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Refer to caption
A glass building facade

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 are named after the material, e.g., a "glass" for drinking, "glasses" for vision correction, and a "magnifying glass".

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.[1]

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

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A graphic showing the lack of periodic arrangement in the microscopic structure of glass
The amorphous structure of glassy silica (SiO2) in two dimensions. No long-range order is present, although there is local ordering to the tetrahedral arrangement of oxygen (O) atoms around the silicon (Si) atoms.
A graphic visually showing the difference between the microscopic arrangement of single crystals, polycrystals, and amorphous solids, as explained in the caption
Microscopically, a single crystal has atoms in a near-perfect periodic arrangement; a polycrystal is composed of many microscopic crystals; and an amorphous solid such as glass has no periodic arrangement even microscopically.
Main article: Structure of liquids and glasses

The standard definition of a glass (or vitreous solid) is a non-crystalline solid formed by rapid melt quenching.[2][3][4][5] 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.[5][6]

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.[7][8][9] 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.[10] 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,[11] 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.[6][12]

Formation from a supercooled liquid

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Main articles: Glass transition and Glass formation
Unsolved problem in physics
What is the nature of the transition between a fluid or regular solid and a glassy phase? "The deepest and most interesting unsolved problem in solid state theory is probably the theory of the nature of glass and the glass transition." —P.W. Anderson[13]
More unsolved problems in physics

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 ability of a material to form glass during rapid cooling is known as its glass-forming ability. This ability can be predicted by the rigidity theory.[14] 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.[15]

Glass is sometimes considered to be a liquid due to its lack of a first-order phase transition[8][16] 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.[3] 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.[5][6]

Occurrence in nature

[edit]
Main articles: Volcanic glass, Impactite, and Fulgurite

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.[17] 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.[18] Vitrification of quartz can also occur when lightning strikes sand, forming hollow, branching rootlike structures called fulgurites.[19] Trinitite is a glassy residue formed from the desert floor sand at the Trinity nuclear bomb test site.[20] 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.[21]

History

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Main article: History of glass
Refer to caption
Roman cage cup from the 4th century

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.[22][23]

Glassmaking dates back at least 6000 years, long before humans had discovered how to smelt iron.[22] Archaeological evidence suggests that the first true synthetic glass was made in Lebanon and the coastal north Syria, Mesopotamia or ancient Egypt.[24][25] 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.[1]

Early glass was rarely transparent and often contained impurities and imperfections,[22] and is technically faience rather than true glass, which did not appear until the 15th century BC.[26] 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.[27][28]

During the Late Bronze Age, there was a rapid growth in glassmaking technology in Egypt and Western Asia.[24] Archaeological finds from this period include coloured glass ingots, vessels, and beads.[24][29]

Much early glass production relied on grinding techniques borrowed from stoneworking, such as grinding and carving glass in a cold state.[30]

The term glass has its origins in the late Roman Empire, in the Roman glass making centre at Trier (located in current-day Germany) where the late-Latin term glesum originated, likely from a Germanic word for a transparent, lustrous substance.[31] Glass objects have been recovered across the Roman Empire[32] in domestic, funerary,[33] and industrial contexts,[34] as well as trade items in marketplaces in distant provinces.[35][36] Examples of Roman glass have been found outside of the former Roman Empire in China,[37] the Baltics, the Middle East, and India.[38] 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.[39]

Elaborate stained glass windows in the choir of the Basilica of Saint Denis
Windows in the choir of the Basilica of Saint-Denis, one of the earliest uses of extensive areas of glass (early 13th-century architecture with restored glass of the 19th century)

In post-classical West Africa, Benin was a manufacturer of glass and glass beads.[40] 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.[41] 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,[42] although stained glass had a major revival with Gothic Revival architecture in the 19th century.[43]

