Glass

Although a glass is a substance that is non-crystalline, it is almost completely undeformable and therefore brittle. A glass exists in a state of matter termed a vitreous state. Vitreous substances, when heated, will transform slowly through stages of decreasing viscosity. As a sample of glass is heated, it becomes increasingly deformable, eventually reaching a point where it resembles a very viscous liquid. Ice , on the other hand, does not go through these changes as it is heated.

Excepting sublimation (direct solid to gas transformations) most substances change directly from a solid to a liquid. Ice, therefore, is not a vitreous substance. Glasses are only very slightly deformable. Glasses tend to bend and elongate under their own weight, especially when formed into rods, plates, or sheets. Glasses can be either organic or inorganic materials.

Because solidification is the act of crystallization, the depiction of glass as a non-crystalline solid may not be entirely correct. However, true crystallization occurs when the molecules of a substance arrange themselves in a systematic, periodic fashion. The atoms or molecules of glass do not exhibit this periodicity; this is consistent with the depiction of glass as an extremely viscous, or "supercooled"' liquid.

Glass is often referred to as an amorphous solid. An amorphous solid has a definite shape without the geometric regularity of crystalline solids. Glass can be molded into any shape. If glass is shattered, the resulting pieces are irregularly shaped. A crystalline solid would exhibit regular geometrical shapes when shattered. Amorphous solids tend to hold their shape, but they also tend to flow very slowly. If left undisturbed for a long period of time, a glass will very slowly crystallize. Once it crystallizes, it is no longer considered to be glass. At this point, it has devitrified. This crystallization process is extremely slow and in many cases may never occur.

The chemical make-up of standard window glass, which will be described in greater detail below, is quite similar to the mineral quartz . An x-ray crystallographic picture of quartz would show atoms arranged in an orderly, periodic sequence. X-ray crystallography studies of glass show no such arrangement. The atoms in glass are disordered and show no periodic structure. This irregular arrangement of atoms not only defines a substance as glass, but also determines several of its properties.

The bonds between the molecules or ions in a glass are of varying length, which is why they show no symmetry or periodic structure. Because the bonds are not symmetrical, glass is isotropic and has no definite melting point. The melting of glass instead takes place over a wide temperature range. Changing the state of a substance with asymmetric bonds requires more energy than a crystalline structure would. The tendency of glass to devitrify is a result of the atoms moving from a higher to a lower energy state.

The most common glasses are silicon based. Most glasses are 75% silicate. These glasses are based on the SiO₂molecule. This molecule creates an asymmetric, aperiodic structure. Some of the oxygen atoms are not bridged together, creating ions that need to be neutralized by metal cations. These metal cations are randomly scattered throughout the glass structure, adding to the asymmetry. The oxides of elements other than silicon can also form glasses. These other oxides include Al₂O₃, B₂O₃, P₂O₅, and As₂O₅.

The production of glasses is a complicated process. In general, certain molten materials are cooled in a specific manner so that no crystallization occurs, i.e., they remain amorphous. There are four basic materials that are used in glass production. These materials are the glass-forming substances, fluxes, stabilizers, and secondary components.

A glass-forming substance is any mineral that remains vitreous when cooled. Glass-forming substances are usually silica, boric oxide, phosphorous pentoxide, or feldspars. Sometimes aluminum oxide (Al₂O₃) is used. Silica, as the most commonly used material to make glass, is usually obtained from sand , which is 99.1-99.7% SiO₂. Occasionally, natural silica deposits are discovered that are pure enough to use in glass manufacturing, but these deposits are rare and the silica found in them is usually expensive to obtain. Even the lowest quality sands can be purified rather economically. Impurities in the natural silica are important because they can dramatically alter the quality of the glass produced. The most common impurities found in natural silica are iron sesquioxide (Fe₂O₃), alumina (Al₂O₃), and calcium compounds. Ferric oxide is sometimes found as an impurity. Even if the amount of ferric oxide in a natural silica sample is only 0.1% of the sample, the glass produced would have a deep yellow-green color and the impurity would have detrimental effects on the thermal and mechanical properties of the glass.

