Tempering is a heat treatment. What tempering does: Strengthens glass by 5 times. When tempered glass breaks, it shatters into countless small bits of glass, instead of large, sharp chards. This is why tempered glass is widely known as “Safety Glass”. After a piece of glass is put through the oven, and cooked at a blazing 1250 degrees, it becomes IMPOSSIBLE to alter, cut, or change in any way.