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 Post subject: Study on corundum crystals from Sweden
PostPosted: Mon Apr 08, 2013 9:34 pm 
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Location: Stockholm, Sweden
These corundums may not impress anyone. However, for Sweden, they are not too bad.

Corundum is rare here, in any form really, not to mention in well crystallized habit. These pieces are from a 3 square km-sized area 22km west of Arvika, southern Sweden. It was likely discovered in the late 1980's, and and the locality was described briefly in a norwegian geological magazine in 1991. The above stones are from 1992.

The Swedish Geological Survey has confirmed that no analysis regarding composition or trace elements, actually no (official, at least) studies at all, have been conducted on this corundum. Hence, during the upcoming months, a friend of mine (Anna Langlet; a Gem-A student with a particular interest for chemistry and treatments) will be studying how this material reacts to heating. We are also going to send a few samples to AIGS for EDXRF analysis.

My question is about reference material. Have there been studies made where the reaction of corundum to heat is analyzed with regard to Cr/Ti/Fe content? To stretch it, even before-and-after photos of corundums having undergone well-characterized treatments - in conjunction with some info on their composition - would be of value.
Many treatment researchers must have kept track of the chemistry of their test objects, no? :?

I don't claim to have experience from this field, so input on thinking is welcome. If someone could point me in the direction of a resembling study, I'd also be glad... in order to have a working methodology to relate to. :)

All the best,
Johannes

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 Post subject: Re: Study on corundum crystals from Sweden
PostPosted: Tue Apr 09, 2013 12:32 am 
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Johannes, at the risk of being obvious have you looked into the books of Ted Themelis? http://themelis.com/Books.html

There may be other resources, especially in the periodic literature, but Ted is an expert and has written extensively on the topic of corundum heat treatment with particular attention to the chemical basis for color change via heat.

Best wishes for your project =D>

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 Post subject: Re: Study on corundum crystals from Sweden
PostPosted: Tue Apr 09, 2013 3:04 am 
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Thanks for the reply Rick - yes, Anna bought one of the books recently and is currently reading it :)

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 Post subject: Re: Study on corundum crystals from Sweden
PostPosted: Tue Apr 09, 2013 11:25 am 
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I would be very interested in any references you find.
On Saturday, at the Sinkankas Symposium on Ruby, Shane McClure touched on the relationship of Ti, Fe and Cr in regards to heat treatment.
I specifically asked the question, "What happens to Ti (which causes the blue cores of Mong Hsu rubies) when it is heated."
Shane quickly responded that the valence changes in oxidizing conditions. An audience member chimed in, when heated, the Ti bonds with Fe and changes the valence of both the Fe and Ti, so they no longer impart a blue color. The Mong Hsu ruby I've seen has been fluorescent with LWUV. Iron inhibits fluorescence. So.....how much Fe needs to be available to bond with the Ti? Apparently, not enough to mask fluorescence....or am I missing an integral part of the equation?

OK, another question, which I never got to ask. When heating ruby with the intention of dissolving rutile inclusions to improve clarity, what happens to the titanium? Does it also bond with iron and change valence? If so, wouldn't that infer that the iron content was significant enough to accommodate the titanium looking for a new home? How would this iron effect fluorescence?


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 Post subject: Re: Study on corundum crystals from Sweden
PostPosted: Thu Apr 11, 2013 11:50 pm 
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The oxidation of iron in corundum is probably the oldest heat treatment, having been described by the Arabs more than 1000 years ago. It involves heating a ruby (or another corundum) in an oxidizing atmosphere. This converts Fe2+ to Fe3+. To obtain a blue color, you need Fe2+. Thus this treatment removes blue from corundum.

The Fe2+ - Ti4+ charge transfer is extremely efficient, requiring just a few hundredths of a percent of the elements in the right form and position to cause a deep blue color. This is different from the Cr3+ that produces color in ruby, where you need nearly 1% Cr to produce a rich red color.

When a corundum containing rutile (Ti02) silk is heated to the melting point of rutile or more (1600°C+), the rutile dissolves into the corundum.

The best works on the heat treatment of corundum are those of Emmett, published in G&G and elsewhere. A quick web search will locate them. They have a rigorous scientific foundation that is not found in other works.

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 Post subject: Re: Study on corundum crystals from Sweden
PostPosted: Fri Apr 12, 2013 5:32 am 
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Very interesting questions Barbra. And thanks Richard!

Meanwhile I dive into the material, would you happen to know of a study where the color change is studied as a function of initial Cr/Ti/Fe content specifically?

Johannes

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 Post subject: Re: Study on corundum crystals from Sweden
PostPosted: Fri Apr 12, 2013 10:34 am 
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Thanks Richard, that's very helpful.


Johannes, although, not specifically targeting the relationship of Cr, Ti and Fe in corundum, here are the articles G&G published written by John Emmett :

-Fluxes and the heat treatment of ruby and sapphire,
F99:90-92
-Heat treating the sapphires of Rock Creek, Montana,
W93:250-272
-Beryllium diffusion of ruby and sapphire,
Su03:84-135

Sidebar: How much iron does it take to mask fluorescence in ruby?


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