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.[39] 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.[22] In the 13th, 14th, and 15th centuries, enamelling and gilding on glass vessels were perfected in Egypt and Syria.[44] 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.[39] Ornamental glass objects became an important art medium during the Art Nouveau period in the late 19th century.[39]

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.[45] 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.[46] 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.[22] Modern multi-story buildings are frequently constructed with curtain walls made almost entirely of glass.[47] Laminated glass has been widely applied to vehicles for windscreens.[48] Optical glass for spectacles has been used since the Middle Ages.[49] The production of lenses has become increasingly proficient, aiding astronomers[50] as well as having other applications in medicine and science.[51] Glass is also employed as the aperture cover in many solar energy collectors.[52]

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.[53][54][55]

Physical properties

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Optical

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Main article: Optical glass

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.[56] 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.[56] Optical glass typically has a refractive index of 1.4 to 2.4, and an Abbe number (which characterises dispersion) of 15 to 100.[56] The refractive index may be modified by high-density (refractive index increases) or low-density (refractive index decreases) additives.[57]

Glass transparency results from the absence of grain boundaries which diffusely scatter light in polycrystalline materials.[58] 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.[44][59] 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.[60] The addition of metallic oxides results in different coloured glasses as the metallic ions will absorb wavelengths of light corresponding to specific colours.[60]

Other

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See also: List of physical properties of glass, Corrosion § Corrosion of glass, and Strength of glass
Glass can be fairly easily melted and manipulated with a heat source

In the manufacturing process, glasses can be poured, formed, extruded and moulded into forms ranging from flat sheets to highly intricate shapes.[61] The finished product is brittle but can be laminated or tempered to enhance durability.[62][63] 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.[22][64][65] Nevertheless, although usually highly resistant to chemical attack, glass will corrode or dissolve under some conditions.[64][66] 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.[67][68]

The density of glass varies with chemical composition with values ranging from 2.2 grams per cubic centimetre (2,200 kg/m3) for fused silica to 7.2 grams per cubic centimetre (7,200 kg/m3) for dense flint glass.[69] Glass is stronger than most metals, with a theoretical tensile strength for pure, flawless glass estimated at 14 to 35 gigapascals (2,000,000 to 5,100,000 psi) 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 megapascals (2,000 to 25,400 psi) in most commercial glasses.[60] Several processes such as toughening can increase the strength of glass.[70] Carefully drawn flawless glass fibres can be produced with a strength of up to 11.5 gigapascals (1,670,000 psi).[60]

Further information on the tiny glass flakes formed during glass vial manufacturing: Spicule

Reputed flow

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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.[71] 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).[8]

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 1024 Pa·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.[72][73]

Types

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Silicate glasses

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Close-up photograph of sand
Quartz sand (silica) is the main raw material in commercial glass production

Silicon dioxide (SiO2) is a common fundamental constituent of glass. Fused quartz is a glass made from chemically pure silica.[68] 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.[74] 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.[75]

Soda–lime glass

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Main article: 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.[68][76] Soda–lime–silicate glass is transparent, easily formed, and most suitable for window glass and tableware.[77] However, it has a high thermal expansion and poor resistance to heat.[77] Soda–lime glass is typically used for windows, bottles, light bulbs, and jars.[75]

Borosilicate glass

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Refer to caption
A Pyrex borosilicate glass measuring cup

Borosilicate glasses (e.g. Pyrex, Duran) typically contain 5–13% boron trioxide (B2O3).[75] Borosilicate glasses have fairly low coefficients of thermal expansion (7740 Pyrex CTE is 3.25×10−6/°C[78] as compared to about 9×10−6/°C for a typical soda–lime glass[79]). 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.[75]

Lead glass

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Main article: Lead glass
See also: Lead poisoning

The addition of lead(II) oxide into silicate glass lowers the melting point and viscosity of the melt.[80] 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.[68][81] 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.[74] 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).[82]

