From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


If you have some polaroid sunglasses (the ones that claim to reduce the
glare over water, etc. and are gray colored) you can do the test for stress
in glass.
Start by finding a nice glancing reflection of the sunlight off of a car or
other window.  This glancing reflection polarizes the light and the more of
a glancing reflection, the stronger the polarization.  This is the fist
polarizer on the test.
Next, hold the piece of glass up to the reflection and look at it wearing
the sunglasses, either on the face or holding with the other hand so you
can rotate them.  This is the glass under test rotating the polarization
with the stress in the glass and finally the detection of that polarization
change with the polaroid sunglasses. A good piece of glass will have an
even color and when you do the extinction of the light by crossing the
polarizers, the whole glass will evenly fade in brightness.  A piece of
glass that has stress in it from tempering will have spots and/or lines in
the brightness and will be basically unusable as it will shatter at some
point rather than allow for the grinding of the surface.
FWIW, I've gotten some that have indications of temper but were in fact not
actually tempered and all have been ground well and work fine.  All of the
pieces had no stress when doing this test. Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

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