What is Tempered Glass?

broken tempered glass in womans hands

Tempered glass is one of the most common glass types used in architecture due to its thermal properties, safety, and durability. But do you know what it is and why it’s used? More importantly, when you need to use it? Learn everything you need from how it’s made, why it’s used, its capabilities, and facts you need to know.

How Its Made

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.

How is it Different?

  • Tempered glass is up to five times stronger than annealed glass of comparable size and thickness.
  • Tempered glass can also withstand high temperature changes of up to 250c.
  • Tempering the glass does not change any of the visual qualities of the glass such as solar radiance and light transmission.
  • Tempered glass is considered safety glass and is very difficult to break. If the glass is broken, it shatters into pebbles reducing the risk of serious injury.
  • Lastly, tempered glass cannot be cut, etched or carved, polished, or altered once it is tempered.
 

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.

Do You Need It?

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.

Strength and Safety

  • Up to 5x stronger than annealed
  • Up to 3x stronger than heat treated glass
  • Breaks into small pebbles instead of large shards
  • Perfect for thermal resistance and impacts

Heat Resistance

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.

When to Use

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.

Tempered Decorative Glass

Outside of etching and carving the glass, tempered glass can still be laminated, back painted, and printed on without any ill effects towards the glass.

Properties:

PropertiesTempered Glass
Thermal Shock ResistanceUp to 250c
Mechanical StrengthUp to 5x stronger than annealed
Tensile Strength65 MPa
Bending Strength120-200 N/mm2
Surface Compression>95 MPa
Design stress for architectural projects50 MPa
FragmentationSmall pebbles
FabricationNo cutting or drilling after temper
Standard ANSI Z97.1American National standard for glazing materials used in buildings – safety performance specifications and methods of test.
ASTM C1036Standard specification for flat glass.
ASTMStandard specification for heat treated flat glass.
Compliance
  • Tempered float glass shall comply with ASTM C1048, Type 1, Class 1 (clear), Glass 2 (tinted), Quality Q3, Kind FT.
  • Tempered safety glass products in the US are to comply with CPSC 16 CFR part 1201 for category II materials.

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