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 Post subject: Heat Treating Tourmaline
PostPosted: Thu Apr 27, 2006 5:11 pm 
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Wanted to see if anyone had any info on the procedure to heat treat tourmalines. I used the wayback machine and got some info on temps and rads, but no clear indications on length of heat soak, ramping speed to temp, cool down etc..

anyone know where to go or could outline the info here would be appreciated..

thx!


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 Post subject: Heat treating tourmaline
PostPosted: Fri May 19, 2006 4:28 pm 
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I'll wager no one wants to share that data freely. You may need to experiment with some throwaway samples to come up with your own formulation. The up side of that is that once you have it, its valuable information. Do you have some material to work with? If not, I have quite a lot of dark tourmaline.

I have been researching the same thing, but it appears I am a little behind you. What kind of oven do you have, and what kind of results do you expect to achieve? Is this a commercial venture or is it just a personal experiment?

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PostPosted: Fri May 19, 2006 5:08 pm 
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I have quite a stash of rough tourmaline, so i can cut small pieces off prior to faceting and test them in the oven. I have a burnout oven good to 2250 I think, so not much of a problem there.

The above info should be quite useful, I'm finishing a topaz I have on the wheel, and then i'll look at the tourmaline I have for the experimentation. Was not considering this as a commercial venture, but, if I can get reliable data from the experimentation, I might consider doing some of this. rough is much easier to test than finished goods, as your not at as much risk.

Would really like to see if some of that nigerian purple will go paraiba blue, or does it just go green.. The gia article this month had some excellent information about the chemical species, which coupled with what barb posted could be very interesting..


I'm not about to nuke anything.. although the scientific american amateur scientist cd has a few great articles on building your homemade linear accelerator and atom smasher!


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PostPosted: Fri May 19, 2006 5:37 pm 
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Blue and green tourmaline is often heated to lighten the color (600-700 degrees Celcius). It is usually done after the stones are cut. It is my understanding that only the maximum temperature reached is important, not the amount of time the tourmaline is kept at that temperature. The best colors no not appear when the maximum temperature is reached, but only as the stone cools. The rate of cooling can also be a very important factor. Therefore, the heat treatment of tourmaline requires a great deal of sophistication. Tourmaline that is included can NOT be heat treated. That's why varieties of rubellite will seldom be heated. The brownish purple variety of tourmaline (batata roxa) often heats to a pleasant light rose tone at relatively low temps: 250-400 degrees Celcius.
For me, the color of batata roxa (purple potato) is one of my favorite colors...so I'd never opt to heat it. :wink:


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PostPosted: Fri May 19, 2006 10:30 pm 
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so how are these miners in brazil heat treating it.. isn't that being done at the source??


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PostPosted: Mon May 22, 2006 1:10 am 
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Barbra wrote:
Tourmaline that is included can NOT be heat treated. That's why varieties of rubellite will seldom be heated.


True enough although dangerous inclusions are those that contain liquids or gases that can expand. I did some experiments heating dark Nigerian rubellites after cutting and was disappointed that the treatment lightened them but emphasized brown tones. It's my understanding that many of the reds are irradiated in addition to being heated to bring out optimum color. I know that to be the case with naturally orangy-pink tourmalines from the prolific Morro Redondo deposit in Brazil that are turned into wonderful bright pure pinks via treatment. An Afghanistan tourmaline supplier has told me most of the reds from that region are irradiated but I don't know the details. As you point out, it's a sophisticated process that not only varies from color to color but is site-specific as well.

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PostPosted: Mon May 22, 2006 2:50 am 
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ok,ok...don't fire up the oven yet...

...I thought it was Barbra that said that you heat your sand bath to 600c to evaporate all moisture. After cool down, the stone is then heated to 400c with a controlled cool down? for cuprian tourlamines....

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PostPosted: Mon May 22, 2006 10:05 am 
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Ux4 wrote:
ok,ok...don't fire up the oven yet...

...I thought it was Barbra that said that you heat your sand bath to 600c to evaporate all moisture. After cool down, the stone is then heated to 400c with a controlled cool down? for cuprian tourlamines....


I'm not sure who you're responding to Ux4 but since my post was last .... What Barbra wrote about cuprian tourmalines may be true: I have no experience with them. But non-cuprian stones are routinely heated for a number of reasons, usually to lighten color.

Heating the sand to drive off moisture is a good idea; I used casting investment for my rubellite experiments, and did the same since it's well known for attracting water (hygroscopic). I place stones so they're separated at least an inch from each other and make sure there's an inch or so of investment over the top ones.

I put the stone container into a cold muffle furnace, ramp up the heat slowly in 50-degree increments (starting around 100 F., allowing at least half an hour for heat-soaking at each level (depending on size of container). When the desired temp is reached I heat-soak for two hours to make sure all areas of the container are the same temp, then turn the power off. The highest temp I've reached was 900 F. (approx. 480 C.) At a certain point some tourmalines will become colorless. I did shatter one batch of 'olive oil' color stones that showed no change after heating to 900F. I took them to 1100 F. -- mistake!

I DO NOT OPEN THE FURNACE DOOR FOR AT LEAST 12 HOURS! I let the container become stone cold (sorry!) before digging into it. Waiting to see what's happened is difficult but it develops character :lol:

The above is my own approach, based on reading and whatever morsels of information I've been able to find. Suggestions for improvement are most welcome, as are exchanges on the results/techniques of others.

ROM


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 Post subject: Heat treating Tourmaline
PostPosted: Thu May 25, 2006 12:16 am 
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1100 degrees is getting too close to the stabiity limit of the crystal structure. If nothing else, you would want to increaase the confining pressure in order work that range. Mineralogists have ovens they call "bombs" for that kind of experiment - and a very appropriate name that is.

Looking at the crystal structure, I see that the manganese and di- and tri- valent iron occupy the interior wall of a circular cavity (hence the name "cyclo-silicate". The interior of that cavity is occupied by a hydroxyl or flourine ion. Given that the dark coloration in minerals typically comoes from tri-valent iron, it makes sense that lightening comes from transitioning of the tri-valent iron into its di-valent state. It can do this by grabbing an electron from the hydroxyl radical or the flourine. But there's a price to pay. The more iron-rich the tourmaline is, the more of this ion is required, and these atoms are naturally gasous at those temperatures. Pressure is bound to build up in the crystal, and coupled with its increasingly weaker strength... well you can guess the rest.

As a rule I would say, the harder it is to heat treat, the more likely to fracture. But also, I have seen some clear evidence of chemical "domains" in these minerals under the microscope. These could lead to fracturing at even lower temperatures -even without the chemical change. The regions have differing expansion coefficients.

All this points to very careful experimentation with trial batches of lower quality material from the same locale.

Hope this helps.

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PostPosted: Fri May 26, 2006 2:10 am 
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[quote="ROM

I'm not sure who you're responding to Ux4 but since my post was last .... [/quote]

ROM...

jleb wrote: ...Would really like to see if some of that nigerian purple will go paraiba blue, or does it just go green....

I assumed that he was refering to the cuprian material. True, jleb?

Super info from you, 'tase and lizardite.

JLEB, let us know what happens! Blow by blow....

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PostPosted: Fri May 26, 2006 3:39 am 
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Yes you have it right.. that's what I'm aiming for.. got a few other items stacked ahead of it, but it's on the list


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