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Description: heat-treatment process - Nigerian Tourmaline

Discussion in 'Colored Stones' started by Roger Dery, Apr 1, 2011.

  1. Roger Dery
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    by Roger Dery » Apr 1, 2011
    Hello gemaholics and other gem enthusiasts.
    From time to time there has been reference to various treatment processes placed on the red, purple-red, brownish-red and pinkish-red Tourmalines from Nigeria. I was recently tasked by a major east coast gemstone firm to heat-treat some of their Tourmaline collection that they had faceted in 2002. They acquired the rough from Nigerians who traveled with it to New York city, and then arranged to have it faceted in Bangkok with their contract cutter there. I do not own these, and they are not for sale as far as I know.

    The heating process is where the gem is placed inside a ceramic crucible that has been filled with casting investment powder. The powder is used as a heat sink to reduce the possibility of thermal shock. I performed the heat treatment over several days as we wanted to be certain that the stone was not damaged due to heating or cooling too quickly. The heating sessions took place over roughly a 1/2 day each. About 4 hours to ramp up, holding for 30 minutes and about 3 hours to ramp down.

    The stats:
    6.54ct previously untreated Nigerian Tourmaline
    11.2 x 11.2 x 7.5mm

    Heating to 400*C - no change
    Heating to 425*C - no discernible change
    Shown below: Heating to 450*C - no discernible change
    [​IMG]
    Shown below: Heating to 475*C - a slight lightening of tone and a very slight alteration of hue
    [​IMG]
    Shown below: Heating to 500*C - a confirmed change, with lightening of tone, alteration of hue as well
    [​IMG]
    The owners may ask me to take the heating another step, and if so, I will do a follow-up report as to how it turns out.

    *even though I have the capability to perform such heat treatments, as a general rule, I am not in favor of them as I would rather have my gems be in their most pristine condition. I hope someone finds this information useful.
     
  2. chrono
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    by chrono » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Roger,
    Thank you for sharing the experiment and resulting pictures with us. Yes, I've long known that dark red tourmalines have been heated to lighten it into a more saleable (and more expensive) and pleasing pink/light red tourmaline, unfortunately, without disclosure.
     
  3. Kismet
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    by Kismet » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Neat! Thanks for sharing that. I'm assuming that you can't see any change under a microscope after it's been heated, yes?
     
  4. T L
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    by T L » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Wow, what a dramatic difference!! Thanks for sharing that.
     
  5. stylish1
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    by stylish1 » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Usually after heating tourmaline to the degree it was heated, possibly inclusions that are found in the stone could be changed or reduced at the cost of a little brighter material and also making the stone a little more brittle.
     
  6. PrecisionGem
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    by PrecisionGem » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Those temperatures are not going to reduce any inclusions. If anything, and inclusion will grow with the heating.

    You can see the trouble it it to heat, as Roger needed to perform this over several days each time changing the temperature. What complicates this further is that stones from different locations will turn at different temperatures.
     
  7. Roger Dery
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    by Roger Dery » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Hi Chrono,
    Well not *everyone* forgoes the disclosure of treatment(s) :geek: .

    Though it is expected from US gem suppliers, there are many that do not, most unfortunately. As for other producing countries, most gem operations look at it as a necessary part of the process and do not see any reason to disclose. This too, in unfortunate.
     
  8. T L
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    by T L » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Now, can you nuke it and see what happens??? :lol:

    And I don't mean putting it in the microwave.
     
  9. Roger Dery
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    by Roger Dery » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Hi Kismet,
    So far, I have not been able to discern any internal changes with the microscope. Other than it is now lighter in color. It was from the beginning a very clean stone - for Tourmaline.
     
  10. Roger Dery
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    by Roger Dery » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Hey TL,
    I suppose one could try irradiation as a continuing exercise, but I don't see why one would, considering the new color.
     
  11. Arkteia
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    by Arkteia » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Thank you for your post. Very interesting.

    You have mentioned the treatment is not changing the internal structure of the stone. Is there absolutely no way to see any evidence? Any tiny cracks that would serve as evidence? Or, a slight difference in colors at ends of a long stone? Would it make sense to heat rough rather than finished stone?

    Did you use a special oven? Or a regular kitchen oven would do? Not that I plan to repeat it in the privacy of my kitchen, just interesting how much investment one needs for the process.
     
  12. lelser
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    by lelser » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Nice description of the process, Roger!

    I have an article coming out in May in Lapidary Journal that looks at heating Nigerian pinks and greens. You can't say there are "recipes" since if you follow a recipe you'll always get dinner. Heating recipes are only valid for gems from the same part of the same mine, and then only if you're luck. The process of how to approach tourmaline heating is what's important.

