Glass

• 1. An Amorphous Solid

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  Glass is an amorphous solid, which means its atoms do not arrange themselves into the orderly, repeating lattice found in crystals. That single fact sets it apart from nearly every other hard material we handle. We pour water from it, see through it, correct our vision with it, and even name the objects after the substance itself. A drinking vessel becomes a glass, spectacles become glasses, and a lens for reading becomes a magnifying glass.

  Most glass is formed by quenching, the rapid cooling of a molten form so fast that the disordered atoms freeze in place before they can crystallize. But nature made glass long before any furnace existed. Volcanic glass and obsidian were shaped by Stone Age societies into arrowheads and knives. Archaeological evidence suggests human glassmaking reaches back at least 3600 BC in Mesopotamia, Egypt, or Syria.

  How can a material behave like a solid while its atoms resemble a frozen liquid? Why do soda-lime compositions account for around 90% of everything manufactured today? And why does the myth that old window glass slowly flows downward refuse to die? Those questions run through everything that follows.

• 2. Frozen Disorder

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  The standard definition of a glass is a non-crystalline solid formed by rapid melt quenching. Its atomic structure lacks the long-range periodicity that defines crystalline solids. Yet because of chemical bonding constraints, glass still holds a high degree of short-range order in its local atomic polyhedra. It looks like a supercooled liquid up close, but it carries all the mechanical properties of a solid.

  When cooling is fast enough relative to the characteristic crystallization time, crystallization is prevented entirely. The disordered configuration of the supercooled liquid is frozen into the solid at the glass transition temperature, written as Tg. This capacity to form glass during rapid cooling is called glass-forming ability, and it can be predicted by rigidity theory. A glass usually exists in a structurally metastable state with respect to its crystalline form.

  Glass is sometimes called a liquid because it lacks a first-order phase transition, where variables like volume, entropy, and enthalpy would jump discontinuously. The glass transition instead resembles a second-order transition, where thermal expansivity and heat capacity are discontinuous. Even so, the equilibrium theory of phase transformations does not hold for glass. The transition cannot be classed among the classical equilibrium phase transformations in solids.

• 3. Glass The Earth Makes

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  Volcanic magma can become glass on its own. Obsidian, a common volcanic glass with high silica content, forms when felsic lava extruded from a volcano cools rapidly. The Earth has other recipes too. Impactite is glass born from the violence of a meteorite strike, and two of its most notable forms are Moldavite, found in central and eastern Europe, and Libyan desert glass, scattered across the eastern Sahara, the deserts of eastern Libya, and western Egypt.

  Lightning supplies another method. When it strikes sand, the quartz vitrifies into hollow, branching, rootlike structures called fulgurites. Human technology has added its own variant. Trinitite is the glassy residue formed from desert floor sand at the Trinity nuclear bomb test site, fused by the heat of the blast.

  Edeowie glass, found in South Australia, keeps its origins uncertain. It has been proposed to come from Pleistocene grassland fires, lightning strikes, or hypervelocity impact by one or several asteroids or comets. That open question is a reminder that not every piece of natural glass announces how it was made.

• 4. From Beads To Skyscrapers

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  The earliest known glass objects, dating to the mid-third millennium BC, were beads. They may have begun as accidental by-products of metalworking, the slags left behind, or arisen during the production of faience, a pre-glass vitreous material made by a process much like glazing. Early glass was rarely transparent, often full of impurities, and technically faience rather than true glass, which did not appear until the 15th century BC. Red-orange beads from the Indus Valley Civilization, dated before 1700 BC, predate sustained production.

  The word itself comes from the late Roman Empire. At the Roman glassmaking center at Trier, in present-day Germany, the late-Latin term glesum arose, likely from a Germanic word for a transparent, lustrous substance. The Romans perfected cameo glass, etching and carving through fused layers of different colors to raise a design in relief. Their glass has been found as far as China, the Baltics, the Middle East, and India.

  From the 10th century onward, glass filled the stained windows of churches and cathedrals, with famous examples at Chartres Cathedral and the Basilica of Saint-Denis. By the 13th century, the island of Murano in Venice became a glassmaking center, where makers developed the clear colorless glass cristallo, named for its resemblance to natural crystal. Around 1675, George Ravenscroft invented lead crystal glass. In the 1950s, Pilkington Bros. in England developed the float glass process, producing distortion-free flat sheets by floating molten glass on molten tin, and modern towers now wear curtain walls made almost entirely of glass.

• 5. The Myth Of Flow

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  Old windows are sometimes found thicker at the bottom than the top, and that observation has fed a persistent claim that glass flows like a liquid over centuries. The idea is wrong. Once solidified, glass stops flowing. The sags and ripples in old panes were there the day they were made, the product of past manufacturing that produced imperfect surfaces and non-uniform thickness. The near-perfect float glass used today only became widespread in the 1960s.

