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 Post subject: Red/Pink Tourmaline heat treatment
PostPosted: Sun Aug 03, 2008 3:00 pm 
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Hi gang,
I've looked through the archives and have not found what I was looking for.... so I will ask.

Heat treating Tourmaline
I've cut several darker purplish-red Tourmalines from Nigeria. Has anyone had success is lightening the color using heat? If so, are you willing to share how you accomplished this?

Already have a burn-out oven and accessories to accomplish the heat treatment process, but would appreciate knowing what others have experienced as I don't want to experiment on pricey pieces.

Thanks!

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 Post subject: Heating tourmaline
PostPosted: Sun Aug 03, 2008 3:11 pm 
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Hi Roger,

My experiments with heating red/reddish tourmaline have largely been done with Mozambique material so I dont know if this is relevant. All the pieces I have heated have either gone colorless or turned a very light pink. The one that turned pink was an incredibly deeply saturated piece that was almost black. Long and short, I'm done heating red tourmaline (at least from that local) until I find a better way to do it.

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PostPosted: Sun Aug 03, 2008 4:08 pm 
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J,

Don't give up. Sounds like you're close to getting it. I, along with Roger, am interested in heat treating dark red tourmalines, I've got a batch of em. Every bit of info I've garnered from stones turning too light or losing all color is from over treatment. Either too much heat, too long at the holding point or both of the above. So what was your procedure? I'm about a mm away from purchasing a computer controlled small oven. it appears that with the right procedure this can be done. I do know that 700*C is terminal for most, if not all. Right now it looks to me that somewhere close to 500*C or slightly above will do the trick. So once again at what temp were you?

Dale


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PostPosted: Sun Aug 03, 2008 5:35 pm 
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Roger,

I am curious if the tourmaline you have has any peach tones. I have a rather large tourmaline from Nigeria that is being studied right now for possibly being heated. It has some peachy/salmon colored tones which are almost secondary undertones.

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 Post subject: Re: Heating tourmaline
PostPosted: Sun Aug 03, 2008 5:47 pm 
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brimsjewelry wrote:
My experiments with heating red/reddish tourmaline have largely been done with Mozambique material so I don't know if this is relevant. All the pieces I have heated have either gone colorless or turned a very light pink. The one that turned pink was an incredibly deeply saturated piece that was almost black. Long and short, I'm done heating red tourmaline (at least from that local) until I find a better way to do it.

Jason,
Thanks for the response and info. Can you share any details of what took place with the goods you heated?

Type of kiln?
How long to ramp up?
What was the final holding tempurature you reached?
Before and after photos?
Original color?
Or any other relevant information...

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 Post subject: re: heat treating red/pink Tourmaline
PostPosted: Sun Aug 03, 2008 5:51 pm 
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Stonecollector wrote:
I am curious if the tourmaline you have has any peach tones. I have a rather large tourmaline from Nigeria that is being studied right now for possibly being heated. It has some peachy/salmon colored tones which are almost secondary undertones.

Stonecollector,
The Tourmalines I have an interest in heating are well-saturated reds with a strong 'burgundy' overtone (under daylight 5500 Kelvin bulbs). Under typical incandescent bulbs, they have a hint of brown.

So..... no peach!

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PostPosted: Sun Aug 03, 2008 5:55 pm 
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Ok, Roger just sent me on a 2.5 hour quest, since heat treating is one of those alchemical arts I just can't resist! :)

So, let's see what's going on..

from CalTech we can see that the coloring ion in pink/red tourmaline is Mn+3 . That's a Manganese "ion" which is an atom missing 3 outer shell electrons (the +3).

From the various articles on heat treating paraiba tourmaline and our own experiments here, we get the following:

Quote:
The purplish pink specimen lightened slightly at 400'C, and its counterpart faded to colorless at 5000C (figure A-1).

Pink coloration related to Mn 3+ absorption at about 515 nm
disappeared when the manganese was reduced to MnI2+ during heat treatment (figure A-2).

Heating of a light violet-blue specimen resulted in "neon" blue
coloration at 500C. This violet-blue sample exhibited a broad
band centered at about 690 nm that is related to Cu 2+; the band remained unchanged after heating, though a broad
band related to Mn3+ absorption centered at about 515 nm was removed.

The color of a light green sample intensified slightly with heating,
whereas a yellow specimen remained unchanged by the heating process.

No significant changes were seen in the visible-range
absorption spectra of the light green and yellow samples after heating to 500'C, but the absorption peaks did shift toward the ultraviolet.
Gems and Gemology, Spring 2006, pg4++


In a personal communication w/ Josh at Pala he suggested i start at 400 deg. C or even 375C and creep up on the heat treatment. The point is, that the color change/lightening occurs as a result of the reduction of Mn+3 to Mn+2 . Obviously there is a gradation of heating that will lighten the stone. That would include temperature and time, so time should be held constant and temperature adjusted.

So, what I can deduce from the above, is that at 400 degrees C lightening of the material will occur. From my experiments on purple Mn+3 tourmaline 489 C will bleach it.

What I would do is get my trim saw, and slice off a 2mm x 1mm chunk, split that into 3 pieces and try 375C, 400C, and 425C to see how much bleaching occurs w/ the material.

If of course, you have already cut stones, I'd first check them with a microscope for 2 or 3 phase inclusions ( it sounds like .22 going off in the oven when the blow, you KNOW it's just happened) so you don't blow it as the water turns to steam.