Aluminosilicate glass

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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.[75] Aluminosilicate glass is extensively used for fibreglass,[83] used for making glass-reinforced plastics (boats, fishing rods, etc.), top-of-stove cookware, and halogen bulb glass.[74][75]

Other oxide additives

[edit]

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.[84] 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.[85] 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.[86]

Glass-ceramics

[edit]
Main article: Glass-ceramic
A cooktop with two of its eyes turned on
A high-strength glass-ceramic cooktop with negligible thermal expansion

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.[87] 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.[87]

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.[88][87]

Fibreglass

[edit]
Main articles: Fiberglass and Glass wool

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.[89][90][91] 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.[92][89][91]

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.[92] 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.[60]

Non-silicate glasses

[edit]
A CD
A CD-RW (CD). Chalcogenide glass forms the basis of rewritable CD and DVD solid-state memory technology.[93]

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.[6] 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.[94][95]

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.[96][97][98]

Amorphous metals

[edit]
Main article: Amorphous metal
Refer to caption
Samples of amorphous metal, with millimetre scale

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.[99][100]

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.[101]

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.[102][103]

Polymers

[edit]

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.[104] 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.[105]

Molecular liquids and molten salts

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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.[106][107] Examples include LiCl:RH2O (a solution of lithium chloride salt and water molecules) in the composition range 4<R<8.[108] sugar glass,[109] or Ca0.4K0.6(NO3)1.4.[110] 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.[111]

Production

[edit]
Main articles: Glass production, Float glass, and Glassblowing
A red hot piece of glass being blown
Industrial robots unloading float glass

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.[76] 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.[112] 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.[113] Container glass for common bottles and jars is formed by blowing and pressing methods.[114] This glass is often slightly modified chemically (with more alumina and calcium oxide) for greater water resistance.[115]

Once the desired form is obtained, glass is usually annealed for the removal of stresses and to increase the glass's hardness and durability.[116] 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[117]), or optical properties (insulated glazing, anti-reflective coating).[118]

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).[119] 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.[119][120]

Colour

[edit]
Main article: Glass coloring and color marking

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.[121] Manganese dioxide (MnO2), which gives glass a purple colour, may be added to remove the green tint given by FeO.[122] FeO and chromium(III) oxide (Cr2O3) additives are used in the production of green bottles.[121] Iron (III) oxide, on the other-hand, produces yellow or yellow-brown glass.[123] Low concentrations (0.025 to 0.1%) of cobalt oxide (CoO) produce rich, deep blue cobalt glass.[124] Chromium is a very powerful colouring agent, yielding dark green.[125] Sulphur combined with carbon and iron salts produces amber glass ranging from yellowish to almost black.[126] A glass melt can also acquire an amber colour from a reducing combustion atmosphere.[127] Cadmium sulfide produces imperial red, and combined with selenium can produce shades of yellow, orange, and red.[121][123] Addition of copper(II) oxide (CuO) produces a turquoise colour in glass, in contrast to copper(I) oxide (Cu2O) which gives a dull red-brown colour.[128]

Uses

[edit]

Architecture and windows

[edit]
Main articles: Architectural glass and Window

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.[129]

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.[130] 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.[130] Structural glazing systems have their roots in iron and glass conservatories of the nineteenth century[131]

Tableware

[edit]
Main articles: Tableware and List of glassware

Glass is an essential component of tableware and is typically used for water, beer and wine drinking glasses.[51] Wine glasses are typically stemware, i.e. goblets formed from a bowl, stem, and foot. Lead crystal glass may be cut and polished to produce decorative drinking glasses with gleaming facets.[132][133] Other uses of glass in tableware include decanters, jugs, plates, and bowls.[51]

Packaging

[edit]
Main article: Container glass

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.[134] 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.[135]

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,[136][137] including very thin solely glass based encapsulation of integrated circuits and other semiconductors in high manufacturing volumes.[138]

Laboratories

[edit]
Main article: Laboratory glassware

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.[139][140] 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.[141][139] 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.[142]

