Stress testing mirrors

It is widely known to use polarized light to check mirror blanks annealing and look for stress in them.

When checking for stress in a friends mirror I'm making for him...I noticed the strangest patterns in the glass...nothing I've seen before.

Almost looks as if the glass cooled on a waffle sheet. It has been suggested this is *tempered* glass...and to my knowledge will be a bear if not impossible to figure. Do any of the *experts* here have a clue to what these unusual stress marks mean or are?

Thanks

Rob

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Here is one more view looking from the back.

Rob

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Hi Rob:

Long time no see! I hope you are well. I'm no expert, but those marks do look a lot like the "quench marks" you can see in a tempered glass auto window under certain light, and with polarized sun glasses on.

Quench marks show the stress/strain pattern intentionally fabricated into tempered glass by careful heating and cooling, to strengthen the glass, and to make sure it breaks into little squares.

If this is in fact tempered glass, I imagine that making a mirror with it will be impossible because of the designed internal stress/strain zones.

Jim

Probably a poor anneal if the glass is more than an inch thick. Commercial tempered glass rarely is more than 3/4in. You can cook it and slow cool to relieve it. Is it borosilicate or soda lime?
BTW, Schott is producing Borosilicate in 67mm thickness. It is hot rolled and the same material as Borofloat 33.