    Nigerian rubelite heat beautifully, but I'll only do it if they are simply too dark to sell. Any tiny inclusion, even one's that are difficult to see with magification, can expand in the heat and destroy the gem. I had a huge dark red gem that today is a pair of small pink earrings thanks to an unseen inclusion near the culet deciding to make itself known.

    The nuked rubelites are mostly Brazilian, and are heated to colourless from all sorts of original colours, then nuked pink. It can be extremely difficult to see pleochroism in nuked stones, so look for some difference in the colours along the C and A/B axis.

    I ALWAYS disclose treatment. With normal tools it's not possible to detect heat treatment in tourmaline. It's not like sapphires where there are often distinctions between unheated and heated inclusions.

    Cheers,

    Lisa
    www.lisaelser.com
     
  13. chrono
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    by chrono » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Roger,
    I didn't specifically name any names or anything, wanting to keep it as a general statement so I hope you didn't take it to mean I was referring to you, in particular.
     
  14. Roger Dery
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    by Roger Dery » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Hello Crasu,
    To my eyes, there is no visible change going on inside of the gem. My microscope only goes up to 42X. It is possible that at 200X more may be available to make a different determination.

    The heating of rough Tourmaline is not normally advised. Though I am certain it is attempted in many gem cutting centers. In an ideal situation, it is always best to pre-form the roughs into something that resembles a final shape. And in so doing, one would remove [or cut away] any noticeable fractures or fissures. Then, again in an ideal situation, one would be heating very clean goods whether still in pre-form state, or as a finished gem.

    Re: special oven
    Not really special, but one that is commonly used in the jewelry trade. A kitchen oven will likely top out at 550*F, which for the gem and jewelry trade, is not suitable. I happen to have the same model as Gene Flanigan. As for cost, it is possible to pick one up in the $400 range, but there are nicer models with some bells and whistles in the $600 to $800 range.

    I hope you find this helpful.
     
  15. Roger Dery
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    by Roger Dery » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Hi Chrono,
    I understood what you meant, and that it was a general statement and not directed. All is good....
     
  16. Michael_E
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    by Michael_E » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Looking at those images I am left wondering why anyone would want to treat a decent red tourmaline to a less attractive, (to my eyes), pink color. It also looks like that stone could have been cut better as the center looks lifeless. What is your opinion of the difference between recutting that stone to get better life as opposed to the heat treating that was done Roger? Was your customer happy with the end result?
     
  17. LD
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    by LD » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Roger

    Three questions if I may?

    1. Was your heating process limited to red tourmaline please or did you experiment with other colours? I'm just wondering if all colours and species of tourmaline react in the same way i.e. would cuprian tourmaline do the same?
    2. I have been told (by a vendor selling material) that the Nigerian or Mozambique "Rubellites" they were selling were irradiated - not simply heated. With that in mind, why would they go to this trouble if heating will produce a lighter gem please?
    3. Have you personally seen any evidence of irradiated tourmaline?

    Thanks in advance.
     
  18. Roger Dery
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    by Roger Dery » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Hi Michael,
    .... great questions
    The untreated material has a 'brown-mask' to the hue, and is currently unacceptable to the dealer. They have a project they are working on that requires 15 to 20 larger Tourmalines in a 'true pink' and not reddish. So in their eyes, they could have let these sit in the safe another ten years, or heat them and potentially make a sale.

    As for cutting or re-cutting to make them livelier, yes, they are in need of some help. But they feel they can make a sale as they are, albeit lighter in color.

    They have seen the same photos as you all. And I have been instructed to try again at higher temperatures. If something goes haywire, I am not going to be held accountable as they are aware of the risks - they have been in business since immediately after WWII.
     
  19. lelser
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    by lelser » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin


    I did some experiments with various Nigerian material. Two different greens - one more yellow than the other, and some blue/pink stones.

    The pink seems always to lighten first. In the blue/pinks they were quite muddy after cutting, but the pink burned out and left a great blue. The greens sometimes do and sometimes don't lighten. Because the article is due for publication, and those nice people paid for the rights I cannot post photos unfortunately.

    Heating will lighten a red tourmaline gem, but not change it's colour. Irradiation ADDs pink to an otherwise colourless gem. There is no reason to irradiate a naturally pink tourmaline that I can see.

    I've not seen irradiated material out of Nigeria, but I'm buying rough not cut.

    Cheers,

    Lisa
    www.lisaelser.com
     
  20. Roger Dery
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    by Roger Dery » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Hi LD,
    1a-at this particular go-around, it is strictly a pink/red Tourmaline project.
    1b-I have experimented with other colors
    1c-not all colors - even from the same location - react the same way

    2-excellent question! Sometimes a vendor will state they are irradiated, even though they really do not know. To protect themselves, they may 'over-state' what has been done by the gem trade - but not necessarily to their particular stones. I believe you may have answered your own question.... as why would they bother? Especially, when they could look fantastic with a less invasive and less costly technique.

