There is an ancient form of aluminum that is transparent: I have heard
that it has been used for sensor windows in expensive devices.
The advantage of aluminum oxynitride for bullet proof glass is that it
is cheaper than the other form, which is pure aluminum oxide that
people value as a gem: white sapphire.
White sapphire is not white, but transparent.
(By the way, ruby is also aluminum oxide, but with a different
coloring agent. Sapphire contains a little iron; ruby contains a
little chrome. White sapphire contains neither.)
Neither aluminum oxynitride nor aluminum oxide are the aluminum metal
of which many imagine when you speak of transparent aluminum ... then
again, most who wear rubies do not think of themselves as wearing
hunks of aluminum oxide contaminated with chrome ...
--
Robert J. Chassell
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http://www.rattlesnake.com http://www.teak.cc
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