Drilling glass

[rusty]

Tempered glass has been heat treated to make it stronger but when they do that it also makes it brittle when you chip or try to cut it. I believe it has something to do with surface tension. If you have a raw piece of glass look at the edges if the have been ground or rounded off it is a good indicator that it is tempered.Also quite often but not always, they will put a sticker on it indicating that it is tempered. Hope that helps