Technical and not-so-technical questions about heating

[evergreen]

I've got a bunch of questions about heating colored stones, some of which probably have been answered in older threads (but the Google search on here just doesn't work for me). Any help -- answers, or threads where these have maybe been done to death -- would be much appreciated.

1. Are there any good sources for before-and-after shots of heated gems? Particularly corundum, but really any CS; I'd just like to know more about how they "change". How predictable is the color change?

2. I think I've seen that zircon usually goes brown/red-->blue, tanzanite brown-->blue, apatite yellow-->blue... any corrections to that basic algorithm? I haven't figured out a pattern for corundum's color change yet, but I'd be interested to know what the "input" and "output" colors tend to be.

3. How do you know how hot, and how long, to heat for? Can you over-heat a gem?

4. I've seen people mention "basic heat" as a treatment -- are there any less-basic types of heat? Like... EXTREME heat, or something?

5. Are stones sometimes heated because of inclusions? Which inclusions improve with heating?

6. What inclusions or stone characteristics suggest a stone that's vulnerable to cracking when heated?

7. Are there any other perils for stones besides cracking or turning yucky colors when they're heated?

8. Are there any post-heating changes that are NOT permanent?

9. What other things do lapidarists think about when heating a stone?

10. Have you ever heard of someone buying an unheated stone and heating it themselves? Or sending it off to be heated? Those who are experienced in CS, have you ever been tempted by something you thought could improve with heating, or are virtually all potentially-improved stones already heated by the time we consumers see them?

I think that's enough for now... Thanks in advance!

 

[chrono]

You are asking too many questions!

First off, many of the heating temperatures are secret recipes and requires a proper oven, not a candle or kitchen oven. If done incorrectly, the stone can end up colourless! There needs to be a proper controlled heating and cooling process. I think colour of corundum starts to change around the 600 degrees Celcius mark.

"Low" heat is under 1000 degrees Celcius
"High" heat around 1800 degrees Celcius

Aquamarine: 450 degrees Celcius
Amethyst to Citrine:: 450 degrees Celcius
Reddish Brown Zircon to Blue: 1000 degrees C in a reducing (oxygen free) environment
Reddish Brown Zircon to Colorless 900 degrees C in air
Orange Beryl to Morganite: 400 degrees C
Yellow and Brownish yellow topaz to Pink: 550 degrees C
Topaz (all colors) to Colorless:1000 degrees C
Dark Green Namibian Tourmaline will brighten Significantly:370 degrees C
Brownish Green Zoisite to Tanzanite:370 degrees C
Sapphire containing titanium to develop or brighten color:1600 degrees C

Sometimes, inclusions can "heal" with heat whilst others fracture and end up looking worse than before. Corundum and spinel can take higher temperatures and the appearance of inclusions can be lessened with heat. Tourmaline cannot take as high a temperature and can break into pieces if the inclusions fracture.

I recall Pandora and LD trying "chicken kunzite" using their home ovens.

 

[evergreen]

Thanks, Chrono! I hadn't realized some of this might be secret information; I've just got a science/research background and I can't help myself. Aren't you glad I limited myself to ten questions? And I'm note even posting any followup questions... yet.

It sounds like there is some similarity with ceramics firing; low-temperature kilns operate around 800-1000C, and high-fire goes up to 1300C. A home kiln is something I want to do when I'm retired (y'know, in 40 years or so)... noted.

"Chicken kunzite" is exactly the sort of thing I'd love to play with in my spare time!! For those following along at home -- MUCH easier to get good Google results on PS with "chicken kunzite" than "heating", incidentally -- [URL='https://www.pricescope.com/community/threads/chicken-kunzite.153988/']https://www.pricescope.com/community/threads/chicken-kunzite.153988/[/URL] . LOVE this, also the TONS of science that follows in discussion, but not sure if the overarching conclusion was that I shouldn't bother buying & baking some kunzite!

 

[pregcurious]

I recognize the pink tourmaline in the white diamond halo on his webpage as a PSer's ring.

 

[chrono]

I recalled Gene sharing a little heating recipe and found it this morning. Note the overbaked tourmalines too.
[URL='https://www.pricescope.com/community/threads/heating-of-corundum-an-investment.181271/page-5']https://www.pricescope.com/community/threads/heating-of-corundum-an-investment.181271/page-5[/URL]

Quote:
have found that all the zircons seem to heat a bit different. This could be due to coming from different mine area's. Most of the stones I have worked with are from Tanzania and Nigeria. I have had no luck with Nigerian stones. They will heat to a nice color, but then over the course of a few hours drift right back to an ugly brown.

