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Glass

Glass


Glass is a state of matter. It is a solid produced by cooling molten material so that the internal arrangement of atoms, or molecules, remains in a random or disordered state, similar to the arrangement in a liquid. Such a solid is said to be amorphous or glassy. Ordinary solids, by contrast, have regular crystalline structures. The difference is illustrated in Figure 1.

Many materials can be made to exist as glasses. Hard candies, for example, consist primarily of sugar in the glassy state. What the term "glass" means to most people, however, is a product made from silica (SiO2). The

common form of silica is sand, but it also occurs in nature in a crystalline form known as quartz.

Pure silica can produce an excellent glass, but it is very high-melting (1,723oC, or 3,133oF), and the melt is so extremely viscous that it is difficult to handle. All common glasses contain other ingredients that make the silica easier to melt and the hot liquid easier to shape.

Natural Glass

Probably as early as 75,000 b.c.e., long before human beings had learned how to make glass, they had used natural glass to fashion knives, arrowheads, and other useful articles. The most common natural glass is obsidian, formed when the heat of volcanoes melts rocks such as granite, which then become glassy upon cooling. Other natural glasses are pumice, a glassy foam produced from lava; fulgurites, glass tubes formed by lightning striking sand or sandy soil; and tektites, lumps or beads of glass probably formed during meteoric impacts.

Manmade (Synthetic) Glass

When, where, or how human beings discovered how to make glass is not known. Very small dark-colored beads of glass have been dated back to 4000 b.c.e. These may well have been by-products of copper smelting or pottery glazing. By 2500 b.c.e. small pieces of true synthetic glass appeared in areas such as Mesopotamia, but an actual glass industry did not appear until about 1500 b.c.e. in Egypt. By this time various small vases, cosmetic jars, and jewelry items made of glass had begun to appear.

All the ancient glasses were based on silica (sand), modified with considerable amounts of various metal oxides, mainly soda (Na2O) and lime (CaO). This is still the most common glass being used today. It is known as soda lime glass. However, the ancient glass was usually colored and opaque due to the presence of various impurities, whereas most modern glass has the useful property of transparency.

Hundreds of thousands of different glass compositions have been devised, and they have been used in different ways. Much has been learned about which combination of chemicals will make the best glass for a particular purpose. For example, in 1664 an Englishman named Ravenscroft found that adding lead oxide (PbO) to a glass melt produced a brilliant glass that was much easier to melt and to shape. Since that time lead glass has been used to make fine crystal bowls and goblets and many kinds of art glass.

An important kind of glass was developed in the early 1900s to solve a serious problemthe inability of glass to withstand temperature shock. This failure resulted in tragic accidents in the early days of the railroads. Glass lanterns used as signals would get very hot, and then, if it started to rain, the rapid cooling would sometimes cause the glass to break and the signal to fail. The problem was solved by replacing much of the soda in the glass with boron oxide (B2O3). The resulting glass, called borosilicate, contains about 12 percent boron oxide and can withstand a temperature variation of 200oC (392oF). It also has greater chemical durability than soda lime glass. Today borosilicate glass is used in most laboratory glassware (beakers, flasks, test tubes, etc.) and in glass kitchen bakeware.

For even greater heat shock resistance and chemical durability, alumina (Al2O3) can be used instead of boron oxide. The resultant aluminosilicate glass has such resistance to heat shock that it can be used directly on the heating element of the kitchen stovetop. It is also used to make the special bottles used for liquid pharmaceutical prescriptions, and to produce the glass thread that is woven into fiberglass fabric.

High silica glass (96.5100% silica) remains difficult to make because of the very high melting point of pure silica. However, it is made for special purposes because of its outstanding durability, excellent resistance to thermal shock or chemical attack, and ability to transmit ultraviolet light (an ability that ordinary glass does not have). Spacecraft windows, made of 100 percent silica, can withstand temperatures as high as 1,200oC (2,192oF). Table 1 lists the five major types of glass along with properties and uses.

Glass Composition. The making of glass involves three basic types of ingredients: formers, fluxes, and stabilizers. The glass former is the key component in the structure of a glassy material. The former used in most glasses is silica (SiO2). Pure silica is difficult to melt because of its extremely high melting point (1,723oC, or 3,133oF), but fluxes can be added to lower the melting temperature. Other glass formers with much lower melting points (400oC600oC, or 7521,112oF) are boric oxide (B2O3) and phosphorus pentoxide (P2O5). These are easily melted, but because their glass products dissolve in water, they have limited usefulness.

