Tempered glass is about four times stronger than “ordinary,” or annealed glass. And unlike annealed glass, which can shatter into jagged shards when broken, tempered glass fractures into small, relatively harmless pieces.
As a result, tempered glass is used in those environments where human safety is an issue. Applications include side and rear windows in vehicles, entrance doors, shower and tub enclosures, shelving, patio furniture, equipment, microwave ovens and skylights.
To prepare glass for the tempering process, it must first be cut to the desired size. (Strength reductions or product failure can occur if any fabrication operations, such as etching or edging, take place after the heat treatment.) Then the glass begins the heat treatment process in which it travels through a tempering oven. The oven heats the glass to a temperature of more than 600 degrees Celsius. (The industry standard is 620 degrees Celsius.)
The glass then undergoes a high-pressure cooling procedure called “quenching.” During this process, which lasts just seconds, high-pressure air blasts the surface of the glass from an array of nozzles. 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.
Glass in tension breaks about five times more easily than it does in compression. Annealed glass will break at 6,000 pounds per square inch (psi). Tempered glass, according to federal specifications, must have a surface compression of 10,000 psi or more; it generally breaks at approximately 24,000 psi.
Not all glass can be tempered. Some Textured Glass that has lots of surface distortion or glass with bubbles on the inside (example: Seedy Glass) cannot be tempered. Also glass less than 1/8” or 3mm cannot be tempered.
Below is a list of Glass that can be tempered.
1/8″ Gray or bronze
1/4″ Gray or bronze
3/8″ Gray or bronze
1/2″ Gray or bronze
1/4″ Ultrawhite with showerguard
3/8″ Ultrawhite with showerguard
1/2″ Ultrawhite with showerguard
3/8″ P516 (pat#62)
Obscure Flutex 3/16”
Glue Chip 1/4”
Glue Chip 1/8”
Satin Etch 1/8”
Satin Etch 1/4”
Satin Etch 3/8”
Satin Etch 1/2”
Autumn Leaf 3/16”
German Antique 1/4”