Tempered glass is about four times stronger than annealed glass. The greater contraction of the inner layer during manufacturing induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass. Fully tempered 6-mm thick glass must have either a minimum surface compression of 69 MPa (10 000 psi) or an edge compression of not less than 67 MPa (9 700 psi). For it to be considered safety glass, the surface compressive stress should exceed 100 megapascals (15,000 psi). As a result of the increased surface stress, when broken the glass breaks into small rounded chunks as opposed to sharp jagged shards.
Compressive surface stresses give tempered glass increased strength. Annealed glass has almost no internal stress and usually forms microscopic cracks on its surface. Tension applied to the glass can drive crack propagation which, once begun, concentrates tension at the tip of the crack driving crack propagation at the speed of sound through the glass. Consequently, annealed glass is fragile and breaks into irregular and sharp pieces. The compressive stresses on the surface of tempered glass contain flaws, preventing their propagation or expansion.
Any cutting or grinding must be done prior to tempering. Cutting, grinding, and sharp impacts after tempering will cause the glass to fracture.
The strain pattern resulting from tempering can be observed by viewing through an optical polarizer, such as a pair of polarizing sunglasses.