Tempered glass

A vandalized telephone booth with toughened glass

Toughened or tempered glass is glass that has been processed by controlled thermal or chemical treatments. It has increased strength compared with normal glass and will usually shatter into small fragments, rather than sharp shards, when broken.

Properties

Toughened glass is strong, has enhanced thermal resistance, and breaks into small cuboid fragments rather than irregular shards of glass.

Uses

Toughened glass is used when strength, thermal resistance and safety are important considerations. The tempered glass that is most familiar to the average person is the type used for side and rear windows in automobiles. Its most familiar property is to shatter into small cubes rather than sharp-edged shards. Within the home, tempered glass can be found in display case windows, shower doors, glass patio doors, glass dinnerware and drinking glasses. In commercial structures, tempered glass is used for unframed assemblies (such as frameless doors), structurally-loaded applications, and any other application that would become dangerous in the event of a human impact.

Cooking and baking

Some forms of tempered glass are used for cooking and baking. Pyrex uses include borosilicate glass in manufacturing baking dishes, while Corelle produces ceramic-glass dinnerware and ARC International produces these and similar products that also have stovetop uses.

Manufacturing

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 to above its annealing point of about 600 °C. The glass is then rapidly cooled with forced draughts of air while the inner portion of the glass remains free to flow for a short time.

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.

Advantages

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 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 whilst 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 and fully tempered glass will deflect the same amount for the same load.

Disadvantages

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. 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 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 due to the tendency the glass has to shatter completely upon hard impact.

Other information

The first patent on tempered glass was held by chemist Rudolph A. Seiden, born in 1900 in Austria.

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 converted to annealed glass 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.