Most silica glasses contain an added flux, so that the silica can be melted at a much lower temperature (800oC900oC, or 1,4721,652oF). Standard fluxes include soda (Na2O), potash (K2O), and lithia (Li2O). Frequently the flux is added as a carbonate substance (e.g., Na2CO3), the CO2 being driven off during heating. Glasses containing only silica and a flux, however, have poor durability and are often water-soluble.

To make glasses stronger and more durable, stabilizers are added. The most common stabilizer is lime (CaO), but others are magnesia (MgO), baria (BaO), and litharge (PbO). The most common glass, made in largest amounts by both ancient and modern glassmakers, is based on silica as the glass former, soda as the flux, and lime as the stabilizer. It is the glass used to make windows, bottles, jars, and lightbulbs.

Colored Glass. The natural glasses used by the ancients were all dark in color, usually ranging from olive green or brown to jet black. The color was

MAJOR GLASS TYPES AND THEIR USES
Glass Type Properties Limitations Uses
Soda lime Inexpensive; easy to melt and shape; most widely used glass Poor durability; not chemically resistant; poor thermal shock resistance Windows; bottles; light bulbs; jars
Lead glass (often 2030% Pb oxide) High density; brilliant; very easy to melt, shape, cut, and engrave Poor durability; easily scratched Fine crystal radiation windows; TV tube parts
Borosilicate (usually 513% B2O3) Very good thermal shock resistance and chemical durability; easy to Not suitable for long-term high temperature use melt and shape Labware; kitchenware; special light bulbs; glass pipe; sealed beam headlights
Aluminosilicate (usually 510% Al2O3) Excellent thermal resistance; durability More difficult to melt and shape than borosilicate Top-of-stove cookware; high quality fiberglass
High silica (Vycor 96.5%; fused quartz 100%) Outstanding thermal resistance Difficult to make; very expensive Spacecraft windows; labware; fiber optics

due to the presence of significant amounts of metal impurities, especially iron. Even today the ubiquitous presence of iron in nature causes most ordinary glass to have a slight greenish cast.

Many standard glass colorants are oxides of metals such as cobalt (blue), chromium (green), and manganese (violet). Yellow glass is usually made with cadmium sulfide, and red or pink glass usually contains selenium, although some ruby-colored glass has had gold added. The coloring of glass is not a simple subject. Glass color depends not only on which elements are added, but also on the composition of the glass, and on whether the furnace used was in an oxidizing or reducing mode. Copper, for example, can produce blue, green, or opaque red glass, depending on melting conditions.

The Egyptians of 1500 b.c.e. knew that they could make brightly colored glasses by adding certain metals (or their compounds) to the glass melt. The ancient Romans continued the science of making colored glass and expanded it. By the fourth century c.e. the Romans had learned how to produce a dichroic (two-color) glass. The most famous dichroic glass article left by the Romans is the Lycurgus Cup (now at the British Museum). It is green in reflected light (with the lamp in front of the cup), but red in transmitted light (the lamp behind the cup). This unusual glass contains microscopic particles of gold and silver.

Glass Forming

Ancient Methods. Shaping hot, molten glass into useful articles has long been a challenge. Molten glass is extremely hot, caustic, sticky, and difficult to handle. In the period extending from about 2000 b.c.e. to 50 b.c.e., there were three basic methods used to form glass. One of the earliest and most widely used was core forming. This involved distributing molten glass around a clay core on a metal rod. The rod with the clay core could either be dipped into molten glass, or the hot liquid glass could simply be poured over it. The outer glass coating was then rolled (marvered) on a flat stone surface to smooth it. Often the object was decorated by dribbling more glass, sometimes of a different color, onto its surface. The hot glass was then annealed (cooled slowly so as to relieve thermal stress), and the metal rod was removed and the clay core scraped out.

A second method involved sagging and fusing. It called for taking preformed glass rods or canes (which were often of different colors), placing them in or on top of a mold, and then heating the canes until they sagged and fused together and conformed to the shape of the mold. (Sheets of glass could also be sagged over shaped clay molds.)

The third method was casting, which called for pouring hot, molten glass into a mold. A variation on cast glass was faience, made from powdered quartz blended into molten glass. The mixture might be pressed between two molds to make a cast vessel such as a bowl.