Optics

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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.[56]

Modern glass art

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Main articles: Studio glass, Art glass, and Glass art

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.[143]

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.[144]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Glass

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Glass is an amorphous, non-crystalline solid material that is typically transparent or translucent, hard, brittle, and resistant to the natural elements, formed by rapidly cooling a molten mixture primarily consisting of silica (sand), , and lime (calcium oxide) to prevent . This composition, often referred to as soda-lime glass, accounts for the majority of glass produced today and exhibits a where it shifts from a rigid solid to a upon heating, without undergoing a distinct like crystalline materials. Unlike metals or ceramics with ordered atomic structures, glass's disordered atomic arrangement gives it unique , such as high transparency to visible , making it ideal for windows, lenses, and containers. The dates back to around 3500 BCE, with the earliest known manmade examples discovered in and , where it was initially produced as small beads and decorative objects from naturally occurring silica-rich sands fused with plant ash or natron. By the second millennium BCE, glassmaking had evolved into a specialized in the , spreading to the Mediterranean and enabling the production of vessels, jewelry, and seals, often colored with metal oxides like for or iron for hues. The invention of around the first century BCE in the revolutionized the industry, allowing for faster and more efficient creation of intricate shapes, which facilitated widespread use in everyday items like bottles and lamps across the ancient world. Modern glass production involves melting the raw materials—silica (about 70-75%), soda ash (10-15%), and lime (5-10%)—in large furnaces at temperatures exceeding 1500°C, followed by forming techniques such as the float process for flat glass, where molten glass is poured onto a bed of molten tin to create smooth sheets. Additives like metal oxides can modify properties, such as increasing durability with boron for used in laboratory equipment or enhancing strength through chemical tempering for screens. Key properties include low , chemical inertness, and recyclability, as glass can be melted and reformed indefinitely without quality loss, contributing to its in . In contemporary applications, glass serves essential roles in (e.g., energy-efficient windows), (e.g., LCD displays and fiber optic cables for ), and (e.g., photovoltaic panels), leveraging its optical clarity and electrical insulation. Advanced variants, such as that switches opacity for privacy or photonic glasses for high-speed data transmission, highlight ongoing innovations building on glass's versatile atomic structure. Its global production exceeds 130 million metric tons annually as of 2024, underscoring its foundational impact on modern civilization.

Structure and Formation

Atomic and Molecular Structure

Glass is classified as an , featuring a highly disordered atomic arrangement that lacks the long-range periodicity characteristic of crystalline materials, yet maintains short-range order through the arrangement of and oxygen atoms into tetrahedral SiO₄ units in glasses. This structural disorder arises from the rapid cooling of molten material, preventing the atoms from aligning into a crystalline lattice, resulting in isotropic unlike the anisotropic behavior of . In glasses, the short-range order manifests as interconnected silica tetrahedra, where each atom is bonded to four oxygen atoms, forming a network that provides rigidity without extended . The continuous random network (CRN) model, introduced by Zachariasen in , provides a foundational description of this structure in silicate , positing a continuous, randomly connected network of corner-sharing polyhedra—primarily SiO₄ tetrahedra—that extends indefinitely without periodic repetition, featuring rings of various sizes. This model emphasizes the preservation of local coordination geometries similar to those in crystalline counterparts, such as , while allowing for the absence of long-range order, which accounts for the mechanical and thermal similarities between and their crystalline analogs despite structural differences. A key feature of amorphous solids like glass is the glass transition temperature (T_g), defined as the temperature range where the material transitions reversibly from a hard, brittle, rigid state to a soft, viscous state without , corresponding to a dramatic increase in (typically around 10^{12} Pa·s). For pure silica glass, T_g occurs at approximately 1480 K (1207°C), marking the point where enables cooperative atomic rearrangements while the structure remains non-crystalline. The disordered atomic packing in amorphous glass leads to lower density and refractive index compared to crystalline forms, reflecting looser atomic coordination and reduced polarizability. For instance, fused silica has a density of 2.20 g/cm³ and a refractive index of 1.458 at 589 nm, whereas α-quartz exhibits a density of 2.649 g/cm³ and an ordinary refractive index of 1.544 at the same wavelength. These variations arise from the more efficient space-filling in crystalline lattices versus the random voids in amorphous networks, influencing applications where optical clarity and mechanical stability are paramount.

