Toughened glass

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Toughened glass or tempered glass is a type of safety glass that has increased strength and will usually shatter in small, square pieces when broken. It is used when strength, thermal resistance and safety are important considerations.

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 which heats it to above its annealing point of 600 °C. The glass is then rapidly cooled with forced draughts of air. This rapidly cools the glass surface below its annealing point, causing it to harden and contract, while the inner portion of the glass remains free to flow for a short time. The final 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.

The pattern of cooling during the process can be revealed by observing the glass with polarized light, which shows the strain pattern in the glass.

Toughened glass has many household and commercial uses. At home you are likely to find toughened glass in shower and sliding glass patio doors and in glass dinnerware and drinking glasses. In commercial structures it is used in unframed assemblies such as frameless doors, structurally loaded applications and any which would be dangerous in the case of a human impact. Using toughened glass can pose a security risk in some situations due to the tendency the glass has to shatter utterly upon hard impact.

Toughened glass is typically four to six times the strength of annealed glass and withstands heating in your microwave oven. However, this strength comes with a penalty. Due to 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. Shattering may not happen when the damage originally occurs and can be triggered by a minor stress like heat or small drop that would not normally affect the toughened glass. If any toughened glass shows any damage it must be replaced.

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. Also, ironically, the toughened glass surface is not as hard as annealed glass and is slightly more susceptible to scratching.

Toughened glass is usually called "tempered glass" in the dining room and kitchen. It is used for microwave safe and break-resistant clear or coloured glass dinnerware under the brand names Arcoroc & Duralex. It is also used in drinking glasses and stemware in two forms, Fully Tempered and Rim Tempered. Rim tempered indicates a limited area such as the rim of the glass or plate is tempered and is popular in food service. Other forms of tempered glass are Borosilicate glass for baking by Pyrex and Arcuisine and ceramic-glass dinnerware by Corelle and Arcopal and bakeware and stove top pots by Arcoflam.

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 (1619–1682), who was grandson of James I of England, and nephew of Charles I, brought the discovery of what are now known as "Prince Rupert's Drops" to the attention of the King. These are remarkable 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. These were often used by the King as a practical joke.