Corundum with sufficient titanium are good samples for the development of asterism (stars) by heat treatment. Asterism can be developed by diffusing titanium into the corundum crystal lattice with appropriate heat treatment apparatus. The technique is very similar to the production of asterism in Verneuil (flame fusion)-grown synthetic sapphires without titanium content. The technique is applied by many companies to produce stones with asterism (stars). In Thailand both natural, synthetic and treated stones are available.
The process consists of :
1. Diffusion treatment of corundum samples in mixture of aluminum oxide and titanium oxide powder. The temperatures required to facilitate the process (diffusing a thin surface layer) may be in the range of 1700 - 1950 degrees centigrade.
2. The samples are then heat treated in a temperature in the range of 1100 - 1500 degrees centigrade permitting the formation of rutile needles in the surface layer.
The asterism (stars) created by this two step process is caused by needles within only a thin surface layer. When properly done, the visual appearance of treated samples may look very similar to that of untreated natural stones.
With careful examination under the microscope + immersion technique (with sample stones: treated + untreated stones) most treated stones can be identified if they are unset (loose stones).
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