Tempered or toughened glass is a type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering puts the outer surfaces into compression and the interior into tension. Such stresses cause the glass, when broken, to crumble into small granular chunks instead of splintering into jagged shards as plate glass (a.k.a. annealed glass) does. The granular chunks are less likely to cause injury.
As a result of its safety and strength, tempered glass is used in a variety of demanding applications, including passenger vehicle windows, shower doors, architectural glass doors and tables, refrigerator trays, mobile phone screen protectors, as a component of bulletproof glass, for diving masks, and various types of plates and cookware.
Safety approval markings on an automobile vent window made for a Chrysler car by PPG.
Police van with screen protector
Tempered glass is used when strength, thermal resistance, and safety are important considerations. Passenger vehicles, for example, have all three requirements. Since they are stored outdoors, they are subject to constant heating and cooling as well as dramatic temperature changes throughout the year. Moreover, they must withstand small impacts from road debris such as stones as well as automobile accidents. Because large, sharp glass shards would present an additional and unacceptable danger to passengers, tempered glass is used so that if broken, the pieces are blunt and mostly harmless. The windscreen or windshield is instead made of laminated glass, which will not shatter into pieces when broken while side windows and the rear windshield are typically tempered glass.