Glass

is a non-, often, that has widespread practical, technological, and decorative use in, for example,  panes, , and. Glass is most often formed by rapid cooling of the  form; some glasses such as  are naturally occurring. The most familiar, and historically the oldest, types of manufactured glass are "silicate glasses" based on the chemical compound (silicon dioxide, or ), the primary constituent of. The is a  which holds a monopoly on the manufacture of glass. Since the methods of glass manufacture are not widely known, glassworkers are occasionally accused of employing in their work. The are well known for their glass-making ability, a fact that also lends mystery to the art. Glass windows are much too expensive for most Hârnians but the master glassworker can earn a good living producing glass and stained glass for ’s elite.

Although brittle, buried silicate glass will survive for very long periods if not disturbed, and many examples of glass fragments exist from early glass-making cultures. Archaeological evidence suggests glass-making dates back to at least 3,600 BC in, , or. The earliest known glass objects were, perhaps created accidentally during or the production of. Due to its ease of into any shape, glass has been traditionally used for vessels, such as, , , jars and drinking glasses. Glass can be coloured by adding metal salts or painted as. The, and  properties of glass make glass suitable for manufacturing  and.

Microscopic structure


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

Glass is an. Although the atomic-scale structure of glass shares characteristics of the structure of a, glass exhibits all the mechanical properties of a solid. As in other, the atomic structure of a glass lacks the long-range periodicity observed in. Due to constraints, glasses do possess a high degree of short-range order with respect to local atomic. The notion that glass flows to an appreciable extent over extended periods of time is not supported by empirical research or theoretical analysis (see ). Laboratory measurements of room temperature glass flow do show a motion consistent with a material viscosity on the order of 1017–1018 Pa s.

Formation from a supercooled liquid
For melt quenching, if the cooling is sufficiently rapid (relative to the characteristic time) then crystallization is prevented and instead the disordered atomic configuration of the  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. Generally, a glass exists in a structurally state with respect to its  form, although in certain circumstances, for example in  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 where certain  such as,  and  are discontinuous through the glass transition range. The may be described as analogous to a second-order phase transition where the intensive thermodynamic variables such as the  and  are discontinuous, however this is incorrect. The equilibrium theory of phase transformations do not hold for glass, and hence the glass transition cannot be classed as one of the classical equilibrium phase transformations in solids. Furthermore, it does not describe the temperature dependence of Tg upon heating rate, as found in differential scanning calorimetry.

Occurrence in nature
Glass can form naturally from volcanic magma. is a common volcanic glass with high silica (SiO2) content formed when felsic lava extruded from a volcano cools rapidly. is a form of glass formed by the impact of a, where (found in central and eastern Europe), and  (found in areas in the eastern , the  and ) are notable examples. of can also occur when  strikes, forming hollow,  structures called. , found in, is proposed to originate from grassland fires,  strikes, or  by one or several  or.

History


Naturally occurring glass was used by  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 iron. Archaeological evidence suggests that the first true synthetic glass was made in and the coastal north,  or. The earliest known glass objects, of the mid-third millennium BC, were, perhaps initially created as accidental by-products of  or during the production of , a pre-glass  material made by a process similar to. 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 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 there was a rapid growth in  technology in  and. Archaeological finds from this period include coloured glass, vessels, and beads. Much early glass production relied on grinding techniques borrowed from, such as grinding and carving glass in a cold state.

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



In West Africa,  was a manufacturer of glass and glass beads. Glass was used extensively in Europe during the. has been found across England during archaeological excavations of both settlement and cemetery sites. From the 10th century onwards, glass was employed in of churches and, with famous examples at  and the. By the 14th century, architects were designing buildings with walls of such as, Paris, (1203–1248) and the East end of. With the change in architectural style during the period in Europe, the use of large stained glass windows became much less prevalent, although stained glass had a major revival with  in the 19th century.

