How the glass industry

This article is about the material. For other uses, see ).

Studio glass by Tyler Hopkins, demonstrating many of the essential properties of glass

Glass is an amorphous (non-crystalline) solid material that exhibits a glass transition, which is the reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle state into a molten or rubber-like state. Glasses are typically brittleand can be optically transparent. The most familiar type of glass is soda-lime glass, which is composed of about 75% silicon dioxide (SiO₂), sodium oxide (Na₂O) from soda ash, lime (CaO), and several minor additives. The term glass is often used to refer only to this specific use.

Silicate glass generally has the property of being transparent. Because of this, it has a great many applications. One of its primary uses is as a building material, traditionally as small panes set into window openings in walls, but in the 20th-century often as the major cladding material of many large buildings. Because glass can be formed or moulded into any shape, and also because it is a sterile product, it has been traditionally used for vessels:bowls, vases, bottles, jars and glasses. In its most solid forms it has also been used for paperweights, marbles, and beads. Glass is both reflective and refractive of light, and these qualities can be enhanced by cutting and polishing in order to make optical lenses, prisms and fine glassware. Glass can be coloured by the addition of metallic salts, and can also be painted. These qualities have led to the extensive use of glass in the manufacturing of art objects and in particular, stained glass windows. Although brittle, glass is extremely durable, and many examples of glass fragments exist from early glass-making cultures.

In science, however, the term glass is defined in a broader sense, encompassing every solid that possesses a non-crystalline (i.e. amorphous) structure and exhibits a glass transition when heated towards the liquid state. These sorts of glasses can be made of quite different kinds of materials: metallicalloys, ionic melts, aqueous solutions, molecular liquids, and polymers. For many applications (bottles, eyewear) polymer glasses (acrylic glass,polycarbonate, polyethylene terephthalate) are a lighter alternative to traditional silica glasses.

Silicate glass

Silica (the chemical compound SiO₂) is a common fundamental constituent of glass. In nature, vitrification of quartz occurs when lightning strikes sand, forming hollow, branching rootlike structures called fulgurite.

While fused quartz (primarily composed of SiO₂) is used for some special applications, it is not very common due to its high glass transition temperature of over 1200 °C (2192 °F).^[1] Normally, other substances are added to simplify processing. One is sodium carbonate (Na₂CO₃, "soda"), which lowers the glass transition temperature. However, the soda makes the glass water soluble, which is usually undesirable, so lime (calcium oxide [CaO], generally obtained from limestone), some magnesium oxide (MgO) and aluminium oxide (Al₂O₃) are added to provide for a better chemical durability. The resulting glass contains about 70 to 74% silica by weight and is called a soda-lime glass.^[2] Soda-lime glasses account for about 90% of manufactured glass.

Most common glass contains other ingredients added to change its properties. Lead glass or flint glass is more 'brilliant' because the increased refractive index causes noticeably more specular reflection and increased optical dispersion. Adding 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.^{[citation needed]} Iron can be incorporated into glass to absorb infrared energy, for example in heat absorbing filters for movie projectors, while cerium(IV) oxide can be used for glass that absorbs UV wavelengths.^[3]

The following is a list of the more common types of silicate glasses, and their ingredients, properties, and applications:

