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Re: Tempered glass

In a message dated 4/17/2000 01:09:07 Pacific Daylight Time, 
Aquatic-Plants-Owner at actwin_com writes:

> Typically, modern tempered 
>  glass is marked as such in a corner.  Older tempered glass may not be.  An 
>  experienced glass technician can tell by the "ring" of the glass if it is 
>  tempered by tapping it.

There is a more certain way.  Take two photographic polarizing filters.  
Sandwich them together.  Rotate one of the filters.  Light transmitted 
through the filters will fade from lighter to completely blacked out as the 
planes of the polarized material crosses through each other's axis.  Now, try 
the same thing with the piece of glass sandwiched between the two filters, 
and rotate one of the filters.  If the glass in untempered, the light 
transmitted through the filters will act the same way, fading from lighter to 
blacked out.  If the glass IS tempered, an interesting phenomena will occur:  
as the filters are rotated, a black cross will form through the filters.  
It's an unmistakable sign:  if the cross is there, the glass is tempered.