Occasionally impurities are added to the glass-forming substances to give the glass certain qualities such as transparency, fusibility, or stability. Stabilizers also are used to give the finished product particular characteristics. For example, calcium carbonate can be added as a stabilizer that will make the glass produced insoluble in water . Lead oxide added as a stabilizer gives the glass extreme transparency, brightness, and a high refractive index. Lead oxide also makes glass easier to cut. Zinc oxide can be added to glass to make it more resistant to changes in temperature as well as to increase its refractive index (a measure of the ability to bend light). Aluminum oxide can also be added as a stabilizer to increase the physical strength of the glass. Secondary components are added to determine some of the final properties of the glass and to correct any defects in the glass. The secondary components can be classified as decolorants, opacifiers, colorants, or refiners.

The production of glass includes many steps that can be generalized as follows. First, the fluxes, glass-forming substances, and stabilizers are crushed and milled, then blended and mixed together. They are then re-milled and granulated. At this point, the secondary components are added, if needed. The granules are then fused, refined, homogenized, and corrected, using more secondary components if necessary. Finally, the glass is formed and finished.

The final product is one of many hundreds of different types of glass. One popular type of glass, especially in laboratory settings or for use in the kitchen, is borosilicate glass. Some well-known borosilicate glasses are Jena, Pyrex, Durax, and Thermoglass. These glasses contain 12% or more B₂O₃. The addition of the boron oxide increases the softening temperature of the glass, making it more resistant to high temperatures such as those experienced while cooking or while performing laboratory experiments. Borosilicate glasses are also used in the production of thermometers, television tubes, and other objects that need to have constant dimensions or a high softening point.

See also Chemical bonds and physical properties

Glass

Glass is a product of inorganic materials that solidified, but did not crystallize. Glass is mainly composed of silicon dioxide (SiO₂), and is extremely prevalent in everyday life. Often, windows are the most fragile elements of a building or a vehicle, and are thus broken by thieves or criminals in order to penetrate the premises or the vehicle. When glass breaks at the scene of a crime, small particles of glass are projected not only forward, but also backward, onto the perpetrator and into the immediate environment. These particles can later be retrieved and used to establish a link between a suspect and a crime scene.

Glass can be classified either by chemical composition or by use. There are four main chemical compositions of glass: soda-lime, lead, borosilicate, and special glass. While glass is mainly composed of silicon dioxide (SiO₂), it also contains modifiers that are used to vary the quality and properties of the glass. Soda-lime glass is obtained by adding a certain amount of soda (Na₂CO₃) and lime (CaO). It is this glass that constitutes most windows and bottles. Borosilicate glass is made by the addition of boron oxide and is much more resistant to heat. Different colors of glass are achieved by introducing small amounts of additives. For example, chromium (Cr) is used to give a green tint, cobalt (Co) for a blue tint.

Almost all types of glass are commercially available. Window glass is probably the most common type of glass, and is usually found as a flat, transparent piece composed of soda-lime glass. This type of glass does not resist high temperatures, quick temperature changes, or corrosive substances. Most of flat glass is now prepared using the floating process. This consists of laying the molten glass onto a bath of molten tin in an inert atmosphere in order to achieve a perfectly flat surface. Tempered glass is another type of glass that is much stronger than regular glass. This particular strength is achieved by introducing extra forces on both sides of the glass through rapid cooling and heating during the manufacturing process. This glass will shatter in very small pieces when it breaks. It is used on side and rear windows of cars. Laminated glass is a glass composed of multiple sheets of glass bonded together with a plastic film such as polyvinyl butyral.

When a criminal breaks glass during a criminal act, some small particles are projected onto his/her clothing, hair, or shoes. If the suspect is apprehended within a relatively short time span after the crime, these small particles of glass can be found on the hair, clothing, shoes, or inside pockets. At the crime scene, the crime scene investigator usually collects some of the broken glass as evidence for further comparison with any glass fragments found on a suspect. The comparison process might lead to the exclusion of a common origin between the glass from the suspect and the glass from the crime scene. Conversely, it might also show that the characteristics are similar and the two samples cannot be differentiated, thus supporting the hypothesis that the two samples of glass come from the same origin. It is important to apprehend the suspect shortly after the glass was broken, because the number of glass fragments on the clothing or shoes of the suspects diminishes very quickly after the activity. About 90% of glass fragments are shed from clothing within 24 hours.

Glass is characterized according to its physical and chemical characteristics. When investigating glass, the first examination is visual. The investigator observes its color, its thickness (if the fragments are big enough), its patterns, and its fluorescing (light-emitting) properties. Pieces of the glass can often be reassembled, revealing patterns that can be compared to crime scene samples. Demonstration of origin by assembly is the only way the common origin between two fragments of glass can be clearly established. The refractive index of the glass fragments is then measured. This is typically achieved by immersion of the fragment in oil and observing the lines of refraction at different temperatures. Finally, elemental composition of the glass is determined.