1. Fused silica glass, vitreous silica glass: silica (SiO₂). Has very low thermal expansion, is very hard and resists high temperatures (1000–1500 °C). It is also the most resistant against weathering (alkali ions leaching out of the glass, while staining it). It is used for high temperature applications such as furnace tubes, melting crucibles, etc.
2. Soda-lime-silica glass, window glass: silica 72% + sodium oxide (Na₂O) 14.2% + magnesia (MgO) 2.5% + lime (CaO) 10.0% + alumina (Al₂O₃) 0.6%. Is transparent, easily formed and most suitable for window glass. It has a high thermal expansion and poor resistance to heat (500–600 °C). Used for windows, containers, light bulbs, tableware.
3. Sodium borosilicate glass, Pyrex: silica 81% + boric oxide (B₂O₃) 12% + soda (Na₂O) 4.5% + alumina (Al₂O₃) 2.0%. Stands heat expansion much better than window glass. Used for chemical glassware, cooking glass, car head lamps, etc. Borosilicate glasses (e.g. Pyrex) have as main constituents silica and boron oxide. They have fairly low coefficients of thermal expansion (7740 Pyrex CTE is 3.25×10^–6/°C^[4] as compared to about 9×10⁻⁶/°C for a typical soda-lime glass^[5]), making them more dimensionally stable. The lower CTE also makes them less subject to stress caused by thermal expansion, thus less vulnerable to cracking from thermal shock. They are commonly used for reagent bottles, optical components and household cookware.
4. Lead-oxide glass, crystal glass: silica 59% + soda (Na₂O) 2.0% + lead oxide (PbO) 25% + potassium oxide (K₂O) 12% + alumina 0.4% + zinc oxide (ZnO) 1.5%. Has a high refractive index, making the look of glassware more brilliant (crystal glass). It also has a high elasticity, making glassware 'ring'. It is also more workable in the factory, but cannot stand heating very well.
5. Aluminosilicate glass: silica 57% + alumina 16% + boric oxide (B₂O₃) 4.0% + barium oxide (BaO) 6.0% + magnesia 7.0% + lime 10%. Extensively used for fiberglass, used for making glass-reinforced plastics (boats, fishing rods, etc.). Also for halogen bulb glass.
6. Oxide glass: alumina 90% + germanium oxide (GeO₂) 10%. Extremely clear glass, used for fiber-optic wave guides in communication networks. Light loses only 5% of its intensity through 1 km of glass fiber.^[6]

Another common glass ingredient is "cullet" (recycled glass). The recycled glass saves on raw materials and energy; however, impurities in the cullet can lead to product and equipment failure. Fining agents such as sodium sulfate, sodium chloride, or antimony oxide may be added to reduce the number of air bubbles in the glass mixture.^[2] Glass batch calculation is the method by which the correct raw material mixture is determined to achieve the desired glass composition.

Physical properties

Optical properties

Glass is in widespread use largely due to the production of glass compositions that are transparent to visible wavelengths of light. In contrast, polycrystalline materials do not in general transmit visible light.^[7] The individual crystallites may be transparent, but their facets (grain boundaries) reflect or scatter light resulting in diffuse reflection. Glass does not contain the internal subdivisions associated with grain boundaries in polycrystals and hence does not scatter light in the same manner as a polycrystalline material. The surface of a glass is often smooth since during glass formation the molecules of the supercooled liquid are not forced to dispose in rigid crystal geometries and can follow surface tension, which imposes a microscopically smooth surface. These properties, which give glass its clearness, can be retained even if glass is partially light-absorbing—i.e., colored.^[8]

Glass has the ability to refract, reflect, and transmit light following geometrical optics, without scattering it. It is used in the manufacture of lenses and windows. Common glass has a refraction index around 1.5. According to Fresnel equations, the reflectivity of a sheet of glass is about 4% per surface (at normal incidence in air), and the transmissivity of one element (two surfaces) is about 90%. Glass also finds application in optoelectronics—e.g., for light-transmitting optical fibers.

Other properties

In the process of manufacture, silicate glass 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 laminated or specially treated, but is extremely durable under most conditions. It erodes very slowly and can withstand the action of water. It is resilient to chemical attack and is an ideal material for the manufacture of containers for foodstuffs and most chemicals.

Contemporary production

Main articles: Glass production, Float glass, and Glazier

Following the glass batch preparation and mixing, the raw materials are transported to the furnace. Soda-lime glass for mass production is melted in gas fired units. Smaller scale furnaces for specialty glasses include electric melters, pot furnaces, and day tanks.^[2] After melting, homogenization and refining (removal of bubbles), the glass is formed. 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.^[9] Container glass for common bottles and jars is formed by blowing and pressing methods. Further glass forming techniques are summarized in the table Glass forming techniques.

