Re: ATM Porthole glass mirrors

[Anthony Stillman <atmer@flash.net>]

Jerry,

You'll need to test it for stress.  To do this you'll use crossed
polarizers.  It can also be done by looking through polarized glasses at an
oblique reflection off the back surface of the glass.  If you see colors,
it's stressed.  That doesn't mean it can't be used, but rather that you
have a few additional possible outcomes.  If you grind through the stress
layer, the glass may explode.  If you don't grind through this layer, the
glass may not explode, but it may still potato chip.  Finially, the glass
may be prone to odd and large figure changes with temperature.

Recently a porthole showed up at the mirror making working shop.  Examined,
it displayed a fair amount of color around the edge.  Guessing it was
Twyman effect, I stoned the rough edge.  Sure enough, a lot of it went
away.  I left the rest as an exercise to its young owner.

Anthony