Tempered glass is the process of heating annealed glass, using a special furnace, to an even and uniform temperature of about 700c. Once the glass is heated up, it is removed from the furnace and rapidly cooled down. This puts the glass in a state of compression while the core of the glass stays within tension.
Due to tempering, a stress pattern is developed by the glass and the mechanical strength of the tempered glass is up to 5x higher than that of annealed glass.
When tempered glass is broken, all the stored energy from the stress pattern is released at once and the glass breaks into very small pebbles.
One issue with tempered glass is a very low percentage of glass panels can spontaneously break. This is due to nickel sulfide stones increasing in size due to repeated heating and cooling during tempering.
Tempered glass is preferred in areas where strength, safety, and thermal resistant properties are needed.
Tempering glass will increase strength, thermal shock resistance, and safety of the glass itself. Tempered glass also has an increased ability to withstand heat over annealed glass.
Also, your local building code will tell you when and where you will need to use tempered glass in place of annealed. So even if you think you wouldn’t need tempered, code will often dictate if tempered glass is needed or not.
This type of glass can withstand uneven and high temperatures due to amazing edge strength.
Annealed glass is susceptible to thermal breakage. This is caused by direct sunlight or direct heat sources causing the glass to expand and contract at different rates. Tempered glass has a very high edge strength allowing it to withstand thermal breakage.
Tempered glass is not harder or softer, easier to scratch, break, or more porous than annealed, but it is tougher. Tempered glass is designed to use in areas where there is a high risk of contact, temperature changes, high temperatures, and breakage.
You will often find tempered glass in architectural situations like windows, glass railing, wall cladding, shelving, doors, and showers. You will also find tempered glass in ovens, used as stove tops, fireplace screens and in cars.
You are also required to use tempered glass when your local code requires it.
Tempered glass is considered a safety glass product as it will break into smaller pieces reducing the risk of serious injury. So if you are designing an area, especially a public environment where glass could break around people, tempered glass is usually the way to go.
|Thermal Shock Resistance||Up to 250c|
|Mechanical Strength||Up to 5x stronger than annealed|
|Tensile Strength||65 MPa|
|Bending Strength||120-200 N/mm2|
|Surface Compression||>95 MPa|
|Design stress for architectural projects||50 MPa|
|Fabrication||No cutting or drilling after temper|
|Standard ANSI Z97.1||American National standard for glazing materials used in buildings – safety performance specifications and methods of test.|
|ASTM C1036||Standard specification for flat glass.|
|ASTM||Standard specification for heat treated flat glass.|