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

Color

Main article: Glass coloring and color marking

Color in glass may be obtained by addition of electrically charged ions (or color centers) that are homogeneously distributed, and by precipitation of finely dispersed particles (such as inphotochromic glasses).^[10] Ordinary soda-lime glass appears colorless to the naked eye when it is thin, although iron(II) oxide (FeO) impurities of up to 0.1 wt%^[11] produce a green tint, which can be viewed in thick pieces or with the aid of scientific instruments. Further FeO and Cr₂O₃ additions may be used for the production of green bottles. Sulfur, together with carbon and iron salts, is used to form iron polysulfides and produce amber glass ranging from yellowish to almost black.^[12] A glass melt can also acquire an amber color from a reducing combustion atmosphere.Manganese dioxide can be added in small amounts to remove the green tint given by iron(II) oxide. When used in art glass or studio glass glass is colored using closely guarded recipes that involve specific combinations of metal oxides, melting temperatures and 'cook' times. Most colored glass used in the art market is manufactured in volume by vendors who serve this market although there are some glass makers with the ability to make their own color from raw materials.

History of silicate glass

Main article: History of glass

See also: Architectural glass and Stained glass

The term glass developed in the late Roman Empire. It was in the Roman glassmaking center at Trier, now in modern Germany, that the late-Latin term glesum originated, probably from aGermanic word for a transparent, lustrous substance.^[13]

Roman Cage Cup from the 4th century CE

Naturally occurring glass, especially the volcanic glass obsidian, has been used by many Stone Age societies across the globe for the production of sharp cutting tools and, due to its limited source areas, was extensively traded. But in general, archaeological evidence suggests that the first true glass was made in coastal north Syria, Mesopotamia or Ancient Egypt.^[14] The earliest known glass objects, of the mid third millennium BCE, were beads, perhaps initially created as accidental by-products of metal-working (slags) or during the production of faience, a pre-glass vitreous material made by a process similar to glazing.^[15]

Glass remained a luxury material, and the disasters that overtook Late Bronze Age civilizations seem to have brought glass-making to a halt. Indigenous development of glass technology in South Asia may have begun in 1730 BCE.^[16] In ancient China, though, glassmaking seems to have a late start, compared to ceramics and metal work. In the Roman Empire, glass objects have been recovered across the Roman Empire in domestic, industrial and funerary contexts.

Bohemian flashed and engraved ruby glass (19th-century)

Glass was used extensively during the Middle Ages. Anglo-Saxon glass has been found across England during archaeological excavations of both settlement and cemetery sites. Glass in the Anglo-Saxon period was used in the manufacture of a range of objects including vessels, beads, windows and was also used in jewellery. 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)^[17] and the East end of Gloucester Cathedral.^[18] Stained glass had a major revival with Gothic Revival architecture in the 19th-century. With the Renaissance, and a change in architectural style, the use of large stained glass windows became less prevalent. The use of domestic stained glass increased until it was general for every substantial house to have glass windows. These were initially of small panes leaded together, but with the changes in technology, glass could be manufactured relatively cheaply in increasingly larger sheets, leading to larger window panes, and, in the 20th-century, to much larger windows in ordinary domestic and commercial premises.

Studio glass by David Patchen. Multiple colors within a single object increases the difficulty of production, as each color has different chemical and physical properties when molten.

In the 20th-century, new types of glass such as laminated glass, reinforced glass and glass bricks have increased the use of glass as a building material and resulted in new applications of glass. Multi-storey buildings are frequently constructed withcurtain walls made almost entirely of glass. Similarly, laminated glass has been widely applied to vehicles for windscreens. While glass containers have always been used for storage and are valued for their hygienic properties, glass has been utilised increasingly in industry. Optical glass for spectacles has been in use since the late Middle Ages. The production of lenses has become increasingly proficient, aiding astronomers as well as having other application in medicine and science. Glass is also employed as the aperture cover in many solar energy systems.

From the 19th century, there was a revival in many ancient glass-making techniques including Cameo glass, achieved for the first time since the Roman Empire and initially mostly used 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 producing colored vases and similar pieces, often in cameo glass, and also using lustre techniques. Louis Comfort Tiffany in America specialized in stained glass, both secular and religious, and his famous lamps. The early 20th-century saw the large-scale factory production of glass art by firms such as Waterfords and Lalique. From about 1960 onwards there have been an increasing number of small studios hand-producing glass artworks, and glass artists began to class themselves as in effect sculptors working in glass, and their works as part fine arts.

