Do you really know tempered glass?

Tempered or toughened glass is a type of safety glass processed through controlled thermal or chemical treatments, which enhances its strength compared to ordinary glass. Tempering creates compressive stress on the outer surface and tensile stress inside the glass. This stress causes the glass to shatter into small granular pieces when broken, rather than sharp, jagged shards like flat glass (also known as annealed glass). The granular fragments are less likely to cause injury.

Due to its safety and strength, tempered glass is used in various demanding applications, including passenger car windows, shower doors, architectural glass doors and tabletops, refrigerator shelves, mobile phone screen protectors, as part of bulletproof glass, for diving masks, and in various types of dishes and cookware.

Tempered glass has approximately four times the strength of annealed ("regular") glass. The greater contraction of the inner layer during manufacturing induces compressive stress on the glass surface and tensile stress within the glass body. Fully tempered glass with a thickness of 6 mm must have a minimum surface compression of 69 MPa (10,000 psi) or edge compression of no less than 67 MPa (9,700 psi). To be classified as safety glass, its surface compressive stress should exceed 100 MPa (15,000 psi). Due to the increased surface stress, if the glass breaks, it only shatters into small, rounded fragments instead of sharp, jagged pieces. This characteristic makes tempered glass safe for high-pressure and explosion-proof applications.

It is this compressive surface stress that gives tempered glass its higher strength. Annealed glass, which has almost no internal stress, typically develops tiny surface cracks; without surface compression, any tension applied to the glass creates tensile stress on the surface, driving crack propagation. Once a crack starts to spread, tension becomes further concentrated at the crack tip, causing it to propagate at the speed of sound in the material. As a result, annealed glass is brittle and breaks into irregular, sharp fragments. On the other hand, the compressive stress on tempered glass contains defects and prevents their propagation or expansion.

Any cutting or grinding must be performed before tempering. Cutting, grinding, or severe impact after tempering can cause the glass to shatter.

The strain patterns caused by tempering can be observed by viewing the glass through an optical polarizer (e.g., a pair of polarized sunglasses).

Tempered glass is used when strength, heat resistance, and safety are key considerations. For example, passenger cars require all three properties. Stored outdoors, they are exposed to constant heating and cooling throughout the year, as well as drastic temperature changes. Additionally, they must withstand road debris such as gravel and minor impacts from road accidents. Since large, sharp glass fragments pose an additional danger to passengers (which is unacceptable), tempered glass is used so that it breaks into blunt, almost harmless pieces. Windshields are made of laminated glass, which does not shatter into fragments when broken, while side windows and rear windshields are typically tempered glass.

Other typical applications of tempered glass include:

Tempered glass is also used for frameless components in buildings (e.g., frameless glass doors), structurally loaded applications, and any other applications that could become dangerous upon human impact. In the United States, building codes require tempered or laminated glass in certain situations, including skylights, areas near doorways and staircases, large windows, windows close to the floor, sliding doors, elevators, fire department access panels, and areas near swimming pools.

Tempered glass is also used in homes. Common household furniture and appliances that use tempered glass include frameless shower doors, glass tabletops, glass shelves, cabinet glass, and fireplace glass.

"Tempered edge" means a limited area (e.g., the edge of a glass or plate) is tempered, which is popular in the food service industry. However, specialized manufacturers also offer fully tempered/toughened drinkware solutions that provide additional benefits in the form of strength and thermal shock resistance. In some countries, these products are specified for venues that require enhanced performance levels or safer glass due to frequent use.

The use of tempered glass in bars and pubs has also increased, particularly in the UK and Australia, to prevent broken glass from being used as a weapon. Tempered glass products can be found in hotels, bars, and restaurants to reduce breakage and improve safety standards.

Certain forms of tempered glass are used for cooking and baking. Manufacturers and brands include Glasslock, Pyrex, Corelle, and Arc International. This is also the type of glass used for oven doors.

Tempered glass can be produced from annealed glass through a thermal tempering process. The glass is placed on a roller conveyor and passed through a furnace, where it is heated to well above its transition temperature of 564°C (1,047°F) to around 620°C (1,148°F). The glass is then rapidly cooled with forced air while the interior remains free-flowing for a short time.

Another chemical toughening process involves compressing a surface layer of at least 0.1 mm thick on the glass surface by ion exchange—replacing sodium ions in the glass surface with potassium ions (30% larger)—followed by immersion in a molten potassium nitrate bath. Compared to thermal tempering, chemical toughening improves toughness and can be applied to glass objects with complex shapes.

Tempered glass must be cut to size or pressed into shape before tempering and cannot be processed further after tempering. Edges or drilled holes in the glass are polished before the tempering process begins. Due to the stress balance in the glass, damage to any part will eventually cause the glass to shatter into thumbnail-sized pieces. The glass is most likely to break due to damage to the edges, where tensile stress is highest, but it can also shatter if subjected to strong impact or concentrated force in the middle of the glass sheet (e.g., striking the glass with a hardened point).

In some cases, the use of tempered glass may pose a safety hazard because it shatters completely upon strong impact, rather than retaining fragments in the frame, which could lead to breakage-related injuries.

If the surface of tempered glass is formed using this process, it may develop surface waves caused by contact with flattening rollers. This waviness is a significant issue in the manufacture of thin-film solar cells. The float glass process can be used to provide low-distortion sheets with very flat and parallel surfaces as an alternative for different glass applications.

Nickel sulfide inclusions can cause tempered glass to spontaneously break years after manufacturing.