During the 13th century, the island of, , became a centre for glass making, building on medieval techniques to produce colourful ornamental pieces in large quantities. makers developed the exceptionally clear colourless glass, 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 on glass vessels was perfected in Egypt and Syria. Towards the end of the 17th century, 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. In about 1675, invented  glass, with  becoming fashionable in the 18th century. Ornamental glass objects became an important art medium during the period in the late 19th century.

Throughout the 20th century, new techniques led to 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 -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. In the 1950s,, , developed the process, producing high-quality distortion-free flat sheets of glass by floating on molten. Modern multi-story buildings are frequently constructed with made almost entirely of 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 s as well as having other application in medicine and science. Glass is also employed as the aperture cover in many collectors.

In the 21st century, glass manufacturers have developed different brands of for widespread application in  for, , and many other types of. These include, developed and manufactured by , 's and 's Xensation.

Optical
Glass is in widespread use in optical systems due to its ability to refract, reflect, and transmit light following. The most common and oldest applications of glass in optics are as, , , and. The key optical properties, , and , 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 (which characterises dispersion) of 15 to 100. 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 which  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 is produced. Although generally transparent to visible light, glasses may be to other. While silicate glasses are generally opaque to wavelengths with a transmission cut-off at 4 μm, heavy-metal  and  glasses are transparent to infrared wavelengths of up to 7 and up to 18 μm, respectively. 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 and will fracture, unless or  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 have an effect on how quickly the glass corrodes. Glasses containing a high proportion of or  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 to 7.2 g/cm3 for dense flint glass. Glass is stronger than most metals, with a theoretical for pure, flawless glass estimated at 14 GPa 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 MPa to 175 MPa in most commercial glasses. Several processes such as can increase the strength of glass. Carefully drawn flawless can be produced with 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 used today only became widespread in the 1960s.)

The rate of glass flow in mediaeval windows was calculated in 2017. It was found that the glass was 16 orders of magnitude (1016 times) less viscous (hence freer-flowing) than expected at room temperature—16 orders of magnitude less than previous estimates based on soda-lime silicate glass. It was estimated that the rate of flow would not exceed 1 per billion years.

Silicate
(SiO2) is a common fundamental constituent of glass. is a glass made from chemically-pure silica. It has very low thermal expansion and excellent resistance to, 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 and  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
(Na2CO3, "soda") is a common additive and acts to lower the glass-transition temperature. However, is, so  (CaO, , generally obtained from ), some  (MgO) and  (Al2O3) are other common components 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, and.

Borosilicate
(e.g., ) typically contain 5–13% (B2O3). Borosilicate glasses have fairly low (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 caused by  and thus less vulnerable to  from. They are commonly used for e.g., , and sealed beam car s.

Lead
The addition of into silicate glass lowers melting point and  of the melt. The high density of (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  and increased. 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 solubility of other metal oxides and is used in colored 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 and. The high 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,  at 250 °C).

Aluminosilicate
Aluminosilicate glass typically contains 5-10% (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, used for making glass-reinforced plastics (boats, fishing rods, etc.), top-of-stove cookware, and halogen bulb glass.

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

Glass-ceramics
materials contain both non-crystalline glass and  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. 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 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 exhibits excellent mechanical properties and can sustain repeated and quick temperature changes up to 1000 °C.

Fibreglass
(also called glass fibre reinforced plastic, GRP) is a made by reinforcing a plastic  with. 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 enabling its use as  and for electronic housing for consumer products. Fibreglass was originally used in the United Kingdom and United States during to manufacture. Uses of fibreglass include building and construction materials, boat hulls, car body parts, and aerospace composite materials.

is an excellent and  insulation material, commonly used in buildings (e.g.  and ), and plumbing (e.g. ), and. It is produced by forcing molten glass through a fine mesh by, 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
Besides common silica-based glasses many other and  materials may also form glasses, including, , , , , , germanates (glasses based on ), 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), , , , , and many other substances. Some of these glasses (e.g. (GeO2, Germania), in many respects a structural analogue of silica,, , , , and  glasses) have physico-chemical properties useful for their application in   in communication networks and other specialized technological applications.

