Toughened or
tempered glass is a type of
safety glassSafety glass is glass with additional safety features. Designs include:* Toughened glass * Laminated glass* Wire mesh glass...
processed by controlled
thermalHeat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass...
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 glassBulletproof glass is a type of strong but optically transparent material that is particularly resistant to being penetrated when struck by bullets, but is not completely impenetrable. It is usually made from a combination of two or more types of glass, one hard and one soft...
, for
diving maskA diving mask is an item of diving equipment that allows scuba divers, free-divers, and snorkelers to see clearly underwater. When the human eye is in direct contact with water as opposed to air, its normal environment, light entering the eye is refracted by a different angle and the eye is unable...
s, and various types of plates and cookware. In the United States, since 1977 Federal law has required glass located within 18 inch of a floor or doorway to be tempered.
Properties
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. However, the toughened glass surface is not as hard as annealed glass and is therefore somewhat more susceptible to scratching. To prevent this, toughened glass manufacturers may apply various coatings and/or laminates to the surface of the glass.
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
polarizedA polarizer is an optical filter that passes light of a specific polarization and blocks waves of other polarizations. It can convert a beam of light of undefined or mixed polarization into a beam with well-defined polarization. The common types of polarizers are linear polarizers and circular...
light or by using a pair of polarizing sun glasses.
Uses
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, used for its characteristic of shattering into small cubes rather than large shards. (The
windscreenThe windshield or windscreen of an aircraft, car, bus, motorbike or tram is the front window. Modern windshields are generally made of laminated safety glass, a type of treated glass, which consists of two curved sheets of glass with a plastic layer laminated between them for safety, and are glued...
or windshield is instead made of
laminated glassLaminated glass is a type of safety glass that holds together when shattered. In the event of breaking, it is held in place by an interlayer, typically of polyvinyl butyral , between its two or more layers of glass. The interlayer keeps the layers of glass bonded even when broken, and its high...
, 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
Some forms of tempered glass are used for
cookingCooking is the process of preparing food by use of heat. Cooking techniques and ingredients vary widely across the world, reflecting unique environmental, economic, and cultural traditions. Cooks themselves also vary widely in skill and training...
and
bakingBaking is the technique of prolonged cooking of food by dry heat acting by convection, and not by radiation, normally in an oven, but also in hot ashes, or on hot stones. It is primarily used for the preparation of bread, cakes, pastries and pies, tarts, quiches, cookies and crackers. Such items...
. Manufacturers include
PyrexPyrex is a brand name for glassware, introduced by Corning Incorporated in 1915.Originally, Pyrex was made from borosilicate glass. In the 1940s the composition was changed for some products to tempered soda-lime glass, which is the most common form of glass used in glass bakeware in the US and has...
,
CorelleCorelle is a brand of glassware and dishware. It is made of Vitrelle, a laminated tempered glass product with three layers of two types of glass. The thermally bonded layers give Corelle its strength, allowing it to be much thinner than other dinnerware. Introduced by Corning Glass Works in 1970,...
, and
Arc InternationalArc International is a French manufacturer and distributor of household goods. The company was established in Arques, Pas-de-Calais, where it is still headquartered, as a glass-making firm under the name Verrerie des Sept Ecluses in 1825...
.
Manufacturing
Toughened glass is made from
annealed glassAnnealing is a process of slowly cooling glass to relieve internal stresses after it was formed. The process may be carried out in a temperature-controlled kiln known as a Lehr. Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change...
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 process involves forcing a surface layer of glass at least 0.1mm thick into compression by
ion exchangeIon exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic 'ion...
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 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
MPaThe pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
(9427
psiThe pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units...
) in pressure-resistance while heat strengthened glass is between 40 and 55
megapascalsThe pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
(5801 and 7977 psi respectively).
It is important to note that the tempering process does not change the stiffness of the glass. Annealed glass deflects the same amount as tempered glass under the same load, all else being equal. But tempered glass will take a larger load, and therefore deflect further at break.
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. 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 the rollers. This waviness is a significant problem in manufacturing of thin film solar cells.
History
The first patent on tempered glass was held by chemist
Rudolph A. SeidenRudolph A. Seiden was an American chemist of Austrian origin.Seiden was born in Austria. In 1935, he and his late wife, Juliette Seiden "foresaw the beginning of the Holocaust in neighboring Germany, and moved to Kansas City, Missouri. Later, they...
, born in 1900 in Austria.
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 BavariaRupert, Count Palatine of the Rhine, Duke of Bavaria, 1st Duke of Cumberland, 1st Earl of Holderness , commonly called Prince Rupert of the Rhine, KG, FRS was a noted soldier, admiral, scientist, sportsman, colonial governor and amateur artist during the 17th century...
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. 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 jokeA practical joke is a mischievous trick played on someone, typically causing the victim to experience embarrassment, indignity, or discomfort. Practical jokes differ from confidence tricks in that the victim finds out, or is let in on the joke, rather than being fooled into handing over money or...
.