Vitrification Process

The transforms a molten into a glass by the liquid below its temperature, thereby kinetically suppressing the formation of an ordered crystalline structure and yielding a metastable . This occurs when the melt is cooled rapidly enough to bypass and , passing through the temperature TgT_g, where the 's increases dramatically—typically by 12 to 14 orders of magnitude—freezing the atomic configuration in a disordered state without significant volume change or molecular rearrangement. The resulting glass is thermodynamically metastable relative to the crystalline form, as it represents a non-equilibrium state trapped by kinetic barriers during cooling. A key distinction exists between equilibrium crystallization and non-equilibrium vitrification: under slow cooling near the melting temperature TmT_m, the system achieves thermodynamic stability by forming a crystalline lattice through ordered atomic packing, releasing . In contrast, vitrification is a kinetically driven process where rapid cooling—often exceeding critical rates on the order of 10^2 to 10^6 K/s depending on composition—prevents atomic necessary for , preserving the liquid's short-range order in a supercooled state below TgT_g. The cooling rate thus determines the glass-forming ability, with faster rates favoring the amorphous phase by reducing the time available for crystalline embryos to develop. In 1932, William H. Zachariasen proposed the random network theory, observing that in oxide glasses arises from the formation of a continuous, three-dimensional network of polyhedra with shared corners but no long-range periodicity, enabling certain compositions to achieve the disordered essential for glass stability. This theory highlights how network-modifying ions disrupt potential crystallinity, facilitating the required for . For common soda-lime-silica glass, the temperature TgT_g is approximately 500–600°C, marking the onset of rigidity during cooling.

Natural Occurrence

Obsidian and Volcanic Glasses

, a prominent natural form of glass, originates from volcanic activity through the rapid quenching of silica-rich . This process occurs when viscous, high-silica erupts and cools abruptly upon exposure to air or , preventing the formation of and resulting in a homogeneous, known as . The material typically appears as sharp, black or dark-colored flows or fragments, with a glossy luster due to its glassy texture. Compositionally, obsidian consists primarily of silicon dioxide (SiO₂), with contents ranging from 65% to 80%, alongside alumina and minor oxides such as sodium, potassium, iron, calcium, and magnesium. This rhyolitic composition contributes to its high viscosity during eruption, facilitating the rapid cooling essential for glass formation. Obsidian's amorphous structure leads to a distinctive conchoidal fracture pattern, producing smooth, curved surfaces with sharp edges when broken, a trait that arises from the lack of crystalline planes in the material. Notable deposits occur in regions with intense volcanic activity, such as Iceland's , where forms in association with rhyolitic eruptions, and Mexico's central highlands, including sources like Otumba, which supplied material for prehistoric artifacts. In prehistoric contexts, 's sharpness made it ideal for crafting tools like blades and points, as evidenced by artifacts from Mesoamerican sites where it was flaked into cutting implements. Other volcanic glasses include variations like and , which also form from quenched silica-rich lavas but exhibit distinct textures. arises from explosive eruptions, trapping gases to create a frothy, lightweight vesicular structure while retaining a glassy composition similar to . , in contrast, develops perlitic cracks from contraction during cooling or subsequent hydration, incorporating 2-5% and often forming through the alteration of obsidian-like glass in volcanic settings.