In the 21st century, scientists, observing the properties of ancient stained glass windows, in which suspended nanoparticles prevent UV light from causing chemical reactions that change image colors, are developing photographic techniques that use similar stained glass to capture true color images of Mars for the 2019 ESA Mars Rover mission

Chronology of advances in architectural glass

• 1226 – "Broad Sheet" first produced in Sussex
• 1330 – "Crown Glass" first produced in Rouen, France. "Broad Sheet" also produced. Both were also supplied for export
• 1620 – "Blown Plate" first produced in London. Used for mirrors and coach plates.
• 1678 – "Crown Glass" first produced in London. This process dominated until the 19th century
• 1843 – An early form of "Float Glass" invented by Henry Bessemer, pouring glass onto liquid tin. Expensive and not a commercial success.
• 1888 – "Machine Rolled" glass introduced allowing patterns to be introduced
• 1898 – "Wired Cast" glass invented by Pilkington for use where safety or security was an issue.
• 1959 – "Float Glass" launched in UK. Invented by Sir Alastair Pilkington

Other types of glass

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).^[21]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 SeO₂. Also, more readily reacting raw materials may be preferred over relatively inert 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 homogeneity is achieved by homogenizing the raw materials mixture (glass batch), by stirring the melt, and by crushing and re-melting the first melt. The obtained glass is usuallyannealed to prevent breakage during processing.^[21][22]

In order to make glass from materials with poor glass forming tendencies, novel techniques are used to increase cooling rate, or reduce crystal nucleation triggers. Examples of these techniques include aerodynamic levitation (cooling the melt whilst it floats on a gas stream), splat quenching (pressing the melt between two metal anvils) and roller quenching (pouring the melt through rollers).

Network glasses

Some glasses that do not include silica as a major constituent may have physico-chemical properties useful for their application in fiber optics and other specialized technical applications. These include fluoride glasses, aluminosilicates, phosphate glasses, borate glasses, and chalcogenide glasses.

There are three classes of components for oxide glasses: network formers, intermediates, and modifiers. The network formers (silicon, boron, germanium) form a highly cross-linked network of chemical bonds. The intermediates (titanium, aluminium, zirconium, beryllium, magnesium, zinc) can act as both network formers and modifiers, according to the glass composition. The modifiers (calcium, lead, lithium, sodium, potassium) alter the network structure; they are usually present as ions, compensated by nearby non-bridging oxygen atoms, bound by one covalent bond to the glass network and holding one negative charge to compensate for the positive ion nearby. Some elements can play multiple roles; e.g. lead can act both as a network former (Pb⁴⁺ replacing Si⁴⁺), or as a modifier.

The presence of non-bridging oxygens lowers the relative number of strong bonds in the material and disrupts the network, decreasing the viscosity of the melt and lowering the melting temperature.

The alkali metal ions are small and mobile; their presence in glass allows a degree of electrical conductivity, especially in molten state or at high temperature. Their mobility, however, decreases the chemical resistance of the glass, allowing leaching by water and facilitating corrosion. Alkaline earth ions, with their two positive charges and requirement for two non-bridging oxygen ions to compensate for their charge, are much less mobile themselves and also hinder diffusion of other ions, especially the alkalis. The most common commercial glasses contain both alkali and alkaline earth ions (usually sodium and calcium), for easier processing and satisfying corrosion resistance.^[24] Corrosion resistance of glass can be achieved by dealkalization, removal of the alkali ions from the glass surface by reaction with e.g. sulfur or fluorine compounds. Presence of alkaline metal ions has also detrimental effect to the loss tangent of the glass, and to its electrical resistance; glasses for electronics (sealing, vacuum tubes, lamps...) have to take this in account.