What I do with the Tanzanian honey brown stones is this. Heat them over the flame, kind of turning them and wiggling them like you would a test tube back in high school chemistry lab. Normally the stones will start going to a deep burnt umber color. This happens pretty quickly, maybe 4 to 6 minutes of heating. Once the go darker, they will quickly start to come lighter. Take them off the heat once this starts. If you heat them until they are the lighter color yellow you are after, you have gone too far, and the stones will continue to turn light and loose too much color. Take them off the heat early, and they will continue to turn. Once they cool they will look fantastic. Now, over the course of a day or so, they may or may not drift back toward the honey brown color. If this happens, repeat the heating. There is almost always some drifting back.

I have found that the red stones generally take longer to heat as they need a higher temperature. Sometimes I'll heat the red stones in my oven. The temperature I use for them is 842 deg F. The yellow stones, if I cook them in the oven are usually around 675 F. It's best to start at lower temperatures, as you can always re-heat. Pretty much any zircon can be cooked to be colorless if you go too high. I have taken some of the ugly brown stones from Nigeria, and cooked them colorless. To do this you might need to get up over 900 F.

 

[evergreen]

Oh, Chrono, thank you for dredging up that post!! That answered a lot of questions for me -- I don't have JewelryLover's flair for the dramatic (nor her tolerance for nasty comments, jeepers) but she asked a lot of the questions I'd had and the discussion was very interesting. It's such a disappointment that she didn't come back and post her results though clearly there was some good rationale behind thinking she might've been embarassed by them in the context of the vitriol she received for her (unshakeable, irrational) optimism.

I do fall into the same category of learns-best-trying-it-myself. I have family near some of the San Diego gem mines, a love of digging in mud, and a retired engineer for a father who'd absolutely design and build me a wee high-precision high-temp oven for my birthday, so the recipe for TROUBLE is all there. I just can't seem to assemble enough information to definitively convince myself it wouldn't be any fun. I love my little (nicely cut, otherwise kinda ugly) tourmalines we dug out of the dirt pile in San Diego a year ago, they're special and shiny reminders of a good time. And I wouldn't consider it a loss at all to systematically study the color outcomes after heating the leftover tourmalines Jerry wouldn't cut (and lo, they are many), even if they turned out yucky. Yuckier. Or baked. Dude.

They're just rocks, after all. What I really like best is science.

Also, I totally think we got some kunzite from that "dig" (hee. sounds so official. from that "afternoon spent muddying ourselves squinting at pans of gravel"). That just flashed into my head. OMG. Next time we're baking something... SCIENCE!

 

[chrono]

There is no harm in playing around with rough material that isn't suitable for cabbing or faceting.

 

[aero]

hello evergreen,
i`ve been living in ratnapura for several years doing the heat thing with rough sapphires.
to have a secret receipe is only the half rent.
because everything starts with gem itself.
you got many different varieties of geuda.
you got silky, milky, young milky, ottu, dark ottu, young silky, thick silky, dot stones etc.
each variety got diffent timing and different temperatures. some of them were yet to be unknown 15 years back.
i mined a white sapphire which occured to be a burn stone. in sri lanka they call it dung.
you can see the diesel smokelike cloud (no geuda) under magnification and with a very thin light beam only.
then you got the dotted stones. this stone can appear in any corundom known colour and pocesses a single or many blue dot/s.
after burn this and the dung variety gives you the nicest cornflower blue you can imagine.
usually every single gem needs a different treatment. every single piece.
dark ottu for example, you can burn with a pipe and coconut shells blowing for one hour. every minute changes the colour.
to know the right treatment to a gem you need to have a big knowledge.
thisknowledge is only available to people doing this for years , daily.
you get no stone twice! there is no common treatment!
nobody would burn two 20 carat blue sapps in the same oven the same way because it is a milky geuda.
in smaller sizes they do lot burn.
BROWN IS DEATH TO BURN STONES
BROWN BECOMES WHITE/UGLY CLOUDY WHITE

regards from berlin

 

[evergreen]

Aero, thanks for your reply! It's VERY cool to hear from someone who does this for a living. I'm afraid I didn't understand many -- any? -- of the terms you used, and even Wikipedia's article on geuda isn't very satisfying. It's clear there are lots of nuances in this area that, as you point out, are really only available by long and no doubt painful experience. It would be very interesting to see some photos, though I understand how a systematic examination (and documentation) of what you look for and how you improve a stone's appearance would be something you'd like to keep private! When you say "in smaller sizes they do lot burn", do you mean they burn (heat?) a lot of small corundum samples all at once just to see whether good stones result?