The interpretation of glass is complicated by the fact that the characteristics exhibited by a large piece of glass (such as a window) might vary from one end to the other. Thus, the analyst needs to determine the extent of the intravariability (variations of characteristics within a same sample) before it can be compared to a different sample. If the variation exhibited between the two samples is greater than the variation exhibited within one sample, then the two samples can be excluded as having a common origin. On the contrary, if the two samples cannot be differentiated, then this supports the hypothesis that they have a common source. However, it does not indicate that they have the exact same common source. Again, the characteristics exhibited by the samples might be very common and found in many other pieces of glass. Thus, the analyst usually expresses his/her findings using statistics.

see also Criminalistics; Minerals; Monochromatic light.

Glass

Glass is a hard, brittle substance that is usually transparent or translucent. It is made by melting together sand (silicon dioxide), soda (sodium carbonate), limestone (calcium carbonate), and other ingredients. The simplest form of glass (containing only sand, soda, and lime) is known today as plate or window glass.

Scholars believe that the first humans to make glass may have been Phoenician sailors living around 5000 b.c. Examples of glass used for

weapons, ornaments, and money from Egypt and Mesopotamia—dating to about 1550 b.c.—still survive.

Humans may well have learned about glass-making by witnessing the natural formation of glass by lightning bolts. When lightning strikes areas where sand, soda, and limestone occur naturally, it can fuse these materials to produce a natural form of glass known as obsidian.

Some Special Kinds of Glass and Their Properties and Uses

The history of glass-making is a long and fascinating one. Artisans in many parts of the world discovered ways to make colored glass and glass with many special properties. Today, a very large variety of glassy materials exists with many different properties and many applications.

Colored glass

Colored glass is made by adding metallic compounds to the basic sand/soda/lime mixture. For instance, red glass is made by adding certain copper oxides or finely divided gold; yellow glass with compounds of uranium and iron; green glass with certain copper oxides or compounds of uranium and iron; blue with copper oxide, cobalt oxide, or finely divided gold; purple with certain manganese oxides and finely divided gold; milky white with calcium fluoride; and opaque with tin oxide.

Originally, glass was used primarily for decorative objects such as beads, ornaments, and stained glass windows. Eventually, though, artisans and chemists found that the properties of glass could be changed dramatically by adding various substances to the basic sand/soda/lime mixture. Those properties also could be altered by changing the way glass is cooled, or annealed.

For example, plate glass is made first by melting together the basic components—sand, soda, and lime. The liquid mixture is then maintained at its melting point for a long period of time, at least three days. Next, the mixture is allowed to cool down very slowly to room temperature. This process assures that strains within the glass are relieved, making the final product less brittle. Tempered glass is cooled even more slowly, giving it very high strength.

glass was a minor industry until 1780, when restrictions on Irish exports were removed (see free trade agitation), and a duty imposed on glass manufacture in Great Britain was not imposed in Ireland. This led to a period of pronounced growth with glasshouses being established in Dublin, Cork, Ballycastle, Drumrea, Belfast, Waterford, Newry, and Derry. By 1785 there were nine glasshouses in Ireland; six were for flint glass, two for bottles, and one for window glass. Three‐quarters of the workers in these houses came from England, bringing with them new techniques. The industry produced predominately for the home market, displacing British imports. Output increased and the number of glasshouses rose to eleven by 1825, Dublin dominating the industry.

Glass designs in Ireland tended to mirror those produced by English factories according to the fashion of the day. This has led to some confusion over attributing glass pieces to a particular factory. The idea that Irish glass has a certain colour or that it has a different ring when struck is totally inaccurate. Some decanters, finger bowls, and jugs, however, have impressed factory marks on the base giving a firm attribution, although recently decanters bearing spurious Cork marks have come on the market. Genuine marks that occur on Irish glass include ‘B. Edwards Belfast’, ‘Cork Glass Co.’, ‘Penrose Waterford’, ‘Francis Collins Dublin’, ‘Mary Carter & Son’, and ‘Waterloo Co. Cork’. Some designs, such as the turnover rim bowl, the canoe shaped bowl, and the piggin, do seem to have been made predominately by Irish glasshouses. The heavy cut decoration normally associated with Irish glass was a later phenomenon, when steam‐driven cutting machines were introduced into the factories c.1820. Earlier Irish glass had much shallower cut designs, giving the pieces a lighter and more delicate appearance.