Addition of lead(II) oxide lowers melting point, lowers viscosity of the melt, and increases refractive index. Lead oxide also facilitates solubility of other metal oxides and therefore is used in colored glasses. The viscosity decrease of lead glass melt is very significant (roughly 100 times in comparison with soda glasses); 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 in the matrix and hinders the movement of other ions; lead glasses therefore have high electrical resistance, about two orders of magnitude higher than soda-lime glass (10^8.5 vs 10^6.5 Ohm·cm, DC at 250 °C). For more details, see lead glass.^[25]

Addition of fluorine lowers the dielectric constant of glass. Fluorine is highly electronegative and attracts the electrons in the lattice, lowering the polarizability of the material. Such silicon dioxide-fluoride is used in manufacture of integrated circuits as an insulator. High levels of fluorine doping lead to formation of volatile SiF₂O and such glass is then thermally unstable. Stable layers were achieved with dielectric constant down to about 3.5–3.7

Amorphous metals

In the past, small batches of amorphous metals with high surface area configurations (ribbons, wires, films, etc.) have been produced through the implementation of extremely rapid rates of cooling. This was initially termed "splat cooling" by doctoral student W. Klement at Caltech, who showed that cooling rates on the order of millions of degrees per second is sufficient to impede the formation of crystals, and the metallic atoms become "locked into" a glassy state. 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). Liquidmetal Technologiessell a number of zirconium-based BMGs. Batches of amorphous steel have also been produced that demonstrate mechanical properties far exceeding those found in conventional steel alloys.^[27][28][29]

In 2004, NIST researchers presented evidence that an isotropic non-crystalline metallic phase (dubbed "q-glass") could be grown from the melt. This phase is the first phase, or "primary phase," to form in the Al-Fe-Si system during rapid cooling. Interestingly, experimental evidence indicates that this phase forms by a first-order transition. Transmission electron microscopy (TEM) images show that the q-glass nucleates from the melt as discrete particles, which grow spherically with a uniform growth rate in all directions. The diffraction pattern shows it to be an isotropic glassy phase. Yet there is a nucleation barrier, which implies an interfacial discontinuity (or internal surface) between the glass and the melt

Electrolytes

Electrolytes or molten salts are mixtures of different ions. 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. The best studied example is Ca_0.4K_0.6(NO₃)_1.4.

Aqueous solutions

Some aqueous solutions can be supercooled into a glassy state, for instance LiCl:RH₂O in the composition range 4<R<8.

Molecular liquids

A molecular liquid is composed of molecules that do not form a covalent network but interact only through weak van der Waals forces or through transient hydrogen bonds. Many molecular liquids can be supercooled into a glass; some are excellent glass formers that normally do not crystallize.

A widely known example is sugar glass.

Under extremes of pressure and temperature solids may exhibit large structural and physical changes that can lead to polyamorphic phase transitions.^[32] In 2006 Italian scientists created an amorphous phase of carbon dioxide using extreme pressure. The substance was named amorphous carbonia(a-CO₂) and exhibits an atomic structure resembling that of silica

Colloidal glasses

Concentrated colloidal suspensions may exhibit a distinct glass transition as function of particle concentration or density

Glass-ceramics

Glass-ceramic materials share many properties with both non-crystalline glass and crystalline ceramics. They are formed as a glass, and then partially crystallized by heat treatment. For example, the microstructure of whiteware ceramics frequently contains both amorphous and crystalline phases. Crystalline grains are often embedded within a non-crystalline intergranular phase of grain boundaries. When applied to whiteware ceramics, vitreousmeans the material has an extremely low permeability to liquids, often but not always water, when determined by a specified test regime.^[37][38]

The term mainly refers to a mix of lithium and aluminosilicates that yields an array of materials with interesting thermomechanical properties. The most commercially important of these have the distinction of being impervious to thermal shock. Thus, glass-ceramics have become extremely useful for countertop cooking. 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

Structure

Main article: Structure of liquids and glasses

As in other amorphous solids, the atomic structure of a glass lacks any long-range translational periodicity. However, due to chemical bonding characteristics glasses do possess a high degree of short-range order with respect to local atomic polyhedra.^[39]

The amorphous structure of glassy silica (SiO₂) in two dimensions. No long-range order is present, although there is local ordering with respect to the tetrahedralarrangement of oxygen (O) atoms around the silicon (Si) atoms.