Other Geological Forms

Fulgurites represent one of the rare non-volcanic natural glasses, formed when strikes silica-rich or soil, instantly melting the material into glassy tubular structures that trace the bolt's path underground. These hollow, branching tubes, often lined with and adhering to external grains, can extend several meters in but typically measure just millimeters to centimeters in diameter, with irregular, vesicular interiors reflecting the rapid cooling of superheated silica. Tektites, another category of impact-derived natural glass, originate from collisions with Earth's surface, where extreme pressures and temperatures vitrify terrestrial rocks into droplets ejected into the atmosphere. The Australasian tektite field, one of the largest such deposits spanning and , dates to approximately 0.8 million years ago and consists of glasses with high silica content (typically 70-80 wt.%) and inclusions of , a pure form of silica glass. Many tektites display distinctive aerodynamic shaping, including rounded, button-like forms and ablated surfaces with pits and flanges, resulting from frictional heating and material loss during high-velocity re-entry through the atmosphere at speeds exceeding 11 km/s. In contrast to more common volcanic glasses, non-volcanic natural glasses such as lightning-formed and impact glasses like tektites often exhibit lower —typically less than 0.1 wt.% for tektites due to during formation—and higher densities, typically 2.45-2.55 g/cm³ for tektites, arising from their iron-enriched, silica-dominated compositions under conditions.

Historical Development

Prehistoric and Ancient Glassmaking

The earliest known precursor to true glass production was Egyptian faience, a nonfired ceramic composite of crushed quartz or sand coated with a vitreous glaze, dating back to around 5000 BCE in predynastic Egypt. Archaeological evidence from Badarian culture sites (c. 5000–3900 BCE) includes faience beads and amulets, where the glaze formed through the reaction of silica with alkali fluxes during sintering, providing a shimmering, glass-like surface that symbolized rebirth and protection in ancient Egyptian culture. This material laid the groundwork for later glassmaking by demonstrating control over vitreous coatings on silica-based bodies. True glassmaking emerged around 2500 BCE in , with the production of small beads and seals from fused silica, soda, and lime mixtures. These early artifacts, found in and other Sumerian sites, represent the first intentional melting of raw materials into a homogeneous, , likely inspired by natural volcanic glasses like but achieved through human-controlled heating in crucibles. By the second millennium BCE, the craft spread to during the New Kingdom (c. 1550–1070 BCE), where glass was shaped into beads, inlays, and vessels, often imitating precious stones like . A key innovation in ancient glass vessel production was the core-forming technique, developed in around 1500 BCE, which involved winding trails of molten glass around a removable clay or dung core attached to a metal rod. This method produced slender, colorful containers and amphorae using soda-lime-silica compositions, with the core later scraped out to leave a hollow vessel; it allowed for intricate designs but was labor-intensive and limited to luxury items. Core-formed vessels proliferated across the and from the 18th to 5th centuries BCE, reflecting technological exchange along trade routes. The invention of in the late 1st century BCE, likely in the Syro-Palestinian region under Roman influence, revolutionized production by enabling the inflation of molten glass on a blowpipe into thin-walled forms. This technique, which spread rapidly through the , facilitated of affordable , bottles, and lamps, shifting glass from an elite rarity to a common household good by the 1st century CE. Throughout prehistoric and ancient periods, glass served as a luxury in networks, prized for its rarity and translucency, with Mesopotamian and Egyptian pieces exchanged as far as the Indus Valley and . Artisans enhanced its appeal by coloring the glass through the addition of metal oxides during melting, such as for turquoise greens, for deep blues, and for purples, creating vessels that evoked the hues of gemstones and held symbolic value in rituals and status display.

