis