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Glass

Glass is an amorphous (non-crystalline) solid. Because it is often transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window panes, tableware, and optics. Some common objects made of glass like "a glass" of water, "glasses", and "magnifying glass", are named after the material.

Glass is most often formed by rapid cooling (quenching) of the molten form. Some glasses such as volcanic glass are naturally occurring, and obsidian has been used to make arrowheads and knives since the Stone Age. Archaeological evidence suggests glassmaking dates back to at least 3600 BC in Mesopotamia, Egypt, or Syria. The earliest known glass objects were beads, perhaps created accidentally during metalworking or the production of faience, which is a form of pottery using lead glazes.

Due to its ease of formability into any shape, glass has been traditionally used for vessels, such as bowls, vases, bottles, jars and drinking glasses. Soda–lime glass, containing around 70% silica, accounts for around 90% of modern manufactured glass. Glass can be coloured by adding metal salts or painted and printed with vitreous enamels, leading to its use in stained glass windows and other glass art objects.

The refractive, reflective and transmission properties of glass make glass suitable for manufacturing optical lenses, prisms, and optoelectronics materials. Extruded glass fibres have applications as optical fibres in communications networks, thermal insulating material when matted as glass wool to trap air, or in glass-fibre reinforced plastic (fibreglass).

Microscopic structure

The standard definition of a glass (or vitreous solid) is a non-crystalline solid formed by rapid melt quenching. However, the term "glass" is often defined in a broader sense, to describe any non-crystalline (amorphous) solid that exhibits a glass transition when heated towards the liquid state.

Glass is an amorphous solid. Although the atomic-scale structure of glass shares characteristics of the structure of a supercooled liquid, glass exhibits all the mechanical properties of a solid. As in other amorphous solids, the atomic structure of a glass lacks the long-range periodicity observed in crystalline solids. Due to chemical bonding constraints, glasses do possess a high degree of short-range order with respect to local atomic polyhedra. The notion that glass flows to an appreciable extent over extended periods well below the glass transition temperature is not supported by empirical research or theoretical analysis (see viscosity in solids). Though atomic motion at glass surfaces can be observed, and viscosity on the order of 1017–1018 Pa s can be measured in glass, such a high value reinforces the fact that glass would not change shape appreciably over even large periods of time.

Formation from a supercooled liquid
For melt quenching, if the cooling is sufficiently rapid (relative to the characteristic crystallization time) then crystallization is prevented and instead, the disordered atomic configuration of the supercooled liquid is frozen into the solid state at Tg. The tendency for a material to form a glass while quenched is called glass-forming ability. This ability can be predicted by the rigidity theory. Generally, a glass exists in a structurally metastable state with respect to its crystalline form, although in certain circumstances, for example in atactic polymers, there is no crystalline analogue of the amorphous phase.

Glass is sometimes considered to be a liquid due to its lack of a first-order phase transition where certain thermodynamic variables such as volume, entropy and enthalpy are discontinuous through the glass transition range. The glass transition may be described as analogous to a second-order phase transition where the intensive thermodynamic variables such as the thermal expansivity and heat capacity are discontinuous. However, the equilibrium theory of phase transformations does not hold for glass, and hence the glass transition cannot be classed as one of the classical equilibrium phase transformations in solids.

Occurrence in nature
Glass can form naturally from volcanic magma. Obsidian is a common volcanic glass with high silica (SiO2) content formed when felsic lava extruded from a volcano cools rapidly. Impactite is a form of glass formed by the impact of a meteorite, where Moldavite (found in central and eastern Europe), and Libyan desert glass (found in areas in the eastern Sahara, the deserts of eastern Libya and western Egypt) are notable examples. Vitrification of quartz can also occur when lightning strikes sand, forming hollow, branching rootlike structures called fulgurites. Trinitite is a glassy residue formed from the desert floor sand at the Trinity nuclear bomb test site. Edeowie glass, found in South Australia, is proposed to originate from Pleistocene grassland fires, lightning strikes, or hypervelocity impact by one or several asteroids or comets.

History
Naturally occurring obsidian glass was used by Stone Age societies as it fractures along very sharp edges, making it ideal for cutting tools and weapons.

Glassmaking dates back at least 6000 years, long before humans had discovered how to smelt iron. Archaeological evidence suggests that the first true synthetic glass was made in Lebanon and the coastal north Syria, Mesopotamia or ancient Egypt. The earliest known glass objects, of the mid-third millennium BC, were beads, perhaps initially created as accidental by-products of metalworking (slags) or during the production of faience, a pre-glass vitreous material made by a process similar to glazing.

Early glass was rarely transparent and often contained impurities and imperfections, and is technically faience rather than true glass, which did not appear until the 15th century BC. However, red-orange glass beads excavated from the Indus Valley Civilization dated before 1700 BC (possibly as early as 1900 BC) predate sustained glass production, which appeared around 1600 BC in Mesopotamia and 1500 BC in Egypt.

During the Late Bronze Age, there was a rapid growth in glassmaking technology in Egypt and Western Asia. Archaeological finds from this period include coloured glass ingots, vessels, and beads.

Much early glass production relied on grinding techniques borrowed from stoneworking, such as grinding and carving glass in a cold state.

The term glass developed in the late Roman Empire. It was in the Roman glassmaking centre at Trier (located in current-day Germany) that the late-Latin term glesum originated, probably from a Germanic word for a transparent, lustrous substance. Glass objects have been recovered across the Roman Empire in domestic, funerary, and industrial contexts, as well as trade items in marketplaces in distant provinces. Examples of Roman glass have been found outside of the former Roman Empire in China, the Baltics, the Middle East, and India. The Romans perfected cameo glass, produced by etching and carving through fused layers of different colours to produce a design in relief on the glass object.

In post-classical West Africa, Benin was a manufacturer of glass and glass beads. Glass was used extensively in Europe during the Middle Ages. Anglo-Saxon glass has been found across England during archaeological excavations of both settlement and cemetery sites. From the 10th century onwards, glass was employed in stained glass windows of churches and cathedrals, with famous examples at Chartres Cathedral and the Basilica of Saint-Denis. By the 14th century, architects were designing buildings with walls of stained glass such as Sainte-Chapelle, Paris, (1203–1248) and the East end of Gloucester Cathedral. With the change in architectural style during the Renaissance period in Europe, the use of large stained glass windows became much less prevalent, although stained glass had a major revival with Gothic Revival architecture in the 19th century.

During the 13th century, the island of Murano, Venice, became a centre for glass making, building on medieval techniques to produce colourful ornamental pieces in large quantities. Murano glass makers developed the exceptionally clear colourless glass cristallo, so called for its resemblance to natural crystal, which was extensively used for windows, mirrors, ships' lanterns, and lenses. In the 13th, 14th, and 15th centuries, enamelling and gilding on glass vessels were perfected in Egypt and Syria. Towards the end of the 17th century, Bohemia became an important region for glass production, remaining so until the start of the 20th century. By the 17th century, glass in the Venetian tradition was also being produced in England. In about 1675, George Ravenscroft invented lead crystal glass, with cut glass becoming fashionable in the 18th century. Ornamental glass objects became an important art medium during the Art Nouveau period in the late 19th century.

Throughout the 20th century, new mass production techniques led to the widespread availability of glass in much larger amounts, making it practical as a building material and enabling new applications of glass. In the 1920s a mould-etch process was developed, in which art was etched directly into the mould so that each cast piece emerged from the mould with the image already on the surface of the glass. This reduced manufacturing costs and, combined with a wider use of coloured glass, led to cheap glassware in the 1930s, which later became known as Depression glass. In the 1950s, Pilkington Bros., England, developed the float glass process, producing high-quality distortion-free flat sheets of glass by floating on molten tin. Modern multi-story buildings are frequently constructed with curtain walls made almost entirely of glass. Laminated glass has been widely applied to vehicles for windscreens. Optical glass for spectacles has been used since the Middle Ages. The production of lenses has become increasingly proficient, aiding astronomers as well as having other applications in medicine and science. Glass is also employed as the aperture cover in many solar energy collectors.

In the 21st century, glass manufacturers have developed different brands of chemically strengthened glass for widespread application in touchscreens for smartphones, tablet computers, and many other types of information appliances. These include Gorilla Glass, developed and manufactured by Corning, AGC Inc.'s Dragontrail and Schott AG's Xensation.

Optical
Glass is in widespread use in optical systems due to its ability to refract, reflect, and transmit light following geometrical optics. The most common and oldest applications of glass in optics are as lenses, windows, mirrors, and prisms. The key optical properties refractive index, dispersion, and transmission, of glass are strongly dependent on chemical composition and, to a lesser degree, its thermal history. Optical glass typically has a refractive index of 1.4 to 2.4, and an Abbe number (which characterises dispersion) of 15 to 100. The refractive index may be modified by high-density (refractive index increases) or low-density (refractive index decreases) additives.

Glass transparency results from the absence of grain boundaries which diffusely scatter light in polycrystalline materials. Semi-opacity due to crystallization may be induced in many glasses by maintaining them for a long period at a temperature just insufficient to cause fusion. In this way, the crystalline, devitrified material, known as Réaumur's glass porcelain is produced. Although generally transparent to visible light, glasses may be opaque to other wavelengths of light. While silicate glasses are generally opaque to infrared wavelengths with a transmission cut-off at 4 μm, heavy-metal fluoride and chalcogenide glasses are transparent to infrared wavelengths of 7 to 18 μm. The addition of metallic oxides results in different coloured glasses as the metallic ions will absorb wavelengths of light corresponding to specific colours.

Other
In the manufacturing process, glasses can be poured, formed, extruded and moulded into forms ranging from flat sheets to highly intricate shapes. The finished product is brittle but can be laminated or tempered to enhance durability. Glass is typically inert, resistant to chemical attack, and can mostly withstand the action of water, making it an ideal material for the manufacture of containers for foodstuffs and most chemicals. Nevertheless, although usually highly resistant to chemical attack, glass will corrode or dissolve under some conditions. The materials that make up a particular glass composition affect how quickly the glass corrodes. Glasses containing a high proportion of alkali or alkaline earth elements are more susceptible to corrosion than other glass compositions.

The density of glass varies with chemical composition with values ranging from 2.2 g/cm3 for fused silica to 7.2 g/cm3 for dense flint glass. Glass is stronger than most metals, with a theoretical tensile strength for pure, flawless glass estimated at 14 to 35 GPa due to its ability to undergo reversible compression without fracture. However, the presence of scratches, bubbles, and other microscopic flaws lead to a typical range of 14 to 175 MPa in most commercial glasses. Several processes such as toughening can increase the strength of glass. Carefully drawn flawless glass fibres can be produced with a strength of up to 11.5 GPa.

Reputed flow
The observation that old windows are sometimes found to be thicker at the bottom than at the top is often offered as supporting evidence for the view that glass flows over a timescale of centuries, the assumption being that the glass has exhibited the liquid property of flowing from one shape to another. This assumption is incorrect, as once solidified, glass stops flowing. The sags and ripples observed in old glass were already there the day it was made; manufacturing processes used in the past produced sheets with imperfect surfaces and non-uniform thickness (the near-perfect float glass used today only became widespread in the 1960s).

A 2017 study computed the rate of flow of the medieval glass used in Westminster Abbey from the year 1268. The study found that the room temperature viscosity of this glass was roughly 1024Pa·s which is about 1016 times less viscous than a previous estimate made in 1998, which focused on soda-lime silicate glass. Even with this lower viscosity, the study authors calculated that the maximum flow rate of medieval glass is 1 nm per billion years, making it impossible to observe in a human timescale.

Silicate glasses
Silicon dioxide (SiO2) is a common fundamental constituent of glass. Fused quartz is a glass made from chemically pure silica. It has very low thermal expansion and excellent resistance to thermal shock, being able to survive immersion in water while red hot, resists high temperatures (1000–1500 °C) and chemical weathering, and is very hard. It is also transparent to a wider spectral range than ordinary glass, extending from the visible further into both the UV and IR ranges, and is sometimes used where transparency to these wavelengths is necessary. Fused quartz is used for high-temperature applications such as furnace tubes, lighting tubes, melting crucibles, etc. However, its high melting temperature (1723 °C) and viscosity make it difficult to work with. Therefore, normally, other substances (fluxes) are added to lower the melting temperature and simplify glass processing.

Soda–lime glass
Sodium carbonate (Na2CO3, "soda") is a common additive and acts to lower the glass-transition temperature. However, sodium silicate is water-soluble, so lime (CaO, calcium oxide, generally obtained from limestone), along with magnesium oxide (MgO), and aluminium oxide (Al2O3), are commonly added to improve chemical durability. Soda–lime glasses (Na2O) + lime (CaO) + magnesia (MgO) + alumina (Al2O3) account for over 75% of manufactured glass, containing about 70 to 74% silica by weight. Soda–lime–silicate glass is transparent, easily formed, and most suitable for window glass and tableware. However, it has a high thermal expansion and poor resistance to heat. Soda–lime glass is typically used for windows, bottles, light bulbs, and jars.

Borosilicate glass
Borosilicate glasses (e.g. Pyrex, Duran) typically contain 5–13% boron trioxide (B2O3). Borosilicate glasses have fairly low coefficients of thermal expansion (7740 Pyrex CTE is 3.25/°C as compared to about 9/°C for a typical soda–lime glass ). They are, therefore, less subject to stress caused by thermal expansion and thus less vulnerable to cracking from thermal shock. They are commonly used for e.g. labware, household cookware, and sealed beam car head lamps.

Lead glass
The addition of lead(II) oxide into silicate glass lowers the melting point and viscosity of the melt. The high density of lead glass (silica + lead oxide (PbO) + potassium oxide (K2O) + soda (Na2O) + zinc oxide (ZnO) + alumina) results in a high electron density, and hence high refractive index, making the look of glassware more brilliant and causing noticeably more specular reflection and increased optical dispersion. Lead glass has a high elasticity, making the glassware more workable and giving rise to a clear "ring" sound when struck. However, lead glass cannot withstand high temperatures well. Lead oxide also facilitates the solubility of other metal oxides and is used in coloured glass. The viscosity decrease of lead glass melt is very significant (roughly 100 times in comparison with soda glass); this allows easier removal of bubbles and working at lower temperatures, hence its frequent use as an additive in vitreous enamels and glass solders. The high ionic radius of the Pb2+ ion renders it highly immobile and hinders the movement of other ions; lead glasses therefore have high electrical resistance, about two orders of magnitude higher than soda–lime glass (108.5 vs 106.5 Ω⋅cm, DC at 250 °C).

Aluminosilicate glass
Aluminosilicate glass typically contains 5–10% alumina (Al2O3). Aluminosilicate glass tends to be more difficult to melt and shape compared to borosilicate compositions but has excellent thermal resistance and durability. Aluminosilicate glass is extensively used for fibreglass, used for making glass-reinforced plastics (boats, fishing rods, etc.), top-of-stove cookware, and halogen bulb glass.

Other oxide additives
The addition of barium also increases the refractive index. Thorium oxide gives glass a high refractive index and low dispersion and was formerly used in producing high-quality lenses, but due to its radioactivity has been replaced by lanthanum oxide in modern eyeglasses. Iron can be incorporated into glass to absorb infrared radiation, for example in heat-absorbing filters for movie projectors, while cerium(IV) oxide can be used for glass that absorbs ultraviolet wavelengths. Fluorine lowers the dielectric constant of glass. Fluorine is highly electronegative and lowers the polarizability of the material. Fluoride silicate glasses are used in the manufacture of integrated circuits as an insulator.

Glass-ceramics
Glass-ceramic materials contain both non-crystalline glass and crystalline ceramic phases. They are formed by controlled nucleation and partial crystallisation of a base glass by heat treatment. Crystalline grains are often embedded within a non-crystalline intergranular phase of grain boundaries. Glass-ceramics exhibit advantageous thermal, chemical, biological, and dielectric properties as compared to metals or organic polymers.

The most commercially important property of glass-ceramics is their imperviousness to thermal shock. Thus, glass-ceramics have become extremely useful for countertop cooking and industrial processes. The negative thermal expansion coefficient (CTE) of the crystalline ceramic phase can be balanced with the positive CTE of the glassy phase. At a certain point (~70% crystalline) the glass-ceramic has a net CTE near zero. This type of glass-ceramic exhibits excellent mechanical properties and can sustain repeated and quick temperature changes up to 1000 °C.

Fibreglass
Fibreglass (also called glass fibre reinforced plastic, GRP) is a composite material made by reinforcing a plastic resin with glass fibres. It is made by melting glass and stretching the glass into fibres. These fibres are woven together into a cloth and left to set in a plastic resin. Fibreglass has the properties of being lightweight and corrosion resistant and is a good insulator enabling its use as building insulation material and for electronic housing for consumer products. Fibreglass was originally used in the United Kingdom and United States during World War II to manufacture radomes. Uses of fibreglass include building and construction materials, boat hulls, car body parts, and aerospace composite materials.

Glass-fibre wool is an excellent thermal and sound insulation material, commonly used in buildings (e.g. attic and cavity wall insulation), and plumbing (e.g. pipe insulation), and soundproofing. It is produced by forcing molten glass through a fine mesh by centripetal force and breaking the extruded glass fibres into short lengths using a stream of high-velocity air. The fibres are bonded with an adhesive spray and the resulting wool mat is cut and packed in rolls or panels.

Non-silicate glasses
Besides common silica-based glasses many other inorganic and organic materials may also form glasses, including metals, aluminates, phosphates, borates, chalcogenides, fluorides, germanates (glasses based on GeO2), tellurites (glasses based on TeO2), antimonates (glasses based on Sb2O3), arsenates (glasses based on As2O3), titanates (glasses based on TiO2), tantalates (glasses based on Ta2O5), nitrates, carbonates, plastics, acrylic, and many other substances. Some of these glasses (e.g. Germanium dioxide (GeO2, Germania), in many respects a structural analogue of silica, fluoride, aluminate, phosphate, borate, and chalcogenide glasses) have physicochemical properties useful for their application in fibre-optic waveguides in communication networks and other specialised technological applications.

Silica-free glasses may often have poor glass-forming tendencies. Novel techniques, including containerless processing by aerodynamic levitation (cooling the melt whilst it floats on a gas stream) or splat quenching (pressing the melt between two metal anvils or rollers), may be used to increase the cooling rate or to reduce crystal nucleation triggers.

Amorphous metals
In the past, small batches of amorphous metals with high surface area configurations (ribbons, wires, films, etc.) have been produced through the implementation of extremely rapid rates of cooling. Amorphous metal wires have been produced by sputtering molten metal onto a spinning metal disk.

Several alloys have been produced in layers with thicknesses exceeding 1 millimetre. These are known as bulk metallic glasses (BMG). Liquidmetal Technologies sells several zirconium-based BMGs.

Batches of amorphous steel have also been produced that demonstrate mechanical properties far exceeding those found in conventional steel alloys.

Experimental evidence indicates that the system Al-Fe-Si may undergo a first-order transition to an amorphous form (dubbed "q-glass") on rapid cooling from the melt. Transmission electron microscopy (TEM) images indicate that q-glass nucleates from the melt as discrete particles with uniform spherical growth in all directions. While x-ray diffraction reveals the isotropic nature of q-glass, a nucleation barrier exists implying an interfacial discontinuity (or internal surface) between the glass and melt phases.

Polymers
Important polymer glasses include amorphous and glassy pharmaceutical compounds. These are useful because the solubility of the compound is greatly increased when it is amorphous compared to the same crystalline composition. Many emerging pharmaceuticals are practically insoluble in their crystalline forms. Many polymer thermoplastics familiar to everyday use are glasses. For many applications, like glass bottles or eyewear, polymer glasses (acrylic glass, polycarbonate or polyethylene terephthalate) are a lighter alternative to traditional glass.

Molecular liquids and molten salts
Molecular liquids, electrolytes, molten salts, and aqueous solutions are mixtures of different molecules or ions that do not form a covalent network but interact only through weak van der Waals forces or transient hydrogen bonds. In a mixture of three or more ionic species of dissimilar size and shape, crystallization can be so difficult that the liquid can easily be supercooled into a glass. Examples include LiCl:RH2O (a solution of lithium chloride salt and water molecules) in the composition range 4<R<8. sugar glass, or Ca0.4K0.6(NO3)1.4. Glass electrolytes in the form of Ba-doped Li-glass and Ba-doped Na-glass have been proposed as solutions to problems identified with organic liquid electrolytes used in modern lithium-ion battery cells.

Production
Following the glass batch preparation and mixing, the raw materials are transported to the furnace. Soda–lime glass for mass production is melted in glass-melting furnaces. Smaller-scale furnaces for speciality glasses include electric melters, pot furnaces, and day tanks. After melting, homogenization and refining (removal of bubbles), the glass is formed. This may be achieved manually by glassblowing, which involves gathering a mass of hot semi-molten glass, inflating it into a bubble using a hollow blowpipe, and forming it into the required shape by blowing, swinging, rolling, or moulding. While hot, the glass can be worked using hand tools, cut with shears, and additional parts such as handles or feet attached by welding. Flat glass for windows and similar applications is formed by the float glass process, developed between 1953 and 1957 by Sir Alastair Pilkington and Kenneth Bickerstaff of the UK's Pilkington Brothers, who created a continuous ribbon of glass using a molten tin bath on which the molten glass flows unhindered under the influence of gravity. The top surface of the glass is subjected to nitrogen under pressure to obtain a polished finish. Container glass for common bottles and jars is formed by blowing and pressing methods. This glass is often slightly modified chemically (with more alumina and calcium oxide) for greater water resistance.

Once the desired form is obtained, glass is usually annealed for the removal of stresses and to increase the glass's hardness and durability. Surface treatments, coatings or lamination may follow to improve the chemical durability (glass container coatings, glass container internal treatment), strength (toughened glass, bulletproof glass, windshields ), or optical properties (insulated glazing, anti-reflective coating).

New chemical glass compositions or new treatment techniques can be initially investigated in small-scale laboratory experiments. The raw materials for laboratory-scale glass melts are often different from those used in mass production because the cost factor has a low priority. In the laboratory mostly pure chemicals are used. Care must be taken that the raw materials have not reacted with moisture or other chemicals in the environment (such as alkali or alkaline earth metal oxides and hydroxides, or boron oxide), or that the impurities are quantified (loss on ignition). Evaporation losses during glass melting should be considered during the selection of the raw materials, e.g., sodium selenite may be preferred over easily evaporating selenium dioxide (SeO2). Also, more readily reacting raw materials may be preferred over relatively inert ones, such as aluminium hydroxide (Al(OH)3) over alumina (Al2O3). Usually, the melts are carried out in platinum crucibles to reduce contamination from the crucible material. Glass homogeneity is achieved by homogenizing the raw materials mixture (glass batch), stirring the melt, and crushing and re-melting the first melt. The obtained glass is usually annealed to prevent breakage during processing.

Colour
Colour in glass may be obtained by addition of homogenously distributed electrically charged ions (or colour centres). While ordinary soda–lime glass appears colourless in thin section, iron(II) oxide (FeO) impurities produce a green tint in thick sections. Manganese dioxide (MnO2), which gives glass a purple colour, may be added to remove the green tint given by FeO. FeO and chromium(III) oxide (Cr2O3) additives are used in the production of green bottles. Iron (III) oxide, on the other-hand, produces yellow or yellow-brown glass. Low concentrations (0.025 to 0.1%) of cobalt oxide (CoO) produces rich, deep blue cobalt glass. Chromium is a very powerful colourising agent, yielding dark green. Sulphur combined with carbon and iron salts produces amber glass ranging from yellowish to almost black. A glass melt can also acquire an amber colour from a reducing combustion atmosphere. Cadmium sulfide produces imperial red, and combined with selenium can produce shades of yellow, orange, and red. The additive Copper(II) oxide (CuO) produces a turquoise colour in glass, in contrast to Copper(I) oxide (Cu2O) which gives a dull brown-red colour.

Architecture and windows
Soda–lime sheet glass is typically used as a transparent glazing material, typically as windows in external walls of buildings. Float or rolled sheet glass products are cut to size either by scoring and snapping the material, laser cutting, water jets, or diamond-bladed saw. The glass may be thermally or chemically tempered (strengthened) for safety and bent or curved during heating. Surface coatings may be added for specific functions such as scratch resistance, blocking specific wavelengths of light (e.g. infrared or ultraviolet), dirt-repellence (e.g. self-cleaning glass), or switchable electrochromic coatings.

Structural glazing systems represent one of the most significant architectural innovations of modern times, where glass buildings now often dominate the skylines of many modern cities. These systems use stainless steel fittings countersunk into recesses in the corners of the glass panels allowing strengthened panes to appear unsupported creating a flush exterior. Structural glazing systems have their roots in iron and glass conservatories of the nineteenth century

Tableware
Glass is an essential component of tableware and is typically used for water, beer and wine drinking glasses. Wine glasses are typically stemware, i.e. goblets formed from a bowl, stem, and foot. Crystal or Lead crystal glass may be cut and polished to produce decorative drinking glasses with gleaming facets. Other uses of glass in tableware include decanters, jugs, plates, and bowls.

Packaging
The inert and impermeable nature of glass makes it a stable and widely used material for food and drink packaging as glass bottles and jars. Most container glass is soda–lime glass, produced by blowing and pressing techniques. Container glass has a lower magnesium oxide and sodium oxide content than flat glass, and a higher silica, calcium oxide, and aluminium oxide content. Its higher content of water-insoluble oxides imparts slightly higher chemical durability against water, which is advantageous for storing beverages and food. Glass packaging is sustainable, readily recycled, reusable and refillable.

For electronics applications, glass can be used as a substrate in the manufacture of integrated passive devices, thin-film bulk acoustic resonators, and as a hermetic sealing material in device packaging, including very thin solely glass based encapsulation of integrated circuits and other semiconductors in high manufacturing volumes.

Laboratories
Glass is an important material in scientific laboratories for the manufacture of experimental apparatus because it is relatively cheap, readily formed into required shapes for experiment, easy to keep clean, can withstand heat and cold treatment, is generally non-reactive with many reagents, and its transparency allows for the observation of chemical reactions and processes. Laboratory glassware applications include flasks, Petri dishes, test tubes, pipettes, graduated cylinders, glass-lined metallic containers for chemical processing, fractionation columns, glass pipes, Schlenk lines, gauges, and thermometers. Although most standard laboratory glassware has been mass-produced since the 1920s, scientists still employ skilled glassblowers to manufacture bespoke glass apparatus for their experimental requirements.

Optics
Glass is a ubiquitous material in optics because of its ability to refract, reflect, and transmit light. These and other optical properties can be controlled by varying chemical compositions, thermal treatment, and manufacturing techniques. The many applications of glass in optics include glasses for eyesight correction, imaging optics (e.g. lenses and mirrors in telescopes, microscopes, and cameras), fibre optics in telecommunications technology, and integrated optics. Microlenses and gradient-index optics (where the refractive index is non-uniform) find application in e.g. reading optical discs, laser printers, photocopiers, and laser diodes.

Art
Glass as art dates to least 1300 BC shown as an example of natural glass found in Tutankhamun's pectoral, which also contained vitreous enamel, that is to say, melted coloured glass used on a metal backing. Enamelled glass, the decoration of glass vessels with coloured glass paints, has existed since 1300 BC, and was prominent in the early 20th century with Art Nouveau glass and that of the House of Fabergé in St. Petersburg, Russia. Both techniques were used in stained glass, which reached its height roughly from 1000 to 1550, before a revival in the 19th century.

The 19th century saw a revival in ancient glassmaking techniques including cameo glass, achieved for the first time since the Roman Empire, initially mostly for pieces in a neo-classical style. The Art Nouveau movement made great use of glass, with René Lalique, Émile Gallé, and Daum of Nancy in the first French wave of the movement, producing coloured vases and similar pieces, often in cameo glass or lustre glass techniques.

Louis Comfort Tiffany in America specialised in stained glass, both secular and religious, in panels and his famous lamps. The early 20th century saw the large-scale factory production of glass art by firms such as Waterford and Lalique. Small studios may hand-produce glass artworks. Techniques for producing glass art include blowing, kiln-casting, fusing, slumping, pâte de verre, flame-working, hot-sculpting and cold-working. Cold work includes traditional stained glass work and other methods of shaping glass at room temperature. Objects made out of glass include vessels, paperweights, marbles, beads, sculptures and installation art.

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Gel electrophoresis

Gel electrophoresis is a method for separation and analysis of biomacromolecules (DNA, RNA, proteins, etc.) and their fragments, based on their size and charge. It is used in clinical chemistry to separate proteins by charge or size (IEF agarose, essentially size independent) and in biochemistry and molecular biology to separate a mixed population of DNA and RNA fragments by length, to estimate the size of DNA and RNA fragments or to separate proteins by charge.

Nucleic acid molecules are separated by applying an electric field to move the negatively charged molecules through a matrix of agarose or other substances. Shorter molecules move faster and migrate farther than longer ones because shorter molecules migrate more easily through the pores of the gel. This phenomenon is called sieving. Proteins are separated by the charge in agarose because the pores of the gel are too large to sieve proteins. Gel electrophoresis can also be used for the separation of nanoparticles.

Gel electrophoresis uses a gel as an anticonvective medium or sieving medium during electrophoresis, the movement of a charged particle in an electric current. Gels suppress the thermal convection caused by the application of the electric field, and can also act as a sieving medium, slowing the passage of molecules; gels can also simply serve to maintain the finished separation so that a post electrophoresis stain can be applied. DNA gel electrophoresis is usually performed for analytical purposes, often after amplification of DNA via polymerase chain reaction (PCR), but may be used as a preparative technique prior to use of other methods such as mass spectrometry, RFLP, PCR, cloning, DNA sequencing, or Southern blotting for further characterization.

Physical basis
Electrophoresis is a process that enables the sorting of molecules based on charge, size, or shape. Using an electric field, molecules (such as DNA) can be made to move through a gel made of agarose or polyacrylamide. The electric field consists of a negative charge at one end which pushes the molecules through the gel, and a positive charge at the other end that pulls the molecules through the gel. The molecules being sorted are dispensed into a well in the gel material. The gel is placed in an electrophoresis chamber, which is then connected to a power source. When the electric field is applied, the larger molecules move more slowly through the gel while the smaller molecules move faster. The different sized molecules form distinct bands on the gel.

The term "gel" in this instance refers to the matrix used to contain, then separate the target molecules. In most cases, the gel is a crosslinked polymer whose composition and porosity are chosen based on the specific weight and composition of the target to be analyzed. When separating proteins or small nucleic acids (DNA, RNA, or oligonucleotides) the gel is usually composed of different concentrations of acrylamide and a cross-linker, producing different sized mesh networks of polyacrylamide. When separating larger nucleic acids (greater than a few hundred bases), the preferred matrix is purified agarose. In both cases, the gel forms a solid, yet porous matrix. Acrylamide, in contrast to polyacrylamide, is a neurotoxin and must be handled using appropriate safety precautions to avoid poisoning. Agarose is composed of long unbranched chains of uncharged carbohydrates without cross-links resulting in a gel with large pores allowing for the separation of macromolecules and macromolecular complexes.

Electrophoresis refers to the electromotive force (EMF) that is used to move the molecules through the gel matrix. By placing the molecules in wells in the gel and applying an electric field, the molecules will move through the matrix at different rates, determined largely by their mass when the charge-to-mass ratio (Z) of all species is uniform. However, when charges are not all uniform the electrical field generated by the electrophoresis procedure will cause the molecules to migrate differentially according to charge. Species that are net positively charged will migrate towards the cathode which is negatively charged (because this is an electrolytic rather than galvanic cell), whereas species that are net negatively charged will migrate towards the positively charged anode. Mass remains a factor in the speed with which these non-uniformly charged molecules migrate through the matrix toward their respective electrodes.

If several samples have been loaded into adjacent wells in the gel, they will run parallel in individual lanes. Depending on the number of different molecules, each lane shows the separation of the components from the original mixture as one or more distinct bands, one band per component. Incomplete separation of the components can lead to overlapping bands, or indistinguishable smears representing multiple unresolved components. Bands in different lanes that end up at the same distance from the top contain molecules that passed through the gel at the same speed, which usually means they are approximately the same size. There are molecular weight size markers available that contain a mixture of molecules of known sizes. If such a marker was run on one lane in the gel parallel to the unknown samples, the bands observed can be compared to those of the unknown to determine their size. The distance a band travels is approximately inversely proportional to the logarithm of the size of the molecule (alternatively, this can be stated as the distance traveled is inversely proportional to the log of samples's molecular weight).

There are limits to electrophoretic techniques. Since passing a current through a gel causes heating, gels may melt during electrophoresis. Electrophoresis is performed in buffer solutions to reduce pH changes due to the electric field, which is important because the charge of DNA and RNA depends on pH, but running for too long can exhaust the buffering capacity of the solution. There are also limitations in determining the molecular weight by SDS-PAGE, especially when trying to find the MW of an unknown protein. Certain biological variables are difficult or impossible to minimize and can affect electrophoretic migration. Such factors include protein structure, post-translational modifications, and amino acid composition. For example, tropomyosin is an acidic protein that migrates abnormally on SDS-PAGE gels. This is because the acidic residues are repelled by the negatively charged SDS, leading to an inaccurate mass-to-charge ratio and migration. Further, different preparations of genetic material may not migrate consistently with each other, for morphological or other reasons.

Types of gel
The types of gel most typically used are agarose and polyacrylamide gels. Each type of gel is well-suited to different types and sizes of the analyte. Polyacrylamide gels are usually used for proteins and have very high resolving power for small fragments of DNA (5-500 bp). Agarose gels, on the other hand, have lower resolving power for DNA but have a greater range of separation, and are therefore used for DNA fragments of usually 50–20,000 bp in size, but the resolution of over 6 Mb is possible with pulsed field gel electrophoresis (PFGE). Polyacrylamide gels are run in a vertical configuration while agarose gels are typically run horizontally in a submarine mode. They also differ in their casting methodology, as agarose sets thermally, while polyacrylamide forms in a chemical polymerization reaction.

Agarose

Agarose gels are made from the natural polysaccharide polymers extracted from seaweed. Agarose gels are easily cast and handled compared to other matrices because the gel setting is a physical rather than chemical change. Samples are also easily recovered. After the experiment is finished, the resulting gel can be stored in a plastic bag in a refrigerator.

Agarose gels do not have a uniform pore size, but are optimal for electrophoresis of proteins that are larger than 200 kDa. Agarose gel electrophoresis can also be used for the separation of DNA fragments ranging from 50 base pair to several megabases (millions of bases), the largest of which require specialized apparatus. The distance between DNA bands of different lengths is influenced by the percent agarose in the gel, with higher percentages requiring longer run times, sometimes days. Instead high percentage agarose gels should be run with a pulsed field electrophoresis (PFE), or field inversion electrophoresis.

"Most agarose gels are made with between 0.7% (good separation or resolution of large 5–10kb DNA fragments) and 2% (good resolution for small 0.2–1kb fragments) agarose dissolved in electrophoresis buffer. Up to 3% can be used for separating very tiny fragments but a vertical polyacrylamide gel is more appropriate in this case. Low percentage gels are very weak and may break when you try to lift them. High percentage gels are often brittle and do not set evenly. 1% gels are common for many applications."

Polyacrylamide
Polyacrylamide gel electrophoresis (PAGE) is used for separating proteins ranging in size from 5 to 2,000 kDa due to the uniform pore size provided by the polyacrylamide gel. Pore size is controlled by modulating the concentrations of acrylamide and bis-acrylamide powder used in creating a gel. Care must be used when creating this type of gel, as acrylamide is a potent neurotoxin in its liquid and powdered forms.

Traditional DNA sequencing techniques such as Maxam-Gilbert or Sanger methods used polyacrylamide gels to separate DNA fragments differing by a single base-pair in length so the sequence could be read. Most modern DNA separation methods now use agarose gels, except for particularly small DNA fragments. It is currently most often used in the field of immunology and protein analysis, often used to separate different proteins or isoforms of the same protein into separate bands. These can be transferred onto a nitrocellulose or PVDF membrane to be probed with antibodies and corresponding markers, such as in a western blot.

Typically resolving gels are made in 6%, 8%, 10%, 12% or 15%. Stacking gel (5%) is poured on top of the resolving gel and a gel comb (which forms the wells and defines the lanes where proteins, sample buffer, and ladders will be placed) is inserted. The percentage chosen depends on the size of the protein that one wishes to identify or probe in the sample. The smaller the known weight, the higher the percentage that should be used. Changes in the buffer system of the gel can help to further resolve proteins of very small sizes.

Starch
Partially hydrolysed potato starch makes for another non-toxic medium for protein electrophoresis. The gels are slightly more opaque than acrylamide or agarose. Non-denatured proteins can be separated according to charge and size. They are visualised using Napthal Black or Amido Black staining. Typical starch gel concentrations are 5% to 10%.

Denaturing

Denaturing gels are run under conditions that disrupt the natural structure of the analyte, causing it to unfold into a linear chain. Thus, the mobility of each macromolecule depends only on its linear length and its mass-to-charge ratio. Thus, the secondary, tertiary, and quaternary levels of biomolecular structure are disrupted, leaving only the primary structure to be analyzed.

Nucleic acids are often denatured by including urea in the buffer, while proteins are denatured using sodium dodecyl sulfate, usually as part of the SDS-PAGE process. For full denaturation of proteins, it is also necessary to reduce the covalent disulfide bonds that stabilize their tertiary and quaternary structure, a method called reducing PAGE. Reducing conditions are usually maintained by the addition of beta-mercaptoethanol or dithiothreitol. For a general analysis of protein samples, reducing PAGE is the most common form of protein electrophoresis.

Denaturing conditions are necessary for proper estimation of molecular weight of RNA. RNA is able to form more intramolecular interactions than DNA which may result in change of its electrophoretic mobility. Urea, DMSO and glyoxal are the most often used denaturing agents to disrupt RNA structure. Originally, highly toxic methylmercury hydroxide was often used in denaturing RNA electrophoresis, but it may be method of choice for some samples.

Denaturing gel electrophoresis is used in the DNA and RNA banding pattern-based methods temperature gradient gel electrophoresis (TGGE) and denaturing gradient gel electrophoresis (DGGE).

Native

Native gels are run in non-denaturing conditions so that the analyte's natural structure is maintained. This allows the physical size of the folded or assembled complex to affect the mobility, allowing for analysis of all four levels of the biomolecular structure. For biological samples, detergents are used only to the extent that they are necessary to lyse lipid membranes in the cell. Complexes remain—for the most part—associated and folded as they would be in the cell. One downside, however, is that complexes may not separate cleanly or predictably, as it is difficult to predict how the molecule's shape and size will affect its mobility. Addressing and solving this problem is a major aim of preparative native PAGE.

Unlike denaturing methods, native gel electrophoresis does not use a charged denaturing agent. The molecules being separated (usually proteins or nucleic acids) therefore differ not only in molecular mass and intrinsic charge, but also the cross-sectional area, and thus experience different electrophoretic forces dependent on the shape of the overall structure. For proteins, since they remain in the native state they may be visualized not only by general protein staining reagents but also by specific enzyme-linked staining.

A specific experiment example of an application of native gel electrophoresis is to check for enzymatic activity to verify the presence of the enzyme in the sample during protein purification. For example, for the protein alkaline phosphatase, the staining solution is a mixture of 4-chloro-2-2methylbenzenediazonium salt with 3-phospho-2-naphthoic acid-2'-4'-dimethyl aniline in Tris buffer. This stain is commercially sold as a kit for staining gels. If the protein is present, the mechanism of the reaction takes place in the following order: it starts with the de-phosphorylation of 3-phospho-2-naphthoic acid-2'-4'-dimethyl aniline by alkaline phosphatase (water is needed for the reaction). The phosphate group is released and replaced by an alcohol group from water. The electrophile 4- chloro-2-2 methylbenzenediazonium (Fast Red TR Diazonium salt) displaces the alcohol group forming the final product Red Azo dye. As its name implies, this is the final visible-red product of the reaction. In undergraduate academic experimentation of protein purification, the gel is usually run next to commercial purified samples to visualize the results and conclude whether or not purification was successful.

Native gel electrophoresis is typically used in proteomics and metallomics. However, native PAGE is also used to scan genes (DNA) for unknown mutations as in single-strand conformation polymorphism.

Buffers
Buffers in gel electrophoresis are used to provide ions that carry a current and to maintain the pH at a relatively constant value. These buffers have plenty of ions in them, which is necessary for the passage of electricity through them. Something like distilled water or benzene contains few ions, which is not ideal for the use in electrophoresis. There are a number of buffers used for electrophoresis. The most common being, for nucleic acids Tris/Acetate/EDTA (TAE), Tris/Borate/EDTA (TBE). Many other buffers have been proposed, e.g. lithium borate, which is rarely used, based on Pubmed citations (LB), isoelectric histidine, pK matched goods buffers, etc.; in most cases the purported rationale is lower current (less heat) matched ion mobilities, which leads to longer buffer life. Borate is problematic; Borate can polymerize, or interact with cis diols such as those found in RNA. TAE has the lowest buffering capacity but provides the best resolution for larger DNA. This means a lower voltage and more time, but a better product. LB is relatively new and is ineffective in resolving fragments larger than 5 kbp; However, with its low conductivity, a much higher voltage could be used (up to 35 V/cm), which means a shorter analysis time for routine electrophoresis. As low as one base pair size difference could be resolved in 3% agarose gel with an extremely low conductivity medium (1 mM Lithium borate).

Most SDS-PAGE protein separations are performed using a "discontinuous" (or DISC) buffer system that significantly enhances the sharpness of the bands within the gel. During electrophoresis in a discontinuous gel system, an ion gradient is formed in the early stage of electrophoresis that causes all of the proteins to focus on a single sharp band in a process called isotachophoresis. Separation of the proteins by size is achieved in the lower, "resolving" region of the gel. The resolving gel typically has a much smaller pore size, which leads to a sieving effect that now determines the electrophoretic mobility of the proteins.

Visualization
After the electrophoresis is complete, the molecules in the gel can be stained to make them visible. DNA may be visualized using ethidium bromide which, when intercalated into DNA, fluoresce under ultraviolet light, while protein may be visualised using silver stain or Coomassie brilliant blue dye. Other methods may also be used to visualize the separation of the mixture's components on the gel. If the molecules to be separated contain radioactivity, for example in a DNA sequencing gel, an autoradiogram can be recorded of the gel. Photographs can be taken of gels, often using a Gel Doc system. Gels are then commonly labelled for presentation and scientific records on the popular figure-creation website, SciUGo.

Downstream processing
After separation, an additional separation method may then be used, such as isoelectric focusing or SDS-PAGE. The gel will then be physically cut, and the protein complexes extracted from each portion separately. Each extract may then be analysed, such as by peptide mass fingerprinting or de novo peptide sequencing after in-gel digestion. This can provide a great deal of information about the identities of the proteins in a complex.

Applications

 * Estimation of the size of DNA molecules following restriction enzyme digestion, e.g. in restriction mapping of cloned DNA.
 * Analysis of PCR products, e.g. in molecular genetic diagnosis or genetic fingerprinting
 * Separation of restricted genomic DNA prior to Southern transfer, or of RNA prior to Northern transfer.

Gel electrophoresis is used in forensics, molecular biology, genetics, microbiology and biochemistry. The results can be analyzed quantitatively by visualizing the gel with UV light and a gel imaging device. The image is recorded with a computer-operated camera, and the intensity of the band or spot of interest is measured and compared against standard or markers loaded on the same gel. The measurement and analysis are mostly done with specialized software.

Depending on the type of analysis being performed, other techniques are often implemented in conjunction with the results of gel electrophoresis, providing a wide range of field-specific applications.

Nucleic acids

In the case of nucleic acids, the direction of migration, from negative to positive electrodes, is due to the naturally occurring negative charge carried by their sugar-phosphate backbone.

Double-stranded DNA fragments naturally behave as long rods, so their migration through the gel is relative to their size or, for cyclic fragments, their radius of gyration. Circular DNA such as plasmids, however, may show multiple bands, the speed of migration may depend on whether it is relaxed or supercoiled. Single-stranded DNA or RNA tends to fold up into molecules with complex shapes and migrate through the gel in a complicated manner based on their tertiary structure. Therefore, agents that disrupt the hydrogen bonds, such as sodium hydroxide or formamide, are used to denature the nucleic acids and cause them to behave as long rods again.

Gel electrophoresis of large DNA or RNA is usually done by agarose gel electrophoresis. See the "chain termination method" page for an example of a polyacrylamide DNA sequencing gel. Characterization through ligand interaction of nucleic acids or fragments may be performed by mobility shift affinity electrophoresis.

Electrophoresis of RNA samples can be used to check for genomic DNA contamination and also for RNA degradation. RNA from eukaryotic organisms shows distinct bands of 28s and 18s rRNA, the 28s band being approximately twice as intense as the 18s band. Degraded RNA has less sharply defined bands, has a smeared appearance, and the intensity ratio is less than 2:1.

Proteins
Proteins, unlike nucleic acids, can have varying charges and complex shapes, therefore they may not migrate into the polyacrylamide gel at similar rates, or all when placing a negative to positive EMF on the sample. Proteins, therefore, are usually denatured in the presence of a detergent such as sodium dodecyl sulfate (SDS) that coats the proteins with a negative charge. Generally, the amount of SDS bound is relative to the size of the protein (usually 1.4g SDS per gram of protein), so that the resulting denatured proteins have an overall negative charge, and all the proteins have a similar charge-to-mass ratio. Since denatured proteins act like long rods instead of having a complex tertiary shape, the rate at which the resulting SDS coated proteins migrate in the gel is relative only to their size and not their charge or shape.

Proteins are usually analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), by native gel electrophoresis, by preparative native gel electrophoresis (QPNC-PAGE), or by 2-D electrophoresis.

Characterization through ligand interaction may be performed by electroblotting or by affinity electrophoresis in agarose or by capillary electrophoresis as for estimation of binding constants and determination of structural features like glycan content through lectin binding.

Nanoparticles
A novel application for gel electrophoresis is the separation or characterization of metal or metal oxide nanoparticles (e.g. Au, Ag, ZnO, SiO2) regarding the size, shape, or surface chemistry of the nanoparticles. The scope is to obtain a more homogeneous sample (e.g. narrower particle size distribution), which then can be used in further products/processes (e.g. self-assembly processes). For the separation of nanoparticles within a gel, the key parameter is the ratio of the particle size to the mesh size, whereby two migration mechanisms were identified: the unrestricted mechanism, where the particle size << mesh size, and the restricted mechanism, where particle size is similar to mesh size.

History

 * 1930s – first reports of the use of sucrose for gel electrophoresis; moving-boundary electrophoresis (Tiselius)
 * 1950 – introduction of "zone electrophoresis" (Tiselius); paper electrophoresis
 * 1955 – introduction of starch gels, mediocre separation (Smithies)
 * 1959 – introduction of acrylamide gels; discontinuous electrophoresis (Ornstein and Davis); accurate control of parameters such as pore size and stability; and (Raymond and Weintraub)
 * 1965 – introduction of free-flow electrophoresis (Hannig)
 * 1966 – first use of agar gels
 * 1969 – introduction of denaturing agents especially SDS separation of protein subunit (Weber and Osborn)
 * 1970 – Lämmli separated 28 components of T4 phage using a stacking gel and SDS
 * 1972 – agarose gels with ethidium bromide stain
 * 1975 – 2-dimensional gels (O’Farrell); isoelectric focusing, then SDS gel electrophoresis
 * 1977 – sequencing gels (Sanger)
 * 1981 – introduction of capillary electrophoresis (Jorgenson and Lukacs)
 * 1984 – pulsed-field gel electrophoresis enables separation of large DNA molecules (Schwartz and Cantor)
 * 2004 – introduction of a standardized polymerization time for acrylamide gel solutions to optimize gel properties, in particular gel stability, during electrophoresis (Kastenholz)

A 1959 book on electrophoresis by Milan Bier cites references from the 1800s. However, Oliver Smithies made significant contributions. Bier states: "The method of Smithies ... is finding wide application because of its unique separatory power." Taken in context, Bier clearly implies that Smithies' method is an improvement.

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title: Gel electrophoresis


text:
Gary Lineker

Gary Winston Lineker (born 30 November 1960) is an English sports broadcaster and former professional footballer. Regarded as one of the best English forwards of the 1980s, Lineker is the only player to have been the top goalscorer in England with three clubs: Leicester City, Everton and Tottenham Hotspur. He also played for Barcelona in Spain, and won 80 caps for England. His media career began with the BBC, where he has presented the flagship football programme Match of the Day since the late 1990s, the longest tenure of any MOTD presenter. Lineker is also the BBC's lead presenter for live football matches, including coverage of international tournaments such as the FIFA World Cup. He has also worked for Al Jazeera Sports, Eredivisie Live, NBC Sports Network, and BT Sport's coverage of the UEFA Champions League.

Lineker began his football career at Leicester City in 1978, and finished as the First Division's joint top goalscorer in 1984–85. He then moved to league champions Everton, where he won both the PFA Players' Player of the Year and FWA Footballer of the Year awards in his debut season, before moving to Spanish club Barcelona. Lineker placed 2nd in the 1986 Ballon d’Or for his performances at Everton after transferring to Barcelona. With the Spanish side, he won the 1987–88 Copa del Rey and the 1989 European Cup Winners' Cup. He joined Tottenham Hotspur in 1989, and won his second FWA Footballer of the Year and the FA Cup, his first and only major trophy in English football. Lineker's final club was Nagoya Grampus Eight; he retired in 1994 after two seasons at the Japanese side.

Lineker made his England debut in 1984, earning 80 caps and scoring 48 goals over an eight-year international career, which made him England's second-highest international goalscorer on his retirement. He remains England's fourth-highest scorer, behind Harry Kane, Wayne Rooney and Bobby Charlton, and his goals-to-games ratio remains one of the best for the country. His six goals in the 1986 FIFA World Cup made him the tournament's top scorer, receiving the Golden Boot. Lineker was again integral to England's progress to the semi-finals of the 1990 World Cup, scoring another four goals. He still holds England's record for goals in the FIFA World Cup.

Lineker never received a yellow or red card during his career, and he also never won a top-flight league title. He was honoured in 1990 with the FIFA Fair Play Award. In a senior career which spanned 16 years and 654 competitive games, Lineker scored a total of 331 goals, including 283 goals at club level. After his retirement from football he was inducted into the English Football Hall of Fame. A keen supporter of Leicester City, he led a consortium in 2002 that invested in his old club, saving it from bankruptcy, and was appointed honorary vice-president.

Early life
Gary Winston Lineker was born on 30 November 1960 in Leicester, the son of Margaret P. (Abbs) and Barry Lineker. He was given his middle name in honour of Winston Churchill, with whom he shares a birthday. He has one brother, Wayne, who is two years his junior. Lineker grew up with his family in the city, playing football with Wayne. Their father was a greengrocer, as were their grandfather William and great-grandfather George, in Leicester. Barry Lineker ran Lineker's fruit and veg stall in Leicester Market, and as a child and a young player Gary regularly helped out on the stall. Lineker, who is white, received racial abuse as a child for his dark features.

Lineker first attended Caldecote Road School (Caldecote Juniors), Braunstone in Leicester (east of the Meridian Centre). He then went to the City of Leicester Boys' Grammar School (now City of Leicester College) on Downing Drive in Evington, owing to his preference for football rather than rugby, which was the main sport of most schools near his home. Lineker was equally talented at both football and cricket. From the ages of 11 to 16 he captained the Leicestershire Schools cricket team, and had felt that he had a higher chance of succeeding at it rather than football. He later stated on They Think It's All Over that as a teenager he idolised former England captain David Gower, who was playing for Leicestershire at the time. During his youth he played for Aylestone Park Youth, later becoming the club's president.

Lineker left school with four O Levels. One of his teachers wrote on his report card that he "concentrates too much on football" and that he would "never make a living at that". He then joined the youth academy at Leicester City in 1976.

Leicester City
Lineker began his career at his hometown club Leicester City after leaving school in 1977, turning professional in the 1978–79 season and making his senior debut on New Year's Day 1979 in a 2–0 win at Filbert Street over Oldham Athletic in the Second Division. He earned a Second Division title medal a year later with 19 appearances, but played just nine league games in 1980–81 as Leicester went straight back down.

However, he became a regular player in 1981–82, scoring 19 goals in all competitions that season. Although Leicester missed out on promotion, they reached the semi-finals of the FA Cup, and clinched promotion a year later as Lineker scored 26 times in the Second Division. In 1983–84, he enjoyed regular First Division action for the first time and was the division's second-highest scorer with 22 goals, although Leicester failed to finish anywhere near the top of the league. He was the First Division's joint top scorer in 1984–85 with 24 goals, and was enjoying a prolific partnership with Alan Smith. However, by this stage he was attracting the attention of bigger clubs, and a move from Filbert Street was looking certain.

Everton
In the 1985 close season, defending league champions Everton signed Lineker for £800,000; he scored 40 goals in 57 games for his new team in the 1985–86 season. Lineker's first game for Everton happened to be away to Leicester City; at half time, he walked into the Leicester dressing room by mistake. He was again the First Division's leading goal scorer, this time with 30 goals (including three hat-tricks), and helped Everton finish second in the league. While at Everton, they reached the FA Cup final for the third consecutive year but lost 3–1 to Liverpool, despite Lineker giving them an early lead when he outpaced Alan Hansen to score. Liverpool had also pipped Everton to the title by just two points. "I was only on Merseyside a short time, nine or 10 months in total really, but it was still a happy time personally, while professionally it was one of the most successful periods of my career", he says. "I still have an affinity towards Everton."

Lineker scored three hat-tricks for Everton; at home to Birmingham City in a 4–1 league win on 31 August 1985, at home to Manchester City in a 4–0 win on 11 February 1986, and then in the penultimate league game of the season on 3 May 1986, when they kept their title hopes alive with a 6–1 home win over Southampton. On his final league appearance, he scored twice in a 3–1 home win over West Ham United whose own title hopes had just disappeared. However, he and his colleagues were denied title glory as Liverpool also won their final league game of the season at Chelsea. Lineker has consistently stated since retiring from football that this Everton team was the best club side he ever played in.

Barcelona
After winning the Golden Boot at the 1986 World Cup in Mexico, Lineker was signed by Barcelona for £2.8 million. Barcelona were being managed by former Queens Park Rangers manager Terry Venables, who had also brought in Manchester United and Wales striker Mark Hughes. Barcelona gave Lineker his first chance of European football, as Leicester had never qualified for Europe while he played for them, and Everton were denied a place in the European Cup for 1985–86 due to the ban on English clubs in European competitions following the Heysel disaster.

Lineker's Golden Boot-winning performance at the World Cup generated much anticipation of success at the Camp Nou, and he scored 21 goals in 41 games during his first season, including a hat-trick in a 3–2 win over archrivals Real Madrid. He made his Barcelona debut against Racing Santander, scoring twice. Barcelona went on to win the Copa del Rey in 1988 and the European Cup Winners' Cup in 1989. Lineker played in Barcelona's shock home and away defeats to Dundee United. Barcelona manager Johan Cruyff decided to play Lineker on the right of the midfield and he was eventually no longer an automatic choice in the team.

With 42 goals in 103 La Liga appearances, Lineker became the highest scoring British player in the competition's history, but was later surpassed by Gareth Bale in March 2016.

Tottenham Hotspur
Manchester United manager Alex Ferguson attempted to sign Lineker to partner his ex-Barcelona teammate Mark Hughes in attack, but Lineker instead signed for Tottenham Hotspur in July 1989 for £1.1 million. Over three seasons, he scored 67 goals in 105 league games and won the FA Cup while playing for the club. He finished as top scorer in the First Division in the 1989–90 season, scoring 24 goals as Spurs finished third.

He finally collected an English trophy when he won the 1991 FA Cup Final with Spurs, who beat Nottingham Forest 2–1. This was despite Lineker having a goal controversially disallowed for offside and also having a penalty saved by goalkeeper Mark Crossley. Lineker had contributed to Tottenham's run to the final. In the semi-final he scored twice in a 3–1 win over North London rivals Arsenal.

He was the top division's second-highest goalscorer in 1991–92 with 28 goals from 35 games, behind Ian Wright, who scored 29 times in 42 games. Despite Lineker's personal performance, Tottenham finished this final pre-Premier League season in 15th place. His last goal in English football came on the last day of the season in a 3–1 defeat to Manchester United at Old Trafford.

Nagoya Grampus Eight
In November 1991, Lineker accepted an offer of a two-year contract from J1 League club Nagoya Grampus Eight. The transfer fee paid to Tottenham Hotspur was £2 million. He officially joined Nagoya Grampus Eight after playing his final game for Spurs on 2 May 1992, when he scored the consolation goal in a 3–1 defeat by Manchester United on the last day of the season. Shortly before accepting the offer from Nagoya Grampus Eight, Tottenham had rejected an offer from ambitious Second Division club Blackburn Rovers, who had recently been taken over by steel baron Jack Walker.

Having scored 9 goals in 23 appearances over two injury impacted seasons for Nagoya Grampus Eight, he announced his retirement from playing in September 1994. The English national media had previously reported that he would be returning to England to complete his playing career at Middlesbrough or Southampton.

International career
Lineker was capped once by the England B national team, playing in a 2–0 home win over New Zealand's B team on 13 November 1984. He first played for the full England team against Scotland in 1984. He played five games in the 1986 World Cup and was top scorer of the tournament with six goals, winning the Golden Boot, making him the first English player to have done so. He scored the second quickest hat-trick ever at a FIFA World Cup tournament against Poland, the second English player to score a hat-trick at a World Cup, and scored two goals against Paraguay in the second round. He played most of the tournament wearing a lightweight cast on his forearm. He scored for England in the World Cup quarter-final against Argentina, but the game ended in defeat as Diego Maradona scored twice for the opposition (the first goal being the "Hand of God" handball, and the second being the "Goal of the Century"). In 1988, Lineker played in Euro 88, but failed to score as England lost all three Group games. It was later established that he had been suffering from hepatitis.

In the 1990 World Cup, he scored four goals to help England reach the semi-finals. He was unwell during the tournament, and accidentally defecated in his shorts during the opening group game against the Republic of Ireland. After Andreas Brehme sent England 1–0 down in the semi-final, Lineker received a pass from Paul Parker and escaped two West German defenders on his way to scoring the equaliser, but the West Germans triumphed in the penalty shoot-out and went on to win the trophy. Later he said: "Football is a simple game; 22 men chase a ball for 90 minutes and at the end, the Germans win." Lineker's equaliser appears in the popular England national team anthem, "Three Lions", with the lyric "When Lineker scored".

He retired from international football with eighty caps and forty-eight goals, one fewer goal than Sir Bobby Charlton's England record (which Charlton accrued over 106 caps). In what proved to be his last England match, against Sweden at Euro 92, he was substituted by England coach Graham Taylor in favour of Arsenal striker Alan Smith, ultimately denying him the chance to equal—or even better—Charlton's record. He had earlier missed a penalty that would have brought him level, in a pre-tournament friendly against Brazil. He was visibly upset at the decision, not looking at Taylor as he took the bench.

He scored four goals in an England match on two occasions and is one of very few players never to have been given a yellow card or a red card in any type of game.

Sports presenting
Following retirement from professional football, Lineker developed a career in the media, initially on the BBC for Radio 5 Live and as a football pundit. He appeared as a team captain on the sports game show They Think It's All Over from 1995 to 2003. He also presented Grandstand in the London studio while then-presenter Desmond Lynam was in Aintree when the Grand National was abandoned because of a bomb alert at the racecourse in 1997. In 1999, he replaced Lynam as the BBC's anchorman for football coverage, including its flagship football television programme Match of the Day, becoming BBC Sport's highest paid presenter. Following the departure of Steve Rider from the BBC in 2005, Lineker, who is a keen recreational golfer with a handicap of four, became the new presenter for the corporation's golf coverage. Despite receiving some criticism from his peers, he continued to front the BBC's coverage of the Masters and The Open.

In 2005, Lineker was sued for defamation by Australian footballer Harry Kewell over comments Lineker had made writing in his column in The Sunday Telegraph about Kewell's transfer from Leeds United to Liverpool. However, the jury was unable to reach a verdict. It became known during the case that the article had actually been ghost-written by a journalist at The Sunday Telegraph following a telephone interview with Lineker.

In May 2010, Lineker resigned from his role as columnist for The Mail on Sunday in protest over the sting operation against Lord Triesman that reportedly jeopardised England's bid to host the 2018 World Cup. Triesman resigned as chairman of the bid and the FA on 16 May 2010 after the publication of a secret recording of a conversation between the peer and a former ministerial aide, during which he claimed that Spain and Russia were planning to bribe referees at the World Cup in South Africa. Lineker then began working as an anchor for the English language football coverage for Al Jazeera Sport, which is broadcast throughout most of the Middle East. He left the Qatar-based network in 2012.

In 2013, Lineker began working for NBCSN as part of their Premier League coverage and contributing to the US version of Match of the Day. On 9 June 2015, Lineker was unveiled as the lead presenter of BT Sport's Champions League coverage. On 13 August 2016, Lineker presented the first Match of the Day of the 2016–17 season wearing only boxer shorts. Believing it would simply not happen, he had promised in a tweet from December 2015 that if Leicester City won the Premier League, he would "present Match of the Day in just my undies".

As of July 2022, Lineker is the highest-paid BBC presenter and has been for several years; receiving payments of £1.75–1.76 million each year between 2016 and 2020, and approximately £1.35 million in 2020-21. His pay has been criticised by Julian Knight, chair of the parliamentary Digital, Culture, Media and Sport Committee, and Dame Esther Rantzen. The BBC's Director General, Tim Davie, stated that Lineker's pay was justified "because of the value of analysis to the viewing audience".

In March 2023, Lineker was required to step back from presenting on the BBC for three days due to a controversy over his criticism of the British government's immigration policy on Twitter.

Walkers commercials
Lineker has appeared in television commercials for the Leicester-based snack company Walkers. Originally signing a £200,000 deal in 1994, his first advert was 1995's "Welcome Home" (Lineker had recently returned to England having played in Japan). Walkers temporarily named their salt and vinegar crisps after Lineker, labelling them 'Salt & Lineker', in the late 1990s. In 2000, Lineker's Walkers commercials were ranked ninth in Channel 4's poll of "The 100 Greatest TV Ads".

Other media appearances
Lineker participated in Prince Edward's charity television special The Grand Knockout Tournament in 1987. He also appeared in the 1991 play An Evening with Gary Lineker by Arthur Smith and Chris England, which was adapted for television in 1994. He presented a six-part TV series for the BBC in 1998 (directed by Lloyd Stanton) called Golden Boots, with other football celebrities. It was an extensive history of the World Cup focusing on the 'Golden Boots' (top scorers).

In 2006, Lineker took on an acting role as the voice of Underground Ernie on the BBC's children's channel, CBeebies. In December 2008, Lineker appeared on the ITV1 television programme Who Wants to Be a Millionaire? where he and English rugby union player Austin Healey won £50,000 for the Nicholls Spinal Injury Foundation. In 2009, Lineker and his wife Danielle hosted a series of the BBC's Northern Exposure, following on from Laurence Llewelyn-Bowen from the previous year in visiting and showcasing locations throughout Northern Ireland.

Lineker has made a number of cameo appearances in TV shows and movies, such as the 2002 film Bend It Like Beckham, the 2014 BBC TV film Marvellous, and a number of appearances in the Apple TV+ football comedy TV series Ted Lasso. In 2013, Lineker participated in the genealogical programme Who Do You Think You Are? during which he discovered an ancestor who was a poacher, and another who was a legal clerk. In 2021, Lineker started hosting the ITV game show Sitting on a Fortune.

Goalhanger Films and Podcasts
In May 2014, Lineker established his own production company Goalhanger Films Ltd. with former ITV Controller Tony Pastor. During the 2014 FIFA World Cup, Lineker presented several short videos produced by Goalhanger Films on YouTube with the title Blahzil. In May 2015, the company produced a 60-minute-long documentary presented by Lineker titled Gary Lineker on the Road to FA Cup Glory for the BBC.

He also operates Goalhanger Podcasts, which produces Leading, The Rest is History, The Rest is Politics and his own podcast, The Rest is Football, which he hosts with Alan Shearer and Micah Richards.

Personal life

Lineker married Michelle Cockayne in 1986. In May 2006, Cockayne filed for divorce on the grounds of her husband's alleged "unreasonable behaviour", with documents submitted to the court claiming that Lineker's actions in their marriage had caused her "stress and anxiety". Lineker and she have four sons. The couple subsequently stated that the situation was amicable.

In November 1991, Lineker's eldest son survived a rare form of leukaemia whilst he was a baby, and was treated at Great Ormond Street Hospital in London. Lineker now supports children's cancer charity CLIC Sargent and has appeared in promotional clips encouraging people to give blood. Lineker has been actively involved with other cancer charities such as Leukaemia Busters, where between 1994 and 2005 Gary and Michelle were the charity's patrons. He has also been involved with the Fight for Life and Cancer Research UK charities.

Lineker was made a freeman of the City of Leicester in 1995 and he has been referred to as "Leicester's favourite son".

In October 2002, Lineker backed a £5 million bid to rescue his former club Leicester City, which had recently gone into administration, describing his involvement as "charity" rather than an "ego trip". He stated that he would invest a six-figure sum and that other members of his consortium would invest a similar amount. Lineker met with fans' groups to persuade them to try and raise money to rescue his former club. The club was eventually saved from liquidation. He is now honorary Vice President of Leicester City F.C. Lineker married Danielle Bux on 2 September 2009, in Ravello, Italy. On 13 January 2016, Lineker and Bux announced they were divorcing, after six years of marriage, the reason given being Gary not wanting more children.

In 1985, Lineker was best man at snooker player Willie Thorne's wedding and their close friendship was the subject of the VHS production, Best of Friends – The Official Story of Gary Lineker & Willie Thorne.

In November 2017, Lineker was named in the Paradise Papers in connection with a tax avoidance scheme relating to property owned in Barbados and a company set up in the British Virgin Islands.

Lineker speaks Spanish, which he learnt during his time playing for FC Barcelona, and is an advocate for the teaching of foreign languages in schools.

In April 2020, during the COVID-19 pandemic, Lineker announced that he was donating £140,000 to the British Red Cross towards research into the virus.

On 28 March 2023 he won an appeal against HM Revenue and Customs over a bill that totalled £4.9 million. The authority had pursued him over taxes on income from BBC and BT Sport from 2013–14 to 2017–18 on the grounds he was an employee of both organisations at the time. A judge ruled he was a freelancer and had contracts with both broadcasters.

Political views
Lineker has been noted for political views which he shares on Twitter. In December 2016, he was described by Angus Harrison of Vice News as "the British Left's Loudest Voice" for being "both staunchly liberal and resolutely unafraid of making his views known". Using a football analogy, Lineker defined his ideological position as "I make more runs to the left than the right, but never felt comfortable on the wing". After the 2017 United Kingdom general election, in which Theresa May led the Conservatives and Jeremy Corbyn led Labour, Lineker wrote "Anyone else feel politically homeless? Everything seems far right or way left. Something sensibly centrist might appeal?"

Lineker endorsed a Remain vote in the 2016 United Kingdom European Union membership referendum. In July 2018, he announced his support for People's Vote, a campaign group calling for a public vote on the final Brexit deal between the UK and the European Union.

On 18 October 2016, Lineker tweeted a rebuttal to a statement made by MP David Davies where Davies suggested refugees entering the UK should undergo dental checks to verify their age: "The treatment by some towards these young refugees is hideously racist and utterly heartless. What's happening to our country?" This led The Sun to call for Lineker's sacking from Match of the Day, accusing him of breaching BBC impartiality guidelines.

In December 2018, Lineker was criticised by the BBC's cricket correspondent Jonathan Agnew for expressing his political views on Twitter. Agnew said, "You are the face of BBC Sport. Please observe BBC editorial guidelines and keep your political views, whatever they are and whatever the subject, to yourself. I'd be sacked if I followed your example." A BBC spokesperson said, "Gary is not involved in any news or political output for the BBC and as such, any expression of his personal political views does not affect the BBC's impartiality."

In October 2022, a complaint about a tweet from Lineker that referred to donations to the Conservative Party was upheld by the BBC, on the grounds that it breached social media use guidelines and failed to meet editorial standards of impartiality. During the 2022 World Cup, Qatari lawyer Hassan Al-Thawadi criticised Lineker for covering human rights violations in Qatar by stating that he did not talk about such issues with other host countries. In response, Lineker countered on The News Agents podcast that he covered issues in other host countries, and characterised the United States, which is hosting the 2026 World Cup, as an "extraordinarily racist country". His statements on the United States were criticised by Culture Secretary Michelle Donelan in January 2023, who said they were "very derogatory questionable comments".

2023 controversy and suspension from Match of the Day
In March 2023, Lineker criticised the British government's asylum policy via Twitter. Commenting on a video message by the Home Secretary, Suella Braverman, about stopping migrants crossing the English Channel in small boats, he said the message was "beyond awful" and called the government's policy "an immeasurably cruel policy directed at the most vulnerable people in language that is not dissimilar to that used by Germany in the 30s". The comments received condemnation from some Conservative politicians, including Braverman herself, and a spokesperson for Labour leader Keir Starmer said comparisons with 1930s Germany "aren't always the best way" to make an argument. Lineker received support from other political figures, including Baron Dubs, Alastair Campbell, and Angela Rayner. A BBC source said the corporation was taking the matter "seriously" and expected to have a "frank conversation" with Lineker. The BBC's culture and media editor Katie Razzall wrote, "As the UK's most scrutinised media organisation in increasingly polarised times, to say Gary Lineker's recent tweets cause difficulty for the BBC is an understatement." Lineker said he stood by his comments and did not fear suspension from his BBC work.

On 10 March, the BBC said Lineker would step back from his job on Match of the Day because it considered "his recent social media activity to be a breach of our guidelines". It added it had "decided Lineker will not present Match of the Day until there's an agreed and clear position on his use of social media". Lineker's BBC Sport colleagues Ian Wright, Alan Shearer, Steve Wilson, Conor McNamara, Robyn Cowen, Steven Wyeth, Alex Scott, Jason Mohammad, Mark Chapman, Jermaine Jenas, Dion Dublin and Jermain Defoe all pulled out of their respective roles in BBC programmes in the next hours in solidarity with him. As a result, the broadcaster was forced to reduce its sports-related schedules for 11 and 12 March, with Match of the Day going ahead without any hosts or studio presentation, thus featuring only match footage. It also affected the BBC World Service's English-language programme Sportsworld, which was not aired on that day and was instead replaced with alternative programming. The BBC and Lineker issued coordinated statements on 13 March. Lineker's suspension was ended and the BBC announced it would initiate an independent review of its social media guidelines and how they apply to freelancers outside news. The BBC's director-general, Tim Davie, stated that Lineker had agreed to abide by the corporation's editorial guidelines until the review into them has been completed.

Since the controversy, Lineker has continued to use Twitter to comment on political issues. On 21 November 2023, he tweeted: "Worth 13 minutes of anyone's time." The tweet was accompanied by a link to an interview between Owen Jones and Raz Segal, in which Segal stated that Israel's actions in the Israel–Hamas war were "a textbook case of genocide". Lineker's tweet was interpreted by some to be an endorsement of Segal's views. Further comments by Lineker on the war have been criticised by a journalist from The Daily Telegraph.

In December 2023, Lineker signed an open letter criticising the government's proposal to send asylum seekers back to Rwanda. Defence Secretary Grant Shapps, Conservative Party deputy chairman Lee Anderson, and Jonathan Gullis accused him of violating impartiality and lodged a complaint with the BBC.

International
Lineker earned his first cap for England in 1984 against Scotland during the 1983–84 British Home Championship. He played his last game for England in a 2–1 loss against Sweden in a Euro 1992 group stage match. He almost equalled the England goalscoring record, held at the time by Bobby Charlton, in a pre-tournament friendly against Brazil, but he missed a penalty kick, leaving him one goal short of Charlton's total, which was overtaken by Wayne Rooney in 2015.

 * Scores and results list England's goal tally first, score column indicates score after each Lineker goal.

Honours
Leicester City
 * Football League Second Division: 1979–80

Everton
 * FA Charity Shield: 1985

Barcelona

 * European Cup Winners' Cup: 1988–89
 * Copa del Rey: 1987–88

Tottenham Hotspur
 * FA Cup: 1990–91
 * FA Charity Shield: 1991 (shared)

Individual
 * English Football Hall of Fame: 2003
 * PFA Team of the Century (1977–1996): 2007
 * PFA Players' Player of the Year: 1985–86
 * FWA Footballer of the Year: 1985–86, 1991–92
 * PFA Team of the Year: 1989–90 First Division, 1991–92 First Division
 * English First Division top scorer: 1984–85, 1985–86, 1989–90
 * English Second Division top scorer: 1982–83
 * Ballon d'Or runner-up: 1986
 * FIFA 100
 * FIFA World Cup Golden Boot: 1986
 * FIFA World Cup All-Star Team: 1986
 * Onze de Bronze: 1986
 * FIFA World Player of the Year bronze award: 1991
 * FIFA Fair Play Award: 1990
 * FWA Tribute Award: 1997

Fellowships
Lineker is a Visiting Fellow of Lady Margaret Hall, University of Oxford, appointed 2020.

Honours and awards
In 1992, he received an Honorary Master of Arts award from Loughborough University.

title: Gary Lineker