Originally Posted by bradedup
It really depends on the components are in the glass...aluminum oxides and such...and what sort of heat treating/etc has been done to the glass. Like...bulletproof glass.
over all the biggest thing is if it is annealed or tempered or lack there of. if it is annealed it will have a lot of strength but if it breaks it will have sharper edges, if it is tempered any deep scratch will make it basically explode, and if its not either of those it may just break randomly.
usually bullet proof glass isnt glass IIRC, at least most of the newer ones are a polymer.
the chemical composition can make a difference your right but the usual issues are probably improper annealing, any way there are really only 2 main categories of glass borosilicate (pyrex) and soda lime (most hand blown stuff) the differences in the chemicals change the strength some but the big part they do is make it stay soft to a cooler temp when working it; pyrex will harden up at really hot temps where as the soda lime glass that is hand blown will stay soft down to like 1200*f