The imposition of excise duty on Irish glass from 1825 signalled the start of a decrease in production. The number of glasshouses fell from ten in 1832 to six by 1835, and only three (flint glasshouses in Dublin and Belfast, and a bottle works in Dublin) by 1852. The craft‐based Irish industry was steadily undermined by cheaper mass produced goods from larger English glasshouses which utilized machinery to a greater degree. Gatchell's of Waterford (the most prestigious works) closed in 1851. The Pugh brothers maintained the production of flint glass in Dublin between 1854 and 1893, but their craft‐based concern was unable to survive against British and continental competition. With the closure of the works in 1893, the only type of glass which continued to be manufactured was bottles in Dublin and Belfast.

Table glass manufacture in Ireland was revived in 1951 when the Waterford factory was reopened. The success of the modern heavy cut glass produced by Waterford Crystal has led to other glasshouses being established, including the lead crystal factories at Cavan in 1969, and Tyrone in 1970. As with ceramics in Ireland, there has been a growth in the rise of individual studio glassmakers in recent years. Simon Pearce was one of the first in 1972 to set up a workshop for blowing studio glass in Co. Kilkenny. He was followed by Keith Leadbetter, who set up Jerpoint glass in 1978.

Bibliography

Boydell, M. , Irish Glass (1976)
Warren, P. , Irish Glass (1981)
Westropp, D. , Irish Glass (1920; 2nd edn., ed. M. Boydell, 1978)

Andrew Bielenberg/ and Kim Mawhinney

glass (natural and industrial) Glass is an amorphous material which lacks the structure that characterizes crystals; glass is formed when molten materials are cooled too rapidly for crystallization to occur. Natural silica-rich liquids readily form glasses, and acid volcanic activity frequently generates the glassy rock known as obsidian, which has a distinctive black appearance and conchoidal fracture.

Basic magmas must be cooled much more rapidly if glasses are to be formed. In consequence, the only basaltic glasses known have been quenched by contact with water (e.g. rims on pillow lavas) or air (spatter). Glasses formed by meteorite impact are also known. Because of their tendency to crystallize (devitrify), most natural terrestrial glasses are geologically young. The presence of glass 4 billion years old on the Moon indicates that water is crucial to the rate of devitrification.

Any liquid can be quenched to form a glass, and technologically important glasses are very varied in composition. The most familiar type of glass used in windows and bottles is made from sand, sodium carbonate (obtained from salt), and limestone, but specialist glasses can contain components as diverse as boron, lead, rare-earth elements, and fluorine.

The colours in industrial glass are usually due to small additions of impurities. These impurities can dissolve either as ions (e.g. Co²⁺ (blue), Cr³⁺ (green) and Ni²⁺ (yellow or purple)), single atoms, or tiny particles (e.g. gold or copper, which give a ruby colour).

S. C. Kohn

glass / glas/ • n. 1. a hard, brittle substance, typically transparent or translucent, made by fusing sand with soda, lime, and sometimes other ingredients and cooling rapidly. It is used to make windows, drinking containers, and other articles: a piece of glass | [as adj.] a glass door. ∎  any similar substance that has solidified from a molten state without crystallizing. 2. a thing made from, or partly from, glass, in particular: ∎  a container to drink from: a beer glass. ∎  glassware. ∎  greenhouses or cold frames considered collectively. ∎ chiefly Brit. a mirror. ∎ archaic an hourglass. 3. a lens, or an optical instrument containing a lens or lenses, in particular a monocle or a magnifying lens. 4. the liquid or amount of liquid contained in a glass; a glassful: a glass of lemonade I'll have another glass, please. • v. [tr.] 1. cover or enclose with glass: the inn has a long balcony, now glassed in. 2. (esp. in hunting) scan (one's surroundings) with binoculars: the first day was spent glassing the rolling hills. 3. poetic/lit. reflect in or as if in a mirror: the opposite slopes glassed themselves in the deep dark water. DERIVATIVES: glass·ful / -ˌfoŏl/ n. (pl. -fuls.) glass·less adj. glass·like / -ˌlīk/ adj.