Formation from a supercooled liquid[edit]

Main article: Glass transition

In physics, the standard definition of a glass (or vitreous solid) is a solid formed by rapid melt quenching.^{[40][41][42][43][44]} However, the term glass is often used to describe any amorphous solid that exhibits a glass transition temperature T_g. 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 T_g. The tendency for a material to form a glass while quenched is called glass forming ability. This ability can be predicted by therigidity theory.^[45] Generally, the structure of a glass exists in a 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.^[46]

Some people consider glass to be a liquid due to its lack of a first-order phase transition^[47][48] where certain thermodynamic variables such as volume,entropy and enthalpy are discontinuous through the glass transition range. However, 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.^[49] Despite this, the equilibrium theory of phase transformations does not entirely hold for glass, and hence the glass transition cannot be classed as one of the classical equilibrium phase transformations in solids.^[43][44]

Glass is an amorphous solid. It exhibits an atomic structure close to that observed in the supercooled liquid phase but displays all the mechanical properties of a solid.^[47][50] The notion that glass flows to an appreciable extent over extended periods of time is not supported by empirical research or theoretical analysis (see viscosity of amorphous materials). Laboratory measurements of room temperature glass flow do show a motion consistent with a material viscosity on the order of 10¹⁷–10¹⁸ Pa s.^[51]

Although the atomic structure of glass shares characteristics of the structure in a supercooled liquid, glass tends to behave as a solid below its glass transition temperature.^[52] A supercooled liquid behaves as a liquid, but it is below the freezing point of the material, and in some cases will crystallize almost instantly if a crystal is added as a core. The change in heat capacity at a glass transition and a melting transition of comparable materials are typically of the same order of magnitude, indicating that the change in active degrees of freedom is comparable as well. Both in a glass and in a crystal it is mostly only the vibrational degrees of freedom that remain active, whereas rotational and translational motion is arrested. This helps to explain why both crystalline and non-crystalline solids exhibit rigidity on most experimental time scales.

Behavior of antique glass

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 was once uniform but has flowed to its new shape, which is a property of liquid.^[54] However, this assumption is incorrect; once solidified, glass stops flowing. The reason for the observation is that in the past, when panes of glass were commonly made by glassblowers, the technique used was to spin molten glass so as to create a round, mostly flat and even plate (the crown glass process, described above). This plate was then cut to fit a window. The pieces were not, however, absolutely flat; the edges of the disk became a different thickness as the glass spun. When installed in a window frame, the glass would be placed with the thicker side down both for the sake of stability and to prevent water accumulating in the lead cames at the bottom of the window.^[55] Occasionally such glass has been found thinner side down or thicker on either side of the window's edge.^[56]

Mass production of glass window panes in the early twentieth century caused a similar effect. In glass factories, molten glass was poured onto a large cooling table and allowed to spread. The resulting glass is thicker at the location of the pour, located at the center of the large sheet. These sheets were cut into smaller window panes with nonuniform thickness, typically with the location of the pour centered in one of the panes (known as "bull's-eyes") for decorative effect. Modern glass intended for windows is produced as float glass and is very uniform in thickness.

Several other points can be considered that contradict the "cathedral glass flow" theory:

• Writing in the American Journal of Physics, materials engineer Edgar D. Zanotto states "... the predicted relaxation time for GeO₂ at room temperature is 10³² years. Hence, the relaxation period (characteristic flow time) of cathedral glasses would be even longer."^[57] (10³² years is many times longer than the estimated age of the Universe.)
• If medieval glass has flowed perceptibly, then ancient Roman and Egyptian objects should have flowed proportionately more—but this is not observed. Similarly, prehistoric obsidian blades should have lost their edge; this is not observed either (although obsidian may have a different viscosity from window glass).^[47]
• If glass flows at a rate that allows changes to be seen with the naked eye after centuries, then the effect should be noticeable in antique telescopes. Any slight deformation in the antique telescopic lenses would lead to a dramatic decrease in optical performance, a phenomenon that is not observed.^[47]
• There are many examples of centuries-old glass shelving that has not bent, even though it is under much higher stress from gravitational loads than vertical window glass.^{[citation needed]}

The above does not apply to materials that have a glass transition temperature close to room temperature, such as certain plastics used in daily life like polystyrene and polypropylene.

