Tempered glass is a type of safety glass that is four to five times stronger than annealed or standard glass. Unlike ordinary non-tempered glass, when tempered glass fails, it shatters into tiny little oval-shaped pebbles. Whereas, annealed glass will break into large pieces of glass making it a major human safety hazard.
Tempered glass is manufactured through a process of heating followed by rapid cooling or quenching. Quenching cools the outer surfaces of the glass much more quickly than the center. As the center of the glass cools, it tries to pull back from the outer surfaces. As a result, the center remains in tension, and the outer surfaces go into compression, which gives tempered glass its strength.
Optical image distortion can also occur for tempered glass. There are many parameters including glazing pressure, wind load, temperature and barometric pressure changes that can influence this distortion. The optical reflection of glass from a distance can also become wavy due to contact with rollers as the glass is passed through the oven during heat treatment.
The industry standard specification requirements for tempered glasses are ASTM C1048 which shared the same standards as heat-strengthened glass. Fully tempered glass will 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).