I remember in high school wood class when we made walnut steeple clocks someone brought in a flat piece of glass to be cut for the clock.

We all suspected it was safety glass but there was no markings. So I scored it with the cutter and it wouldn't break. I turned it over and scored the reverse side and it still wouldn't break. The teacher came over and said he had read the sometimes quickly flashing a torch over the glass would make it break. So he lit up the acetylene torch and moved very quickly along the lines. The glass exploded and blew chunks all over all of us.. Yep, it was safety glass.....