History of perfume

The word perfume is used today to describe scented mixtures and is derived from the Latin word, "per fumus", meaning through smoke. Perfumery, or the art of making perfumes, began in ancient Egypt but was developed and further refined by the Romans, the Persians and the Arabs. Although perfume and perfumery also existed in East Asia, much of its fragrances are incense based. The basic ingredients and methods of making perfumes are described by Pliny the Elder in his Naturalis Historia.

Mesopotamia

The world's first recorded chemist is a person named Tapputi, a perfume maker who was mentioned in a Cuneiform tablet from the 2nd millennium BC in Mesopotamia

India

Perfume and perfumery also existed in India, much of its fragrances were incense based. The earliest distillation of Attar was mentioned in the Hindu Ayurvedic text Charaka Samhita. The Harshacharita, written in 7th century A.D. in Northern India mentions use of fragrant agarwood oil.

Cyprus

To date, the oldest perfumery was discovered on the island of Cyprus.^[2] Excavations in 2004-5 under the initiative of an Italian archaeological team unearthed evidence of an enormous factory that existed 4,000 years ago during the Bronze Age.^[3] This covered an estimated surface area of over 4,000m² indicating that perfume manufacturing was on an industrial scale.^[4] The news of this discovery was reported extensively through the world press and many artifacts are already on display in Rome.^[5][6] The Bible describes a sacred perfume (Exodus 30:22-33) consisting of liquid myrrh, fragrant cinnamon, fragrant cane, and cassia. Its use was forbidden, except by the priests. The women wore perfume to present their beauty.

Islamic

See also: Alchemy and chemistry in Islam

Islamic cultures contributed significantly in the development of Western perfumery in both perfecting the extraction of fragrances through steam distillation and introducing new, raw ingredients. Both of the raw ingredients and distillation technology significantly influenced Western perfumery and scientific developments, particularly chemistry.

As traders, Islamic cultures such as the Arabs and Persians had wider access to different spices, herbals, and other fragrance material. In addition to trading them, many of these exotic materials were cultivated by the Muslims such that they can be successfully grown outside of their native climates. Two examples of this are jasmine, which is native to South and Southeast Asia, and various citrus, which is thought to have originated in Southeast Asia. Both of these ingredients remain important in modern perfumery.

In Islamic culture, perfume usage has been documented as far back as the 6th century and its usage is considered a religious duty. Muhammad said:

“

The taking of a bath on Friday is compulsory for every male Muslim who has attained the age of puberty and (also) the cleaning of his teeth with Miswaak (type of twig used as a toothbrush), and the using of perfume if it is available. (Recorded in Sahih Bukhari).

”

Such rituals gave incentives to scholars to search and develop a cheaper way to produce incenses and in mass production. Thanks to the hard work of two talented Arabian chemists: Jābir ibn Hayyān (Geber, born 722, Iraq), and Al-Kindi (Alkindus, born 801, Iraq) who established the perfume industry. Jabir developed many techniques, including distillation, evaporation and filtration, which enabled the collection of the odour of plants into a vapour that could be collected in the form of water or oil.^[7]

Al-Kindi, however, was the real founder of perfume industry as he carried out extensive research and experiments in combining various plants and other sources to produce a variety of scent products. He elaborated a vast number of ‘recipes’ for a wide range of perfumes, cosmetics and pharmaceuticals. His work in the laboratory is reported by a witness who said:

“

I received the following description, or recipe, from Abu Yusuf Ya'qub b. Ishaq al-Kindi, and I saw him making it and giving it an addition in my presence.

