Tempered glass, also called toughened glass, is a type of safety glass processed by controlled thermal treatments to increase its strength compared to ordinary annealed glass. When broken, it crumbles into small granular chunks instead of splintering into sharp shards, significantly reducing the risk of serious injury. This characteristic break pattern is the defining feature that qualifies tempered glass as safety glazing under building codes worldwide.
The Thermal Tempering Process
The tempering process begins with annealed float glass that has already been cut to its final size. This is a critical requirement: tempered glass cannot be cut, drilled, edge-worked, or sandblasted after tempering. Any attempt to do so will cause the glass to shatter completely. All fabrication must be completed before the glass enters the tempering furnace.
Inside the furnace, the glass is heated to its softening point, approximately 620 degrees Celsius. The glass sheets are conveyed on ceramic rollers through a furnace approximately 20 to 30 metres long. At this temperature, the glass is soft enough to deform slightly but not enough to sag under its own weight over the roller span.
The glass then exits the furnace and enters the quench section, where an array of air nozzles blasts cool air at high velocity onto both surfaces of the glass simultaneously. This rapid air quenching cools the surfaces quickly while the core remains hot and plastic. As the core eventually cools and contracts, it pulls the surfaces into compression. The result is a stress profile where the surfaces are under high compression and the interior is under tension. This is the opposite of what would make glass weak; surface compression closes up any micro-cracks that would otherwise propagate under tensile stress.
Strength and Stress Profile
Tempered glass is four to five times stronger than annealed glass of the same thickness. A typical 6 millimetre thick annealed glass has a modulus of rupture around 40 MPa, whereas the same glass after tempering achieves roughly 165 to 200 MPa. The compressive surface stress in fully tempered glass exceeds 10,000 pounds per square inch, or approximately 69 MPa. The standard requirement per ASTM C1048 in the United States and EN 12150 in Europe is a minimum surface compression of 69 MPa for fully tempered glass and between 24 and 52 MPa for heat-strengthened glass, which is a partially tempered product.
This elevated strength means tempered glass can resist impact from human bodies, wind loads, and thermal stress that would shatter annealed glass. The thermal resistance is also notably improved: tempered glass can withstand temperature differentials of up to 250 degrees Celsius, compared to roughly 40 degrees Celsius for annealed glass.
The Safety Break Pattern
When tempered glass does fail, it does so in a specific and predictable way. The tensile stress stored in the interior of the glass is released catastrophically through the entire sheet, causing it to disintegrate into thousands of small, relatively blunt fragments. Building codes typically require that the fragments have a surface area no larger than a specified maximum, often defined as the number of fragments in a 50 by 50 millimetre square. Broken tempered glass looks like a pile of small cubes or pebbles, hence the common term "diced" glass.
It is worth noting a peculiar failure mode: spontaneous breakage. Tempered glass can occasionally break without any apparent cause due to nickel sulphide inclusions. Nickel sulphide can exist in two crystal forms. During heating in the tempering furnace, the inclusion shrinks as it transforms to a high-temperature phase. Rapid cooling traps it in that phase. Over months or years, the inclusion can slowly transform back to the low-temperature phase, expanding in volume by about 2 to 4% and generating enough stress to fracture the glass from within. Heat soak testing, where tempered glass is held at 290 degrees Celsius for several hours, can identify and induce failure in at-risk panels before installation.
Common Applications
Tempered glass is mandated by building codes wherever human impact is a foreseeable risk. In residential construction, this includes shower doors and enclosures, sliding patio doors, windows adjacent to doors, stair railings, and any glazed panel within 18 inches of the floor. In automobiles, tempered glass is used for side windows and rear windows; only the windshield is laminated rather than tempered. In commercial applications, tempered glass is used for frameless glass doors, point-supported canopies, glass floors, and balustrades. Table tops, oven doors, refrigerator shelves, and smartphone screen protectors also frequently use tempered glass because of its combination of strength and safe failure mode.