  Measurement settles the matter. A 2017 study computed the flow rate of the medieval glass at Westminster Abbey, dating from the year 1268. Its room-temperature viscosity came out to roughly 10 to the 24th power Pa-s, about 10 to the 16th power times less viscous than a 1998 estimate based on soda-lime silicate glass. Even at that lower viscosity, the authors calculated a maximum flow rate of 1 nanometer per billion years.

  That figure makes the effect impossible to observe on any human timescale. The general principle holds across the material. Viscosity on the order of 10 to the 17th or 18th power Pa-s can be measured in glass, a value so high it confirms glass would not change shape appreciably even over enormous spans of time.

• 6. A Family Of Recipes

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  Silicon dioxide is the common fundamental constituent, and fused quartz made from chemically pure silica sits at one extreme. It has very low thermal expansion, survives immersion in water while red hot, resists temperatures of 1000 to 1500 degrees Celsius, and stays transparent across the visible, UV, and IR ranges. But its melting temperature of 1723 degrees Celsius makes it hard to work, so fluxes are added to bring that temperature down. The most common flux is sodium carbonate, or soda, which lowers the glass-transition temperature.

  Soda dissolves in water, so lime, magnesia, and alumina are added to restore chemical durability. Soda-lime glasses account for over 75% of manufactured glass and contain about 70 to 74% silica by weight, ideal for window glass and tableware though weak against heat. Borosilicate glasses such as Pyrex and Duran carry 5 to 13% boron trioxide, giving low thermal expansion and resistance to thermal shock for labware and cookware. Lead glass, made by adding lead oxide, gains a high refractive index, a clear ringing sound when struck, and electrical resistance about two orders of magnitude higher than soda-lime glass.

  Beyond the silicates lies a wider world. Glasses can be built from metals, phosphates, borates, chalcogenides, fluorides, and many other substances. Glass-ceramics combine non-crystalline glass with crystalline ceramic phases, prized for imperviousness to thermal shock that lets them sustain quick temperature changes up to 1000 degrees Celsius. Amorphous metals, including bulk metallic glasses sold by Liquidmetal Technologies, extend the idea into alloys, and amorphous steel has shown mechanical properties far exceeding conventional steel.

• 7. Color From Metal

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  Iron(II) oxide impurities tint ordinary soda-lime glass green in thick sections, even though it looks colorless in a thin slice. To erase that green, glassmakers add manganese dioxide, which on its own would give a purple color. Color in glass comes from electrically charged ions distributed evenly through the material, each absorbing the wavelengths of light that correspond to specific colors.

  Low concentrations of cobalt oxide, between 0.025 and 0.1%, produce the rich deep blue of cobalt glass. Chromium is a very powerful coloring agent that yields dark green, while iron(III) oxide produces yellow or yellow-brown. Sulphur combined with carbon and iron salts makes amber ranging from yellowish to nearly black, and a reducing combustion atmosphere can pull the same amber color from a melt.

  Cadmium sulfide produces imperial red, and combined with selenium it spans yellow, orange, and red. Copper splits two ways depending on its oxidation state. Copper(II) oxide produces turquoise, while copper(I) oxide gives a dull red-brown. The same metal, in two forms, lands on opposite sides of the spectrum.

• 8. Where Glass Goes

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  Structural glazing systems use stainless steel fittings countersunk into recesses at the corners of glass panels, letting strengthened panes appear unsupported and giving a flush exterior. These systems trace their roots to the iron and glass conservatories of the nineteenth century, and glass buildings now dominate the skylines of many modern cities. Optical glass typically has a refractive index of 1.4 to 2.4 and an Abbe number of 15 to 100, which is why lenses, mirrors, and prisms remain its oldest applications.

  Glass holds the things we eat and drink because it is inert, impermeable, and slightly more durable in container form, which carries more silica, calcium oxide, and aluminium oxide than flat glass. In the laboratory it becomes flasks, Petri dishes, test tubes, and pipettes, and although most standard glassware has been mass-produced since the 1920s, scientists still employ skilled glassblowers for bespoke apparatus. Glass also locks away danger. Alkali borosilicate glasses immobilise high-level radioactive waste, with the International Simple Glass serving as a standard for durability testing, while some countries, notably the United States and Russia, use phosphate glasses instead.

  In the 21st century, manufacturers built chemically strengthened glass for touchscreens, including Gorilla Glass from Corning, Dragontrail from AGC Inc., and Xensation from Schott AG. Art took the opposite path, reviving ancient methods. René Lalique, Émile Gallé, and Daum of Nancy led a French wave of Art Nouveau, while Louis Comfort Tiffany in America specialised in stained glass and his famous lamps. The same substance that seals a reactor's waste also glows in a Tiffany lampshade, shaped by hands that still blow, cast, and fuse it one piece at a time.