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Borosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion (~3 × 10−6 /°C at 20°C), making them resistant to thermal shock, more so than any other common glass. Such glass is less subject to thermal stress and is commonly used for the construction of reagent bottles. Borosilicate glass is sold under such trade names as Suprax, Kimax, Pyrex, Endural, Schott, or Refmex.
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, the name became a synonym for borosilicate glass in the English-speaking world.
The European manufacturer of Pyrex, Arc International, uses borosilicate glass in its Pyrex glass kitchen products; however, the U.S. manufacturer of Pyrex kitchenware uses tempered soda-lime glass. Thus Pyrex can refer to either soda-lime glass or borosilicate glass when discussing kitchen glassware, while Pyrex, Bomex, Duran, TGI and Simax all refer to borosilicate glass when discussing laboratory glassware.
Most borosilicate glass is colorless. Colored borosilicate, for the studio glass trade, was first widely brought onto the market in 1986 when Paul Trautman founded Northstar Glassworks. In 2000, former Northstar Glassworks employee Henry Grimmett started Glass Alchemy and developed the first cadmium Crayon Colors and aventurine Sparkle colors in the borosilicate palette.
In addition to the quartz, sodium carbonate, and aluminum oxide traditionally used in glassmaking, boron is used in the manufacture of borosilicate glass. The composition of low expansion borosilicate glass such as those laboratory glasses mentioned above is approximately 80% silica, 13% boric oxide, 4% sodium oxide, and 2-3% aluminum oxide. Though more difficult to make than traditional glass due to the high melting temperature required (Corning conducted a major revamp of their operations to make it), it is economical to produce. Its superior durability, chemical and heat resistance finds excellent use in chemical laboratory equipment, cookware, lighting and, in certain cases, windows.
Borosilicate glass is created by adding boric oxide 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 for industrial production. Borrowing from the welding trade, burners combining oxygen with natural gas were required.
The common type of borosilicate glass used for laboratory glassware has a very low thermal expansion coefficient (3.3 x 10−6/K), about one-third that of ordinary soda-lime glass. This reduces material stresses caused by temperature gradients which makes borosilicate a more suitable type of glass for certain applications (see below).
Borosilicate glass is less dense than typical soda-lime glass due to the low atomic weight of boron.
While more resistant to thermal shock than other types of glass, borosilicate glass can still crack or shatter when subjected 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).
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Virtually all modern laboratory glassware is borosilicate glass. It is so widely used in this application due to its chemical and thermal resistance and good optical clarity, but the glass can be reacted with sodium hydride to produce sodium borohydride, a common laboratory reducing agent. Fused quartz is also found in some laboratory equipment when its higher melting point and transmission of UV are required (e.g. for tube furnace liners and UV cuvettes), but the cost and difficulty of working with quartz make it excessive for the majority of laboratory equipment.
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. Borosilicate glass is used for measuring cups, featuring screen printed markings providing graduated measurements, which are widely used in American kitchens.
Many current glass sex toys use high grade Borosilicate glass. The toys are generally hand blown, their beauty combined with the ease of cleaning and a long service life make them a popular material.
Many high-quality flashlights use borosilicate glass for the lens. This allows for a higher percentage of light transmittance through the lens compared to plastics and lower-quality glass.
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 studio glass movement has responded to color. "The availability of colors began to increase when companies such as Glass Alchemy introduced the Crayon Colors, which brought a whole new vivacity to the glass industry." 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 kitchen- and glassware increased durability along with microwave and dishwasher compatibility.
Most astronomical reflecting telescope 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" eyeglass lenses. Ordinary lower-cost borosilicate glass, like that used to make kitchenware and even reflecting telescope mirrors, cannot be used for high-quality lenses because of the striations and inclusions common to lower grades of this type of glass. The maximum working temperature is 515 Fahrenheit (268.3 °C). While it transitions to a liquid starting at 550 degrees Fahrenheit(just before it turns red-hot), it is not workable until it reaches over 1000 fahrenheit. That means that in order to industrially produce this glass, oxygen/fuel torches must be used. Glassblowers borrowed technology and techniques from welders.
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 microelectromechanical systems (MEMS), as part of stacks of etched silica wafers bonded to the etched borosilicate glass.
Lighting manufacturers use borosilicate glass in their refractors.
Organic light emitting diode (for display and lighting purposes) also uses borocilicate glass (BK7). The thicknesses of the BK7 glass substrates are usually less than 1 millimeter for the OLED fabrication. Due to its optical and mechanical characteristics in relation with cost, BK7 is a common substrate in OLEDs. However, depending on the application, sodalime glass substrates of similar thicknesses are also used in OLED fabrication.
Additionally, borosilicate tubing is used as the feedstock for the production of parenteral drug packaging, such as vials and pre-filled syringes, and is also used for the production of ampoules and dental cartridges. The chemical resistance of borosilicate glass minimizes the migration of sodium ions from the glass matrix thus making it well suited for injectable drug applications. This type of glass is typically referred to as USP / EP JP Type I.
Borosilicate glasses are used for immobilisation and disposal of radioactive wastes. In most countries high-level radioactive waste has been incorporated into alkali borosilicate or phosphate vitreous waste forms for many years and vitrification is an established technology. Vitrification is a particularly attractive immobilization route because of the high chemical durability of the vitrified glass product. This characteristic has been used by industry for centuries. The chemical resistance of glass can allow it to remain in a corrosive environment for many thousands and even millions of years.
It was initially thought that borosilicate glass could not be formed into nanoparticles, since an unstable boron oxide precursor prevented successful forming of these shapes. However, in 2008 a team of researchers from the Swiss Federal Institute of Technology at Lausanne were successful in forming borosilicate nanoparticles of 100 to 500 nanometers in diameter. The researchers formed a gel of tetraethylorthosilicate and trimethoxyboroxine. When this gel is exposed to water under proper conditions, a dynamic reaction ensues which results in the nanoparticles.
Borosilicate, or "boro" (as it is often called), is used extensively in the glassblowing process lampworking; the glassworker uses a burner torch to melt and form glass, using a variety of metal and graphite tools to shape it. Borosilicate is referred to as "hard glass" and has a higher melting point (approximately 3,000 degrees Fahrenheit / 1649 degrees Celsius) than "soft glass," which is preferred for glassblowing by beadmakers. Raw glass used in lampworking comes in glass rods for solid work and glass tubes for hollow work tubes and vessels/containers. Lampworking is used to make complex and custom scientific apparatus; most major universities have a lampworking shop to manufacture and repair their glassware. For this kind of "scientific glassblowing", the specifications must be exact and the glassblower must be highly skilled and able to work with precision. Lampworking is also done as art and common items made include goblets, paper weights and pendants.
In recent years, with the resurgence of lampworking as a technique to make handmade glass beads, boro has become a popular material in many glass artists' studios. Boro for beadmaking comes in thin, pencil-like rods. Northstar, Momka's Glass, Trautman Art Glass, and Glass Alchemy are popular manufacturers, although there are other brands available. The metals used to color boro glass, particularly silver, often create strikingly beautiful and unpredictable results when melted in an oxygen-gas torch flame. Because it retains heat longer than soft glass, and because it is stronger than soft glass, boro is particularly suited for sculpting and creating large beads, including figurines. The tools used for making glass beads from boro glass are the same as those used for making glass beads from soft glass. Colored borosilicate lampworking glass is most often considerably more expensive than "soft" lampworking glass, and this is usually reflected in the selling price of handmade artisan borosilicate beads and other creations.
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