But artistic glass blowing can induce extreme residual stress in glass as it cools if done rapidly, especially if using different glass compositions. (This is true for all glass forming processes.) Failing under high stress is not the same as failing under little stress. If you have an I-beam with a large weight on it and it fails, you wouldn't say the metal was unstable, you would say there was too much stress on it. The issue with glass is that you can freeze in stress so even though it is there, you can't see it like that weight on the I-beam. Annealed glass will sit in a room until the end of time and not crack. If you have ever tried to break window glass, it is actually pretty hard. People don't open their cabinet to find one of their glasses has cracked, but if you take a hot glass out of the dishwasher and put cold water in it, you can certainly expect it to crack. It didn't fail under minimal stress, it failed under high stress. I can design a glass that is under extreme stress so that if you add a little more, it will crack. (I can make one that not only will crack, but will explode.) That doesn't mean that glass is inherently unstable, it just means that I stressed the daylights out of it. I can also design a glass that has no stress in it and so you really have to work at it to break it. Pyrex is almost completely heat insensitive for example. The glass they make telescope mirrors out of does not expand or contract with temperature changes and is annealed so that there is no residual stress either. The overarching statement that glass is unstable is just not correct.