”

The writer goes on in the same section to speak of the preparation of a perfume called ghaliya, which contained musk, amber and other ingredients; too long to quote here, but which reveals a long list of technical names of drugs and apparatus. Al-Kindi also wrote in the 9th century a book on perfumes which he named ‘Book of the Chemistry of Perfume and Distillations’. It contained more than a hundred recipes for fragrant oils, salves, aromatic waters and substitutes or imitations of costly drugs. The book also described one hundred and seven methods and recipes for perfume-making, and even the perfume making equipment, like the alembic, still bears its Arabic name.^[8]

The Persian Muslim doctor and chemist Avicenna (also known as Ibn Sina) introduced the process of extracting oils from flowers by means of distillation, the procedure most commonly used today. He first experimented with the rose. Until his discovery, liquid perfumes were mixtures of oil and crushed herbs, or petals which made a strong blend. Rose water was more delicate, and immediately became popular. Both of the raw ingredients and distillation technology significantly influenced western perfumery and scientific developments, particularly chemistry.

Eggs and floral perfumes were brought to Europe in the 11th and 12th centuries from Arabia, through trade with the Islamic world and with the returning Crusaders. Those who traded for these were most often also involved in trade for spices and dyestuffs. There are records of the Pepperers Guild of London, going back to 1179; which show them trading with Muslims in spices, perfume ingredients and dyes

Western

Knowledge of something perfumery came to Europe as early as the 14th century due partially to Arabic influences and knowledge. But it was the Hungarians who ultimately introduced the first modern perfume. The first modern perfume, made of scented oils blended in an alcohol solution, was made in 1370 at the command of Queen Elizabeth of Hungary and was known throughoutEurope as Hungary Water. The art of perfumery prospered in Renaissance Italy, and in the 16th century, Italian refinements were taken to France by Catherine de' Medici's personal perfumer,Rene le Florentin. His laboratory was connected with her apartments by a secret passageway, so that no formulas could be stolen en route.

France quickly became the European center of perfume and cosmetic manufacture. Cultivation of flowers for their perfume essence, which had begun in the 14th century, grew into a major industry in the south of France. During the Renaissance period, perfumes were used primarily by royalty and the wealthy to mask body odors resulting from the sanitary practices of the day. Partly due to this patronage, the western perfumery industry was created. Perfume enjoyed huge success during the 17th century. Perfumed gloves became popular in France and in 1656, the guild of glove and perfume-makers was established. Perfumers were also known to create poisons; for instance, a French duchess was murdered when a perfume/poison was rubbed into her gloves and was slowly absorbed into her skin.

Perfume came into its own when Louis XV came to the throne in the 18th century. His court was called "la cour parfumée" (the perfumed court). Madame de Pompadour ordered generous supplies of perfume, and King Louis demanded a different fragrance for his apartment everyday. The court of Louis XIV was even named due to the scents which were applied daily not only to the skin but also to clothing, fans and furniture. Perfume substituted for soap and water. The use of perfume in France grew steadily. By the 18th century, aromatic plants were being grown in the Grasseregion of France to provide the growing perfume industry with raw materials. Even today, France remains the centre of the European perfume design and trade.

After Napoleon came to power, exorbitant expenditures for perfume continued. Two quarts of violet cologne were delivered to him each week, and he is said to have used sixty bottles of double extract of jasmine every month. Josephine had stronger perfume preferences. She was partial to musk, and she used so much that sixty years after her death the scent still lingered in her boudoir.

England

Perfume reached its peak in England during the reigns of Henry VIII and Queen Elizabeth I. All public places were scented during Queen Elizabeth's rule, since she could not tolerate bad smells. It was said that the sharpness of her nose was equaled only by the slyness of her tongue. Ladies of the day took great pride in creating delightful fragrances and they displayed their skill in mixing scents.

As with industry and the arts, perfume was to undergo profound change in the 19th century. Changing tastes and the development of modern chemistry laid the foundations of perfumery as we know it today. Alchemy gave way to chemistry and new fragrances were created. The industrial revolution had in no way diminished the taste for perfume, there was even a fragrance called "Parfum à la Guillotine". Under the post-revolutionary government, people once again dared to express a penchant for luxury goods, including perfume. A profusion of vanity boxes containing perfumes appeared in the 19th century.

Americas

In early America, the first scents were colognes and scented water by French explorers in New France. Florida water, an uncomplicated mixture of eau de cologne with a dash of oil of cloves, cassia, and lemongrass, was popular