Toughened or tempered glass is glass that has been processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. It will usually shatter into small fragments, rather than sharp shards, when broken.
An alternative chemical 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.
There are two main types of heat treated glass, heat strengthened and fully tempered. Heat strengthened glass is twice as strong as annealed glass whilst 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 above 65 MPa while Heat Strengthened glass is between 40 and 55 MPa.
It is important to note that while the strength of the glass does not change the deflection, being stronger means that it can deflect more before breaking. Annealed glass deflects less than tempered glass under the same load, all else being equal.
Rim tempered indicates a limited area such as the rim of the glass or plate is tempered and is popular in food service.
The strain pattern resulting from tempering can be observed with polarized light or by using a pair of polarized sun glasses.
Tempered glass must be annealed to remove its internal stresses before cutting or grinding.
The greater contraction of the inner layer induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass. This compressive stress on the surface of the glass is typically as high as 50 MPa. It is this compressive stress that gives the toughened glass an 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. However, the toughened glass surface is not as hard as annealed glass and is slightly more susceptible to scratching. To prevent this, toughened glass manufacturers apply various coatings and/or laminates to the surface of the glass.
Though the underlying mechanism was not known at the time, the effects of "tempering" glass have been known for centuries. In the 1640s, Prince Rupert of Bavaria brought the discovery of what are now known as "Prince Rupert's Drops" to the attention of the King. 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. The teardrops were often used by the King as a practical joke.