Borosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are most well known for having very low coefficient of thermal expansion, making them resistant to thermal shock, more so than any other common 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 （however, since 1998 Pyrex kitchen brand is no longer made of borosilicate but of soda-lime glass）.

Most borosilicate glass is clear. Colored borosilicate, for the art glass trade, was first widely brought onto the market in 1986 when Paul Trautman founded Northstar Glassworks. There are now a number of small companies in the U.S. and abroad that manufacture and sell colored borosilicate glass for the art glass market.

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% boron oxide, 8% sodium oxide, 8% potassium oxide, and 1% calcium oxide （lime）. 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.

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 escope 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 107.6 poise.

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）.

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. In addition, borosilicate glass warps minimally when exposed to heat allowing a borosilicate container 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, featuring 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 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 and tobacco pipes 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, spurred largely by the rapid development of a borosilicate color palette at Northstar Glass in the 1980s and 1990s, 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, kitchenware, to sculpture as well as for artistic glass tobacco pipes.

Borosilicate glass is sometimes used for high-quality beverage glassware. Borosilicate glass lends the kitchenware and glassware increased durability along with microwave and dishwasher compatibility.

Most astronomical reflecting escope glass mirror components are made of borosilicate glass because of its low coefficient of expansion with 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.

The optical glass most often used for making instrument lenses is Schott BK-7 （or the equivalent from other makers）, a very finely made borosilicate crown glass. It is also designated as 517642 glass after its 1.517 refractive index and 64.2 Abbe number. Other less costly borosilicate glasses, such as Schott B270 or the equivalent, are used to make "crown glass" eyeglasses lenses. Ordinary lower-cost borosilicate glass, like that used to make kitchenware and even reflecting escope mirrors, cannot be used for high quality lenses because of the striations and inclusions common to lower grades of this type of glass.

Borosilicate is also a material of choice for evacuated tube solar thermal technology, because of its high strength and heat resistance.

Borosilicate glasses also find application in the semiconductor industry in the development of micromechanical devices, known as MEMS, as part of stacks of etched silica wafers bonded to the etched borosilicate glass.

The thermal insulation tiles on the Space Shuttle are coated with Borosilicate glass. Lighting manufacturers use borosilicate glass in their refractors.