All three of these methods were slow, and they generally produced small items that were rather thick. Glass pieces tended to be quite expensive, and, in antiquity, were affordable only by the very wealthy.

Glassblowing. It was probably in the Middle East during the first century b.c.e. that the important technique of glassblowing was discovered. A hollow metal rod (or pipe) was used to pick up a gob of molten glass; the act of blowing into the pipe generated a bubble of glass. If the bubble were blown into a mold, the molten glass could be given a desired shape. Wooden paddles and pincers were used to refine the shape even further. The blowing procedure was used to make glass objects that were larger and thinner than those that had been made previously, and it was much faster than previous glass-forming methods. As glass pieces became easier to make, they became cheaper and more available. The ancient Romans became particularly skillful at glassblowing. More glass was produced and used in the Roman world than in any other civilization of antiquity. During the Middle Ages, there was a great expansion of glassblowing activity, especially in Venice, the Middle East, and European countries such as Spain and Germany.

Some Modern Methods. Since the nineteenth century, many centuriesold glass-forming methods have been mechanized, greatly increasing the rate of production of glass objects, and lowering the prices of these objects. For example, the "ribbon machine," developed in the 1920s for the automatic glassblowing of lightbulbs, is a milestone of mechanical glass forming. In the ribbon machine, puffs of air blow glass bubbles from a rapidly moving ribbon of molten glass into a moving stream of molds that give the bulbs their shape and then release them. Small lightbulb blanks can be made at the rate of 1,000 per minute.

With so many millions of windows in buildings and vehicles everywhere, we tend to take sheets of flat glass for granted. Throughout most of human history, however, there were no sheets of flat, transparent glass. Even as recently as the eighteenth century, glass windows were quite uncommon.

In a very limited way glass windows did start to appear in the Roman world during the third century, but they were generally small glass fragments set in bronze or wooden frames. In that era most windows were not glass, but were thin sheets of translucent horn or marble, or perhaps panes of mica (isinglass). Around 600 c.e., during the Byzantine period, glass windows (usually made of small pieces of colored glass) began to appear in the large churches, but glass windows in houses and other secular buildings remained quite rare until the end of the eighteenth century.

The principal method for making flat glass during the 1700s called for blowing a hot glass bubble, securing an iron rod to the bubble's other side, and then cutting the bubble free from the blowing pipe. The tulip-shaped hot glass was then rotated rapidly around the iron rod axis until the centrifugal force forced the glass tulip to open up and form a disk. The rod was then removed from the glass (leaving a spot in the middle of the glass disk that looked rather like a bull's eye). This method was the source of the old "bull's-eye" windows that can still sometimes be found in English pubs. The windows were limited in size and poor in optical quality (besides having a bull's-eye at their centers).

The chief method for making flat glass during the 1800s was the cylinder method. The first step was to blow a large glass bubble (compressed air was often used); it would then be swung back and forth until the bubble became elongated and acquired a cylindrical shape; finally the cylinder was split lengthwise, reheated, and allowed to flatten on an iron table. The resulting pane of glass was not really flat, and it had a lot of optical distortion, but the method was used widely to make sheet glass. For example, it was used to produce the 300,000 panes of glass that were used to build London's Crystal Palace, the huge greenhouse constructed for the London World's Fair Exhibition of 1851.

By the twentieth century these methods were replaced by an innovative technique invented by a Belgian named Foucault, who had learned how to draw up continuous sheets from a tank filled with molten glass. Even this glass was of nonuniform thickness and had some roughness at its surface, therefore, for high quality flat glass, it had to be ground and polished.

Then, in 1959, the Pilkington Glass Works in Britain introduced the "float glass" process. In the float process, molten glass is allowed to flow continuously onto a mirrorlike surface of molten tin at 1,000oC (1,832oF). At this temperature the glass spreads out and becomes a layer that is about 6 millimeter (1/4 inch) thick. If the layer is stretched as it cools, a thickness of 2 millimeter (0.08 inch) can be achieved. The glass is allowed to advance on the hot liquid tin until, at 600oC (1,112oF), it becomes solid enough to be lifted off the molten tin surface. It is then annealed (heated to relieve any strain) before being cut into desired sheet lengths. The float glass method rapidly replaced the Foucault drawing process, and today it is the standard method for making flat glass. A large modern float glass plant can produce 5,000 tons of glass sheet per week, and it can be operated 24 hours a day, 365 days a year, for several years before serious repairs are apt to be needed. Float glass has uniform thickness and bright fire-polished surfaces that need no grinding or polishing.

