Heat treatment in Sapphires . Question on Rutile

[haagen_dazs]

I have been reading about heat treatment on Sapphires.
Its pretty much acceptable form of treatment (going a long way back) to enhance the colour.

Couple of questions on the Corundum family .
1)
Is Rutile ALWAYS present in unheated natural sapphires?
2)
After heating, does the rutile disintegrate completely or will some of it still be around?
If some rutile is still around, then presence of rutile doesn''t necessarily mean its non-heated right?
3)
What happens to the rutile chemically?
4)
Can rutile be replicated in synthetic sapphires?

Thanks for your help!

 

[morecarats]

!) Rutile inclusions are not present in all corundum. Thai and Cambodian corundum tend not to contain rutile, unlike Burmese and Sri Lankan corundum.

2) Heat treatment of corundom is done at very high temperatures (often as high as 1800 degrees C). Rutile (titanium dioxide) has a lower melting point than corundum and is dissolved by heat treatment.

3) Dissolved rutile is usually absorbed into the corundum, both improving the clarity of the corundum and altering its color.

4) Rutile inclusions could in principle be replicated in synthetic corundum, given the right technology. But almost all synthetic corundum produced by the flame fusion method is pure aluminum oxide, with the addition of coloring agents such as chromium or iron.

 

[chrono]

1. I don’t think so. The presence of rutile is probably what is used to help determine origin.
2. The rutile “melts” when the sapphire is heated. I believe it is completely gone.
3. It is dissolved.
4. Anything is always possible, but if I’m not mistaken, at this point, there are not observable in synthetic sapphires.

 

[zeolite]

Date: 10/28/2009 9:35:18 AM
Author: Chrono
1. I don’t think so. The presence of rutile is probably what is used to help determine origin.
2. The rutile “melts” when the sapphire is heated. I believe it is completely gone.
3. It is dissolved.
4. Anything is always possible, but if I’m not mistaken, at this point, there are not observable in synthetic sapphires.

This chromophore article mentions that the main coloring agents of blue sapphire are iron and titanium. As more carats states, rutile is titanium dioxide. Also we know that sapphire is aluminum oxide (AL2O3).

Many low quality sapphire crystals are nearly white, because the titanium coloring agent is bound up in the rutile crystals.

The point of heating sapphire is not just to reduce a visible inclusion, rutile. It is to remove the titanium from the rutile crystal and allow it to replace AL in the AL203 structure, which intensifies the color.

So no, the titanium is not gone, it is the AL2O3 crystal structure, making the sapphire more intense blue.