    3-no, I have not 'seen' evidence in recent years primarily because it was coming from Nigerian sources, purchased as rough. But then, those who are doing it, are unlikely to come forward and share it all (in today's brave new world). In the 80's and early 90's, when we were handling the classic Rubellites from several of my Brazilian sources (as already cut stones), they were very forthcoming with what they knew. Today, using the AGTA stats, we are expected to declare they are possibly irradiated and heated - even if we know they were probably not done this way. This way, in their opinion, all bases are covered. I don't happen to like this philosophy, but as an AGTA firm member, I must abide by the rules.

    I hope this has been a little helpful.
     
  21. T L
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    by T L » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    I thought they were irradiating rough, as well as cut gems.
     
  22. lelser
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    by lelser » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    While I cannot know for sure, the rubelites I've gotten don't fit the colour profile of the known irradiated material I've seen, and I buy from smaller suppliers who buy closer to the mine and don't have the resources to treat. When I get HOT pink, I check for distinct pleochroism, and look for inclusions or "rind" which would normally blow to bits in the initial heating process.
     
  23. T L
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    by T L » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Can you go more into this color profile specifically? I have told anything and everything in rubellite or pink tourmaline shades/colors can be irradiated. I even posted a Nigerian stone I obtained from an American lapidary who has a Nigerian source, and the rough was shipped from Nigeria to him. Upon looking at the photos I posted (in various light sources), some collectors/dealers even said that looked like some irradiated rubellites, and it also looked like some natural ones as well. It's frustrating, so if someone could provide an accurate "color profile", it would help. It would be nice for indicolite as well.

    Thanks
     
  24. LD
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    by LD » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    I don't know if this helps but the irradiated rubelites I've seen most definitely are NOT hot pink. They were the traditional colour (purplish red) that one would expect from a "normal" rubelite. It's interesting to hear that pleochroism may be less present in the irradiated material. I do wish I'd kept the ones I saw just so I could experiment but alas I didn't. Interestingly the amount of material and clarity that this particular vendor had (not to mention gemstone carat weights) were phenomenal. They were most definitely Tourmaline but it struck me as odd at the time that the clarity was so good. Hence the reason I asked about treatments and was told categorically that "most" of the material was nuked.

    Thanks for your descriptions of your tests heating other colours. Once you've published would you mind very much sharing the link so that we can read?

    Also, a complete aside! How does one get on your mailing list and to view your catalogue please? (sorry for the threadjack)
     
  25. T L
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    by T L » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Richard Homer sells some rubellites on his site and he notes they are irradiated. This was also a confirmed irradiated tourmaline in this photo.

    irradiated_tourmaline.jpg
     
  26. LD
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    by LD » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Hi Roger - thank you so much for your reply. What is interesting about the irradiated Rubelites I've seen is that they were being sold in the UK where there are no regulations (sadly) about stating treatments etc. Therefore, I'm assuming that the vendor didn't have to be so transparent about his gems! I have to say that I would appreciate a vendor saying "this may have had ......" a certain treatment if not known or not apparent during testing but I very much appreciate that this all bases covered approach has certain contraints too.
     
  27. LD
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    by LD » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    ........... and this one does look pink to me! The ones I've seen really didn't!
     
  28. lelser
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    by lelser » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    I'll post a link if they publish it on their website. When they don't, I wait a few months and then put a .pdf up on my site. I'm not allowed to tell you how to get on my mailing list, though :)

    Here's an oldie but goodie, that outlines the colour changes seen when irradiating tourmaline. The only really commercially viable one is in the pinks.

    www.minsocam.org/ammin/AM60/AM60_710.pdf

    Because the hot pink is most commercially desirable, that's where you'll see 90% of the treatment. The photo posted of Richard's stone is classic irradiated rubellite. Heating lightens, but does not intensify the colour.

    I'm absolutely not an expert on irradiation. I do try to know enough to keep things honest and to double check my own purchases.

    Cheers,

    Lisa
    www.lisaelser.com
     
  29. Pandora II
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    by Pandora II » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Many thanks for this post and especially the before and after pics - would love to see any further steps.

    Am wondering about a repeat of Chicken Kunzite... Turkey Tourmaline? :bigsmile: Off to rootle through the gem boxes and check how high my oven goes... :devil:
     
  30. lelser
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    by lelser » Apr 1, 2011
    Re: Description: heat-treatment process - Nigerian Tourmalin

    Probably not 900F which is about where the changes become evident :)

    When I was doing research for the article, even with my very efficient programmable furnace, with it's small heating box, my hydro bill was frigging HUGE. It added about 100 bucks to the monthly bill. That's not normal, but I ran a heat program every night for nearly 2 weeks.

    Cheers,

    Lisa
    www.lisaelser.com
     

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