The drawing of glass fibers had long been of interest, but glass fibers found little use until the twentieth century. Articles such as wedding gowns made from glass fiber cloth were largely curiosities, made for show rather than use. In the 1930s glass researchers learned to feed molten glass into platinum bushings having hundreds of tiny holes. Fine glass filaments of 10 to 50 microns were rapidly drawn downward and assembled as bundles or strands of glass fiber. Today a major use of glass cloth or filaments is to strengthen the plastics used to make fiberglass-reinforced composites. These composites are widely used in making boats, from canoes to yachts, and bodies for cars, such as the Corvette.

An even larger poundage market is that of glass wool insulation. In a process much like that used to make cotton candy, fine glass fibers are spun, sprayed with an organic bonding agent, and then heat-cured and cut into mats of various sizes, to be used for insulating buildings and appliances.

Surely the most significant glass fiber development in recent times is fiber optics, or optical wave guides. These ultrapure, very fine glass fibers are a most crucial part of modern communications technology, wherein glass fibers link telephones, televisions, and computers. A single strand of glass optical fiber that has a protective plastic coating looks much like a human hair. The glass fiber has an inner core of ultrapure fused silica, which is coated with another silica glass that acts as a light-refractive barrier. Lasers are used to convert sound waves and electrical impulses to pulses of light that are sent, static-free, through the inner glass core. Glass fibers can transmit many times more information than can be carried by charges moving in a copper wire. In fact, one pound of glass optical wave guides can transmit as much information as can be transmitted via 200 tons of copper wire. Today millions of miles of optic fibers are crisscrossing not only the United States, but also the entire planet.

Windows need to be cleaned. In 2000 a new glass that largely cleans itself when it comes into contact with rain was introduced. This low-maintenance glass was developed by Pilkington Glass Works, the company that invented the float process. It is made by depositing a microscopically thin coating of titanium dioxide (TiO2) on hot sheet glass during its manufacture in the float process. As dirt collects on the window, the Sun's ultraviolet rays promote a catalytic reaction at the glass surface that breaks down and loosens surface dirt.

see also Ceramics; Gemstones; Minerals.

Kenneth E. Kolb

Bibliography

Brooks, John A. (1973). Glass. New York: Golden Press.

Douglas, R. W., and Susan Frank (1972). A History of Glassmaking. Oxfordshire, England: G. T. Foulis & Co.

Kampfer, Fritz, and Beyer, Klaus G. (1966). Glass: A World History. London: Studio Vista.

Kolb, Kenneth E., and Kolb, Doris K. (1988). Glass: Its Many Facets. Hillside, NJ: Enslow.

Phillips, Phoebe (1981). The Encyclopedia of Glass. New York: Crown Publishers.

Rogers, Frances, and Beard, Alice (1948). 5000 Years of Glass. New York: Lippincott.

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Glass

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

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glass

glass, hard substance, usually brittle and transparent, composed chiefly of silicates and an alkali fused at high temperature.

Composition and Properties of Glass

Most glass is a mixture of silica obtained from beds of fine sand or from pulverized sandstone; an alkali to lower the melting point, usually a form of soda or, for finer glass, potash; lime as a stabilizer; and cullet (waste glass) to assist in melting the mixture. The properties of glass are varied by adding other substances, commonly in the form of oxides, e.g., lead, for brilliance and weight; boron, for thermal and electrical resistance; barium, to increase the refractive index, as in optical glass; cerium, to absorb infrared rays; metallic oxides, to impart color; and manganese, for decolorizing. The term "crystal glass," derived from rock crystal, was at first applied to clear, highly refractive glass; it has come to denote in the trade a high-grade, colorless glass and is sometimes applied to any fine hand-blown glass.

The Process of Glassmaking

The processes of glassmaking have remained essentially the same since ancient times. The materials are fused at high temperatures in seasoned fireclay containers, boiled down, skimmed, and cooled several hundred degrees; then the molten glass (called metal) is ladled or poured into molds and pressed, or is blown (sometimes into molds), or is drawn. The shaped glass is annealed to relieve stresses caused by manipulation, then is slowly cooled. The glass, formerly annealed on shelves in a melting furnace, is now usually carried on rollers through annealing ovens (lehrs).

