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Toughened or tempered glass is a type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering creates balanced internal stresses which cause the glass, when broken, to crumble into small granular chunks instead of splintering into jagged shards. The granular chunks are less likely to cause injury.
As a result of its safety and strength, tempered glass is used in a variety of demanding applications, including passenger vehicle windows, shower doors, architectural glass doors and tables, refrigerator trays, as a component of bulletproof glass, for diving masks, and various types of plates and cookware. In the United States, federal safety laws require that window glass be tempered if each of the following criteria are met: sill height within 18 in (0.457 m) of the floor, top edge greater than 36 in (0.914 m) from the floor, area greater than 9 ft² (0.836 m²), and horizontal distance to nearest walking surface of less than 36 in (0.914 m).
Toughened glass is physically and thermally stronger than regular glass. The greater contraction of the inner layer during manufacturing induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass. For glass to be considered toughened, this compressive stress on the surface of the glass should be a minimum of 69 MPa. For it to be considered safety glass, the surface compressive stress should exceed 100 MPa. The greater the surface stress, the smaller the glass particles will be when broken.
It is this compressive stress that gives the toughened glass increased strength. This is because any surface flaws tend to be pressed closed by the retained compressive forces, while the core layer remains relatively free of the defects which could cause a crack to begin.
Any cutting or grinding must be done prior to tempering. Cutting, grinding, sharp impacts and sometimes even scratches after tempering will cause the glass to fracture. The glass solidified by dropping into water, known as "Prince Rupert's Drops", which will shatter when their "tails" are broken, are extreme examples of the effects of internal tension.
The strain pattern resulting from tempering can be observed with polarized light or by using a pair of polarizing sun glasses.
Toughened glass is used when strength, thermal resistance and safety are important considerations. The most commonly encountered tempered glass is that used for side and rear windows in automobiles.
It is used for its characteristic of shattering into small cubes rather than large shards and is sometimes referred to as safety glass in this context. (The windscreen or windshield is instead made of laminated glass, which will not shatter when broken.)
Toughened glass is also used in buildings for unframed assemblies (such as frameless doors), structurally loaded applications, and any other application that would become dangerous in the event of human impact.
Rim-tempered indicates a limited area such as the rim of the glass or plate is tempered and is popular in food service.
Cooking and baking 
Toughened glass is made from annealed glass via a thermal tempering process. The glass is placed onto a roller table, taking it through a furnace that heats it above its annealing point of about 720 °C. The glass is then rapidly cooled with forced air drafts while the inner portion remains free to flow for a short time.
An alternative chemical toughening process involves forcing a surface layer of glass at least 0.1mm thick into compression by ion exchange of the sodium ions in the glass surface with the 30% larger potassium ions, by immersion of the glass into a bath of molten potassium nitrate. Chemical toughening results in increased toughness compared with thermal toughening, and can be applied to glass objects of complex shape.
The term toughened glass is generally used to describe fully tempered glass but is sometimes used to describe heat strengthened glass as both types undergo a thermal 'toughening' process.
There are two main types of heat treated glass: heat strengthened and fully tempered. Heat strengthened glass is twice as strong as annealed glass while fully tempered glass is typically four to six times the strength of annealed glass and withstands heating in microwave ovens. The difference is the residual stress in the edge and glass surface. Fully tempered glass in the US is generally rated above 65 MPa (9427 psi) in pressure-resistance while heat strengthened glass is between 40 and 55 megapascals (5801 and 7977 psi respectively).
It is important to note that the tempering process does not change the stiffness of the glass. Annealed glass undergoes a similar deflection compared to tempered glass under the same load. But tempered glass can take a higher load and therefore deflects further before breaking.
Toughened glass must be cut to size or pressed to shape before toughening and cannot be re-worked once toughened. Polishing the edges or drilling holes in the glass is carried out before the toughening process starts. Because of the balanced stresses in the glass, damage to the glass will eventually result in the glass shattering into thumbnail-sized pieces. The glass is most susceptible to breakage due to damage to the edge of the glass where the tensile stress is the greatest, but shattering can also occur in the event of a hard impact in the middle of the glass pane or if the impact is concentrated (for example, striking the glass with a point). Using toughened glass can pose a security risk in some situations because of the tendency of the glass to shatter completely upon hard impact rather than leaving shards in the window frame.
The surface of tempered glass does exhibit surface waves caused by contact with flattening rollers, if it has been formed using this process. This waviness is a significant problem in manufacturing of thin film solar cells.
Though the underlying mechanism was not known at the time, the effects of "tempering" glass have been known for centuries. In about 1660, Prince Rupert of the Rhine brought the discovery of what are now known as "Prince Rupert's Drops" to the attention of King Charles II. These are teardrop shaped bits of glass which are produced by allowing a molten drop of glass to fall into a bucket of water, thereby rapidly cooling it. They have the curious ability to withstand a blow from a hammer on the bulbous end without breaking, but the drops will disintegrate explosively if the tail end is even slightly damaged. The teardrops were often used by the King as a practical joke.
- H.G. Pfaender (1996) Schott guide to glass. Chapman and Hall ISBN 0-412-62060-X
- O'Block, Robert L.; Donnermeyer, Joseph F.; Doeren, Stephen E. (1991). Security and Crime Prevention. Butterworth–Heinemann. "The security value of tempered glass, however, is questionable. Although it will resist a brick or rock, it is susceptible to sharp instruments such as ice picks or screwdrivers. When attacked in this manner, tempered glass tends to crumple easily and quietly, leaving no sharp edges."
- W. Walecki, F. Szondy, "Integrated quantum efficiency, topography, and stress metrology for solar cell manufacturing: real space approach" , Proc. SPIE, Vol. 7048, 704804 (2008); doi:10.1117/12.792934 Online Publication Date: 10 September 2008 http://dx.doi.org/10.1117/12.792934