Then I'd get the cheapest/least valuable stone and start it off at 375C for a 1 hour dwell, 2 hr ramp up / down. (I hope you have a digital meter for your oven, the analog one on mine is off by 150 C according to my digital thermometer. $110 from Omega ).

If it isn't light enough, do it at 400C. I wouldn't exceed 450C unless I was working w/ rough chunks.

Well, let me know how it turns out. This is best I can find searching the literature, I'd like to know what temp is the "sweet spot"

-John

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 Post subject: re: red/pink Tourmaline heat treatment
PostPosted: Sun Aug 03, 2008 6:37 pm 
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After placing it in the oven today.... here's what I have. Filled the flask with investment powder, heated to 360*C - taking two hours to ramp up, and held it for two hours. Turned the oven off and waited about four hours. And now you see the photo - it is ever so slightly lighter than when I put it in. I'll attempt a higher temp tomorrow when I have more time.

(I re-shot the photo - hopefully this one does a better job of representing the stone accurately - still has some dust on the crown :P )

Image
By Roger_Dery

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PostPosted: Sun Aug 03, 2008 9:44 pm 
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Here is a stone that was nearly black when I put it in the oven. It was a very saturated pink. It's a gorgeous stone, but not as saturated as I would have liked. Slightly included under the crown facets and weighs 7+ cts. Mozambique cuprian.

Image

Jason

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PostPosted: Sun Aug 03, 2008 10:51 pm 
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Hi Roger --

I bought some Nigerian tourmaline from the nice folks on the 3rd floor of the Holiday Inn/Radison/?? in Tucson a few years ago. The $2 a gram material that was brownish-pink. Followed the directions of a good guy that was also there on how to heat it. Except I used a butane tourch instead of a alcohol lamp. Stone into pyrex test tube, use a cotton stopper to minimize air circulation and watch color change. Tourch worked great and I kept heating (if a little is good, then a lot is ___?) and then the stone split. Darn! End of experiments with that stuff. I suspect that the alcohol lamp would have worked much better and I would have ended up with a nice pink 11 mm, 6.33 ct stone instead of a 11 mm, 6.33 one that had a serious fracture in it. Darned if I know how to post a picture here and am not up to it right now. I'll email you the picture and if you think the stone is worth showing, then go ahead and post it.

Thanks for the help, Roger. I will read the photo faq, but I'm not sure how long the info will stick.
Image

Keith


Last edited by tfw on Mon Aug 04, 2008 9:35 am, edited 1 time in total.

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PostPosted: Mon Aug 04, 2008 12:18 am 
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Roger and All,

Danger, Will Robinson!

A few years ago I experimented quite a bit with heating dark Nigerian red tourmaline. I learned that if there is any significant amount of brown in the original hue, brown will dominate and red will tend to be driven off by heating.

The image at the top is approximately the original color of the stone at the bottom before heating. It had somewhat darker tone, but some brown was apparent, similar to Roger's stone.

My stones were packed in casting investment and ramped up at 50-degree increments to 600 F. first, which didn't do much. I tried at again with a peak temp of 900 F. and saw some lightening. An optimist, I tried again peaking at 1100 F. Damn!

Stones were heat-soaked for 2 hours at peak temp and allowed to cool for 12 hours before opening the oven.

A dealer who handled a lot of the same material told me his prime reds were achieved with a combination of heating and cobalt 60 irradiation, similar to the Brazilian Morro Redondo treatment. I haven't yet found an irradiator who's willing to work with small quantities of stones. I wasn't told which comes first, the heating or irradiation.

This is only my experience and others' mileage may vary.
Image

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 Post subject: re: red/pink Tourmaline heat treatment
PostPosted: Mon Aug 04, 2008 1:43 pm 
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This morning, Monday the 4th, I place the oval Tourmaline back into the kiln, pack it the same way as before with the investment powder in the crucible. Ramped up to 410*C over two hours, and held it for two hours, then allowing it to cool to room tempurature.

It now appears ever-so-slightly lighter in tone, in the hand - don't know if this photo depicts it accurately enough. But here it is...

Image
The only adjustment made in Photoshop was to crop it to an appropriate size for the forum.

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 Post subject: heating Nigerian red tourmaline
PostPosted: Tue Aug 05, 2008 1:16 am 
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I use a programmable furnace, and find that

- a slow ramp of 125c per hour to 520C,
- hold for 2 ours and
- let the oven cool completely

is turning my dark purplish reds into bright pinks.

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 Post subject: re: red/pink Tourmaline heat treatment
PostPosted: Wed Aug 06, 2008 8:47 am 
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Yesterday, the 5th, I place the oval Tourmaline back into the kiln - again, packed it the same way as before with the investment powder in the crucible. But this time, I followed Lisa Elser's directions and ramped up to 520*C over a three hour period. Then, I held it at 520*C for two hours, allowing it to cool to room tempurature over night.

It now appears very pink - and a very desirable color. Photo was taken under 5500 Kelvin lamps - only change in Photoshop was a very slight adjustment in 'Levels' and change to size to fit here. Here it is.... though I think it is slightly richer in color than this photo shows.
Image

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PostPosted: Wed Aug 06, 2008 10:03 am 
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Very nice indeed, Roger! Very nice.

J-

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