Although today most hollow vessels such as light bulbs or containers are machine blown, fine ornamental hollow ware is still made by gathering a mass of glass at the end of a long, iron blowpipe, blowing it into a pear-shaped bulb, which is rolled on an oiled slab (marver), shaped with tools, and then reblown, often into a mold; the glass is reheated periodically in a small furnace (glory hole). It is finally transferred to an iron rod (punty) attached to the base of the vessel, and the lip is shaped and smoothed. Methods of decoration include cutting, copper-wheel engraving, etching with hydrofluoric acid, enameling, gilding, and painting.

Development of the Glass Industry

Humans have used glass since prehistoric times, at first fashioning small objects from natural glass such as obsidian, a volcanic glass, or from rock crystal, a colorless, transparent quartz whose brilliance and clarity are emulated in manufactured glass.

Ancient Glassmaking

The place and date of origin of manufactured glass are not known. The oldest known specimens of glass are from Egypt (c.2000 BC), where the industry was well established c.1500 BC Many varieties of glass were known during Roman times, including cameo glass, such as the Portland vase, and millefiore glass, produced from fused and molded bundles of thin glass rods of many colors. Glass was also used for window panes, mirrors, prisms, and magnifying glasses. Except for the work done in Constantinople, little is now known of the methods of glassmaking used in Europe from the fall of Rome until the 10th cent., when stained glass came into use.

Early European Glassmaking

Venice was the leader in making fine glassware for almost four centuries after the Crusades and attempted to monopolize the industry by strict control at Murano of glassworkers, who were severely penalized for betraying the secrets of the art. After the invention (c.1688) of a process for casting glass, France was for many years supreme in the manufacture of plate glass such as that used to line the Galerie des Glaces at Versailles. Late in the 17th cent. England began to make flint glass, whose lead oxide content imparted a brilliance and softness that made it suitable for cut glass.

Glassmaking in Colonial America

The first glass factory in America was built in 1608, and glass was carried in the first cargo exported to England. Although other glasshouses were operated in the colonies, especially in New Amsterdam, the first successful and enduring large-scale glasshouse was set up by the German-born manufacturer Caspar Wistar in New Jersey in 1739. Some of the finest colonial glassware was produced in the Pennsylvania glasshouses of the German-born manufacturer H. W. Stiegel.

Beginnings of the Modern Era

The invention of a glass-pressing machine (c.1827), used by the American manufacturer Deming Jarves in his Boston and Sandwich Glass Company (1825–88), permitted the manufacturing of inexpensive and mass-produced glass articles. Nevertheless, in the 19th and 20th cent., there has remained a sense of pride in individual craftsmanship. The American artist Louis C. Tiffany was responsible for the design and manufacture of an extraordinary iridescent glass used in a variety of objects in the late 1800s. Exceptionally fine blown glassware has been designed by such artists as René Lalique and Maurice Marinot in France, Edvard Hald and Simon Gate in Sweden, as well as Sidney Waugh in the United States.

Contemporary Applications of Glass

Glass has become invaluable in modern architecture, illumination, electrical transmission, instruments for scientific research, optical instruments, household utensils, and even fabrics. New forms of glass, new applications, and new methods of production have revolutionized the industry. Recently developed forms of glass include safety glass, which is usually constructed of two pieces of plate glass bonded together with a plastic that prevents the glass from scattering when broken; fiberglass, which is made from molten glass formed into continuous filaments and used for fabrics or for electrical insulation; and foam glass, which is made by trapping gas bubbles in glass to yield a spongy material for insulating purposes. Certain uses of glass are now being superseded by newly developed plastics.

See also window.

Bibliography

See G. O. Jones, Glass (2d ed. 1971); L. D. Pyle et al., Introduction to Glass Science (1972); R. H. Doremus, Glass Science (1973); I. Fanderlik, Optical Properties of Glass (1983); P. Bansal, Handbook of Glass Properties (1986).

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Glass

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.