Medieval to Modern Advancements

In the , Venetian glassmakers on the island of pioneered the production of cristallo, a nearly colorless and transparent glass achieved through refined purification of silica and sources, marking a significant advancement over earlier tinted varieties. This innovation, developed in the mid-15th century, relied on high-purity and from beechwood , allowing for clearer vessels that mimicked rock crystal and elevated Venice's dominance in European glass trade. By the 17th century, English glassmaker George Ravenscroft introduced lead crystal in 1674, patenting a formula that incorporated lead oxide into the molten glass batch to create a denser, more brilliant material with superior refractive qualities. This addition, detailed in Ravenscroft's royal patent granted by King Charles II, reduced defects like crizzling and enabled the production of fine , shifting English glass from imitation of Venetian styles toward a distinct national style. The 19th century saw the widespread adoption of the cylinder blowing method for flat sheet glass, an evolution of earlier hand-blown techniques that involved elongating a blown glass bubble into a cylinder, slitting it lengthwise, and flattening it into sheets. This process, mechanized in Britain and France during the mid-1800s, improved uniformity and scale for window glass production, bridging manual craftsmanship with emerging industrial demands. Concurrently, the Owens automatic bottle-making machine, invented by Michael J. Owens in 1903, revolutionized container production by automating the forming, blowing, and annealing of glass bottles from molten gobs, increasing output from a few thousand to over 2,500 per day per machine and reducing labor-intensive hand-blowing. The early 20th century introduced electric melting furnaces, with full electric operation first achieved in the 1920s using graphite electrodes to generate heat directly within the glass melt, offering precise temperature control and energy efficiency over traditional coal or gas-fired batch systems. This adoption facilitated the transition from intermittent batch production—where raw materials were melted in discrete loads—to continuous processes, enabling uninterrupted flow of molten glass for higher throughput and consistency. The pinnacle of this shift came in 1959 with the Pilkington float glass process, in which molten glass is poured onto a bed of molten tin to form a continuous, flat ribbon of uniform thickness without grinding or polishing. Developed by Sir Alastair Pilkington after years of research at Pilkington Brothers, this method dramatically scaled sheet glass production, accounting for over 90% of flat glass worldwide by the late 20th century.

Physical Properties

Optical Characteristics

Glass exhibits high transparency in the visible spectrum primarily due to the wide bandgap of its primary component, silica (SiO₂), which is approximately 9 eV, preventing absorption of photons in the 400–700 nm range. This electronic bandgap structure ensures minimal electronic transitions in the visible light wavelengths, allowing over 90% transmission through typical thicknesses of soda-lime glass, the most common variety. The amorphous nature of glass further contributes to this clarity by lacking crystalline defects that could scatter light, resulting in a material that appears colorless and highly transmissive under standard conditions. A key optical property of glass is its , which measures how much light bends when entering the material; for soda-lime glass, this value is approximately 1.5 at visible wavelengths, higher than air (n ≈ 1.0) but lower than many other solids. This leads to at glass-air interfaces, governed by : n1sin⁡θ1=n2sin⁡θ2n_1 \sin \theta_1 = n_2 \sin \theta_2 where n1n_1 and n2n_2 are the refractive indices of the two media, and θ1\theta_1 and θ2\theta_2 are the angles of incidence and , respectively. Dispersion, the wavelength-dependent variation in refractive index, causes different colors of light to bend by slightly different amounts, with shorter wavelengths (e.g., ) refracting more than longer ones (e.g., red). The , a measure of this dispersion, is typically around 60 for soda-lime glass, indicating low suitable for lens design in optical systems. As an isotropic amorphous material, glass lacks inherent , meaning it does not split incoming into two polarized rays with different refractive indices, unlike crystalline materials. This uniformity simplifies its use in polarization-insensitive . Additionally, soda-lime glass has a UV around 300 nm, beyond which transmission drops sharply due to absorption by impurities and the bandgap edge, limiting its utility in deep applications.

Mechanical and Thermal Properties

Glass exhibits brittle behavior under mechanical loading, failing suddenly without significant deformation due to its and lack of dislocations. This leads to initiated by the propagation of pre-existing surface flaws under tensile stress, governed by principles such as the KIC=YσπcK_{IC} = Y \sigma \sqrt{\pi c}
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