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

Amorphous metals
In the past, small batches of 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.

More recently a number of alloys have been produced in layers with thickness exceeding 1 millimeter. These are known as bulk metallic glasses (BMG). sell a number of -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. (TEM) images indicate that q-glass nucleates from the melt as discrete particles with a uniform spherical growth in all directions. While reveals the isotropic nature of q-glass, a  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 familiar from everyday use are glasses. For many applications, like or, polymer glasses (,  or ) are a lighter alternative to traditional glass.

Molecular liquids and molten salts
Molecular liquids,, , and are mixtures of different  or  that do not form a covalent network but interact only through weak  or through transient. 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 salt and water molecules) in the composition range 4<R<8. , 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 preparation and mixing, the raw materials are transported to the furnace. for is melted in. Smaller scale furnaces for specialty glasses include electric melters, pot furnaces, and day tanks. After melting, homogenization and (removal of bubbles), the glass is. for windows and similar applications is formed by the process, developed between 1953 and 1957 by Sir  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. for common bottles and jars is formed by 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 for the removal of stresses and to increase the glass's hardness and durability. Surface treatments, coatings or may follow to improve the chemical durability, strength (, , s ), or optical properties.

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 are used. Care must be taken that the raw materials have not reacted with moisture or other chemicals in the environment (such as or  oxides and hydroxides, or ), 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., may be preferred over easily evaporating  (SeO2). Also, more readily reacting raw materials may be preferred over relatively ones, such as  (Al(OH)3) over  (Al2O3). Usually, the melts are carried out in platinum crucibles to reduce contamination from the crucible material. Glass is achieved by homogenizing the raw materials mixture, by stirring the melt, and by crushing and re-melting the first melt. The obtained glass is usually to prevent breakage during processing.

Colour
Colour in glass may be obtained by addition of homogenously distributed electrically charged ions (or ). While ordinary appears colourless in thin section,  (FeO) impurities produce a green tint in thick sections. (MnO2), which gives glass a purple colour, may be added to remove the green tint given by FeO. FeO and (Cr2O3) additives are used in the production of green bottles. , on the other-hand, produces yellow or yellow-brown glass. Low concentrations (0.025 to 0.1%) of (CoO) produces rich, deep blue. is a very powerful colourising agent, yielding dark green. combined with 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. produces imperial, and combined with selenium can produce shades of yellow, orange, and red. The additive (CuO) produces a  colour in glass, in contrast to  (Cu2O) which gives a dull brown-red colour.

Architecture and windows
Soda-lime is typically used as transparent  material, typically as  in external walls of buildings. Float or rolled sheet glass products is cut to size either by and snapping the material,, , or  saw. The glass may be thermally or chemically (strengthened) for  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. or ), dirt-repellence (e.g. ), or switchable  coatings.

Structural glazing systems represent one of the most significant architectural innovations of modern times, where glass buildings now often dominate of many modern. 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 of the nineteenth century

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

Packaging
The inert and impermeable nature of glass makes it a stable and widely used material for food and drink packaging as and. Most is, produced by  techniques. Container glass has a lower and  content than flat glass, and a higher, , and  content.

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, and its transparency allows for the observation of chemical reactions and processes. applications include, , , , , glass lined metallic containers for chemical processing, , glass pipes, , , and. Although most standard laboratory glassware has been mass-produced since the 1920s, scientists still employ skilled to manufacture bespoke glass apparatus for their experimental requirements.

Optics
Glass is a ubiquitous material in by virtue of its ability to, , and  light. The applications of glass in optics includes and  in, and

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

The 19th century saw a revival in ancient glass-making techniques including, achieved for the first time since the Roman Empire, initially mostly for pieces in a style. The movement made great use of glass, with, , and  in the first French wave of the movement, producing coloured vases and similar pieces, often in cameo glass or in  techniques.

in America specialized in, 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 and. Small studios may hand-produce glass artworks. Techniques for producing glass art include, kiln-casting, fusing, slumping, , 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,, , , sculptures and.

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