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glass

glass. Semi- or fully transparent hard, brittle, lustrous material made by igneous fusion of silica (usually sand) with an alkaline sodium or potassium salt and added ingredients, such as lime, alumina, lead oxide, etc. Colour may be added by the addition of metallic oxides. It appears to have come into use for glazing the windows of grander buildings during the Roman Empire. In the Middle Ages coloured and painted glass, used in small pieces because of the difficulties of manufacturing larger expanses, was set in lead cames, commonly in churches: surviving examples (e.g. Chartres Cathedral, King's College Chapel, Cambridge, and Fairford, Glos.) are among the glories of medieval art. Glass used for domestic architecture also had to be set in lead cames or in sashes subdivided by glazing-bars.

Types of glass include:acid-etched: glass treated with wax or a similar substance into which a design is cut, then subjected to action by hydrofluoric acid which etches into the unprotected surface to create a design. Much used from Victorian times for decorative windows for public-houses, etc.;armourplate: thick toughened polished plate glass used for large windows, doors, etc.;broad: glass blown in cylinders which are then cut open and flattened, called muff or window-glass;Crown: blown from a glass tube into a bulb then opened and spun rapidly, the outer part thin, clear, and lustrous, but the centre or hub of thick glass (bottle, bullion, or bull's eye) largely opaque and never used for good work. It was the commonest type of glass found in British domestic architecture until the mid-1830s;flint: made from white sand, potash, nitre, red lead, and ground window-glass, used where refraction is desired, as on decorative altar-pieces, etc.;ground: with a rough surface, usually ground, to make it lose transparency;iridescent: with a coating to give it the appearance of the surface of a soap-bubble with rainbow colours;jealous: glass roughened to allow the light to pass through, but with a loss of transparency;laminated: toughened glass made by a laminated process;plate: poured on to a cast-iron table, rolled with a heavy roller, and polished on both sides;sheet: made by blowing into a cylinder of glass which increases in diameter before being cut lengthways, flattened, and polished. It was a refinement of broad or muff glass, invented by Messrs. Chance, near Birmingham (1832–8), and had a finish comparable to that of Crown glass, but cheaper and capable of being made in larger sheets, thus making glazing-bars in sashes obsolete;stained: coloured throughout its mass or with the colour applied or flashed. Crimson is produced with oxides of copper or tin, blue with cobalt, purple with manganese, and other colours using various combinations of chemicals;toughened: thick glass made in a variety of ways;wire: with a network of wire enclosed within it to improve security.Glass was used as a major component of external walls first in conservatories and green-houses, and then as a roofing material with glazing-bars of wood and, later, of iron. The evolution of railway termini and major exhibition buildings (such as Paxton's Crystal Palace, London (1851) ) developed the architectural possibilities of glass, leading to Dutert's Galerie des Machines, Paris (1889). Glazed curtain-walls were suggested by conservatories and used by architects such as Ellis, later developed by Behrens, Gropius, and others. Later developments have included solar-reflective and tinted glass for large expanses (as in Norman Foster's work), while blocks of glass, glass tubes, and glass slabs have been used widely in C20.

Bibliography

H. Blaser (1980);
Button & and Pye (1993);
Hix (1996);
Kohlmaier & and von Sartory (1986);
Koppelkamm (1981);
Korn (1967);
McGrath & and Frost (1937);
W. Papworth (1997);
Schild (1983);
Sturgis et al. (1901–2);
Wigginton (1996)

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Glass

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 Mesopotamiadating 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

Type of Glass Composition, Properties, and Uses
Ceramic glass Contains titanium oxide; heat and shock resistant; used in range and stove tops, architectural panels, and telescope mirrors.
Enamel glass Contains lead and borax; resistant to most chemicals used in bottles, tumblers, glass signs, architectural objects.
Fiberglass Formed by forcing melted glass through small openings; used in the manufacture of insulation, fabrics, tire cords, and light transmission (optical fibers).
Heat-resistant glass Contains about %5 borax; trade names include Pyrex® and Vicor®; resistant to shock and sudden changes in temperature; used in laboratory glassware and kitchen utensils.
Laminated glass Consists of layers of glass and plastic to provide strength and make the final product shatterproof; used in automobile windows.
Optical glass Contains either lime (crown glass) or lead (flint glass); used for lenses in cameras, microscopes, eyeglasses, and other applications where refraction of light is important.
Photochromic glass Contains silver halide or borax; changes color when exposed to light; used in eyeglasses that double as sunglasses.

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 componentssand, 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.

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glass

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.

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glass

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.

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glass

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.

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glass

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.

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glass

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.

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glass

glassarse, 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

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