BOROSILICATE GLASS

'Borosilicate glass' is a type of heat-resistant glass. Borosilicate glass was first developed by German glassmaker Otto Schott in the late 19th century and sold under the brand name "Duran" in 1893. After Corning Glass Works introduced Pyrex in 1915, it became a synonym for borosilicate glass in the English-speaking world. Holophane manufactures original equipment lenses for street lights under the Endural brand name. Bomex is a brand of borosilicate glassware made in China for a number of resellers in the United States and abroad.
In addition to the quartz, sodium carbonate, and calcium carbonate traditionally used in glassmaking, boron is used in the manufacture of borosilicate glass. Typically, the resulting glass composition is about 70% silica, 10% boric oxide, 8% sodium oxide, 8% potassium oxide, and 1% calcium oxide. Though somewhat more difficult to make than traditional glass (Corning conducted a major revamp of their operations to make it), it is economical to produce because its superior durability, chemical and heat resistance finds excellent use in chemical laboratory equipment, cookware, lighting, and in certain cases, windows.

Contents

Manufacturing process

Composition and physical characteristics

Usage

References

Weblinks

Manufacturing process

Borosilicate glass is created by adding boron to the traditional glassmaker's "frit" of silicate sand, soda, and ground lime. Since Borosilicate glass melts at a higher temperature than ordinary silicate glass, some new techniques were required to bring it into industrial production. Borrowing from the welding trade, new burners combining oxygen with natural gas were required.

Composition and physical characteristics

Borosilicate glass has a very low thermal expansion coefficient, about one-third that of ordinary glass. This reduces material stresses caused by temperature gradients, thus making it more resistant to breaking. This makes it a popular material for objects like telescope mirrors, where it is essential to have very little deviation in shape. It is also used in the processing of high-level nuclear waste, where the waste is immobilised in the glass through a process known as vitrification (contrast with Synroc).
Borosilicate glass begins to soften around 821 °C (1510 °F); at this temperature, the viscosity of type 7740 Pyrex is 10^7.6 poise. ^[1]
Borosilicate glass is less dense than ordinary glass.
While more resistant to thermal shock than other types of glass, borosilicate glass can still crack or shatter when subject to rapid or uneven temperature variations. When broken, borosilicate glass tends to crack into large pieces rather than shattering (it will snap rather than splinter).
Optically, borosilicate glasses are crown glasses with low dispersion (Abbe numbers around 65) and relatively low refractive indices (1.51 - 1.54 across the visible range).
'Fraction by weight'

Element	Atomic number	Fraction
B	5	0.040064
O	8	0.539562
Na	11	0.028191
Al	13	0.011644
Si	14	0.377220
K	19	0.003321

'Physical characteristics'

Density = 2.23 g/cm³

Mean Excitation Energy = 134.0 eV

Usage

Borosilicate glass's refractory properties and physical strength make it ideal for use in laboratories, where it is used to make high-durability glass lab equipment, such as beakers and test tubes.
Most glassware used in laboratories is made of borosilicate glass, due partly to how little it warps when exposed to heat hence its ability to provide accurate measurements of volume over time.
During the mid-twentieth century borosilicate glass tubing was used to pipe coolants (often distilled water) through high power vacuum tube based electronic equipment such as commercial broadcast transmitters.
Glass cookware is another common usage; a borosilicate glass pie plate is almost the American standard pie dish. Borosilicate glass measuring cups, which featured painted-on markings illustrating graduated measurements, are also widely used in American kitchens.
Aquarium heaters are sometimes made out of borosilicate glass. Due to its high heat resistance, it can tolerate the great temperature differences between water and the nichrome heating element.
Many high quality flashlights, such as those made by Surefire, use borosilicate glass for the lens. This allows for a higher percentage of light transmittance through the lens than compared to plastics and lower-quality glass.
Specialty marijuana pipes (commonly sold as tobacco pipes for reasons of legality) are made from borosilicate glass. The high heat resistance allows the pipe to tolerate a longer period of use, and these pipes are also more durable.
Most premanufactured glass guitar slides are also made of borosilicate glass.
New lampworking techniques led to artistic applications such as contemporary glass marbles. The modern glass art movement has provided vast economic growth for borosilicate glass suppliers. Borosilicate is commonly used in the glassblowing form of lampworking and the artists create a range of products ranging from jewelry, kitchen ware, sculpture as well as for the previously mentioned artistic glass "tobacco" pipes.
Borosilicate glass is sometimes used for high-quality beverage glassware; Bodum, Inc. markets a line of French coffee presses and double-walled beverage glasses made of borosilicate, lending them increased durability and microwave/dishwasher compatibility.
Most astronomical reflecting telescope glass mirror components are made of borosilicate glass due to the low coefficient of expansion due to heat. This makes very precise optical surfaces possible that change very little with temperature, and matched glass mirror components that 'track' across temperature changes and retain the optical system's characteristics. Borosilicate glass is not used for high quality lenses due to striations and inclusions common to this type of glass.
Borosilicate is also a material of choice for evacuated tube solar thermal technology, due to its high strength and heat resistance.

References

1. Melting Point Table

Weblinks

★ http://www.duran-group.com/english/products/duran/properties/index.html zu Borosilicatglas