Although it looks indistinguishable from normal glass, tempered glass is four to five times stronger than standard float glass.
Tempered glass is produced by a heat treatment process that balances the center and outer stresses of the glass and changes the internal structure of the particles. If tempered glass is broken, it will instantly shatter into small, relatively harmless thumbnail sized pebbles, instead of sharp, jagged shards, which could seriously injure a person. Tempered glass is widely used within the construction, automotive, electronic and kitchen appliance industries.
Storm doors, window panes, glass in building entrances, shower doors, sliding doors, coffee maker carafes, oven windows, tableware, computer screens, cell phone screens, diving masks and microwave oven screens are all made of tempered glass, as they are often used and surrounded by human activity. Building codes often require public buildings to use tempered glass in construction of their windows or decorative features.
Tempered glass breaks at about 24 thousand psi, but shattering will still occur at that point. One downside of tempered glass is the tendency to shatter without warning if a small knick occurs. Instead of partially breaking or becoming shards that stay in place, tempered glass explodes into thousands of small pieces instantly, even if a small section of the glass pane breaks.
Many glass fabricators temper glass through a process involving extreme heating and rapid cooling. First, a pane of standard glass is cut, washed, sanded and inspected, because after glass has been tempered, any altering to its shape will result in shattering. Then the glass is heated to 620º C in a tempering oven for a short time. When it exits the oven, the glass is promptly quenched with high pressure blasts of cold air.
This cooling process reduces the temperature of the glass’ exterior, while the center remains heated. Therefore, the center remains in tension while the outer goes into compression, thus balancing out the glass tension and significantly increasing the force required to shatter the glass. Although tempered glass is much stronger than normal glass, it is used only for the rear and side windows in cars.
The front windshield is typically made of laminated glass and composed of 2 outer layers of glass and an inner layer of transparent vinyl. If broken, the glass will cling to the vinyl instead of falling onto passengers. Coatings and laminates are sometimes applied to prevent scratching.