glass An amorphous, metastable solid with the atomic structure of a silicate liquid. Glass can be formed by quenching a silicate melt, the short time-scale for cooling or pressure reduction preventing the reorganization of the random liquid structure into an ordered crystalline structure. Since cohesion between atoms in the liquid silicate increases with increasing silica content, melts with high silicate contents are most likely to form glasses. Natural igneous glasses of rhyolite composition (70% SiO₂) are termed ‘obsidians’. A wide variety of glasses, formed by meteoritic impact into the lunar regolith, exist on the lunar surface. Shapes include spheres averaging 100 μm in diameter, tear-drops, dumb-bells, etc., typical of rotational shapes assumed by splashed liquids. They do not resemble meteoritic chondrules. Volcanic glasses, formed by fire fountains during eruption of mare basalts, also occur locally on the Moon. Their compositions match those of local surface rocks, soils, or minerals. No tektite compositions are found.

glass, the seaman's name for a telescope, a barometer, and—before clocks suitable for use on board ship were developed—a sand-glass. Although the word telescope is as old as the instrument itself, being used by Galileo in 1611 to describe his invention, the name did not become used in maritime circles until very much later. In 1619 there is a reference to it in England under the name ‘trunke-spectacle’, but the first naval use of the name appears to have been by Murdoch Mackenzie (b. 1712) in 1744 in his treatise on surveying (see also hydrography). In general maritime use the name glass was an abbreviation of long-glass or spyglass, and Captain Marryat was using that name in his naval novels at least as late as 1844.

The barometer was, and is, almost invariably known as a glass, even when in the form of a barograph, ‘the glass is high, low, rising, falling’ being the usual description of its movements.

glass glass ceiling an unacknowledged barrier to advancement in a profession, especially affecting women and members of minorities. The term was originally (in the early 1980s) coined to denote an invisible but impenetrable barrier enshrining prejudices which were not openly admitted, but as the concept became more familiar, the figurative associations were developed: a glass ceiling was taken as something that could be broken.
glass slipper a slipper made of glass, especially the one lost by Cinderella in the fairy-tale (from a mistranslation of French pantoufle en vair ‘fur slipper’, mistaken for verre ‘glass’).
those who live in glass houses shouldn't throw stones it is unwise to criticize or slander another if you are vulnerable to retaliation. The saying is recorded from the mid 17th century.

glass. The transparent, brittle, man-made material has often provoked the Celtic imagination. The Welsh Caer Wydyr [Fortress of Glass] implied a vision of the Otherworld. Conand's Tower, the Fomorian fortress on Tory Island (off Co. Donegal), is made of glass. Fabulous Irish voyagers such as Bran, St Brendan, and Máel Dúin encounter towers of glass. Merlin goes to sea in a glass house. Welshmen used the name Ynys Wydrin/ Gutrin/Witrin [glass island] for Glastonbury in pre-Saxon times, although the English place-name is not a translation of it. OIr. glaine; ModIr. gloine; ScG glaine; Manx gless; W gwydr; Corn. gweder; Bret. gwer.

glass Brittle, transparent material. It behaves like a solid, but is actually a liquid that is cooled to prevent particles organizing themselves into a regular pattern. It is made by melting together silica (sand), sodium carbonate (soda), and calcium carbonate (limestone). It can only be worked while hot and pliable. There are many types of glass. Soda-lime glass is used in the manufacture of bottles and drinking vessels. Flint glass refracts light well, and is used in lenses and prisms. Toughened glass (laminated with plastic) is used in car windscreens. Glass is also used in fibre optic cables.

glass OE. glæs = OS., (O)HG. glas :- WGmc. *ʒla·-sam, of which a var. *ʒlaza·m is repr. by ON. gler glass; prob. rel. to OE. glǣr, MLG. glār amber.

glass Coloured glass has been excavated at many Palestinian sites, but it was expensive and a luxury until NT times when it became as cheap to produce as pottery.

glass •arse, baas, brass, carse, class, coup de grâce, farce, glass, grass, Grasse, impasse, Kars, kick-ass, kvass, Laplace, Maas, Madras, outclass, pass, sparse, stained glass, surpass, upper class, volte-face •badass • lardass • sandglass •eyeglass, spyglass •wine glass • tooth glass • subclass •hourglass •fibreglass (US fiberglass) • underclass •masterclass • weather glass • bypass •underpass • wheatgrass • ryegrass •knotgrass • sawgrass • bluegrass •goosegrass • smart-arse