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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; alumina, for strength and durability, as in cellphone glass, and thermal resistance; 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.
The place and date of origin of manufactured glass are not known. The oldest known specimens of glass are from Egypt (c.2000 B.C.), where the industry was well established c.1500 B.C. Many varieties of glass were known during Roman times, including cameo glass, such as the Portland vasePortland vase,
a Roman glass vase, known also as the Barberini vase. It is an unusually fine work of the late Augustan era (early 1st cent. B.C.). About 10 in. (25 cm) high and 22 in.
..... Click the link for more information. , 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 glassstained glass,
in general, windows made of colored glass. To a large extent, the name is a misnomer, for staining is only one of the methods of coloring employed, and the best medieval glass made little use of it.
..... Click the link for more information. 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 windowwindow,
in architecture, the casement or sash, fitted with glass, which closes an opening in the wall of a structure without excluding light and air. It may have a square, round, or pointed head; may be single, double, or grouped; in relation to the wall, it may be flush,
..... Click the link for more information. .
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).
a solid amorphous material obtained by supercooling a melt. Glass is characterized by reversibility of the transition from a liquid state to a metastable, glassy state. Under certain temperature conditions, glass will crystallize. Unlike crystalline solids, it does not fuse but rather softens, gradually changing from a solid to a plastic material and then to a liquid. With respect to state of aggregation, glass occupies an intermediate position between liquid and crystalline substances. Its elastic properties suggest a similarity to crystalline solids, but the absence of crystallographic symmetry (and associated isotropy) invites comparison with liquids. The tendency to form glass is characteristic of many substances, including selenium, sulfur, silicates, and borates.
The term “glass” is also applied to certain glass items, such as window glass, container glass, and laboratory glass. Glass products may be transparent or opaque and colorless or colored; they may luminesce when exposed to, for example, ultraviolet or ?y-radiation; and they may either transmit or absorb ultraviolet rays. Inorganic glass, which is characterized by good mechanical, thermal, and chemical properties, is most common. Most inorganic glass is used in construction, in particular, sheet glass, and the manufacture of containers. This type of glass is based on silicon dioxide (soda-lime-silica glass); other oxide types of glass, whose composition includes oxides of phosphorus, aluminum, boron, and other elements, also find application. The oxygen-free types of inorganic glass include glass based on the chalcogen-ides of such elements as arsenic (As2S3) and antimony (Sb2Se3) and on the halides of beryllium (BeF2) and other elements.
According to function, distinctions are made between structural glass (window glass, patterned glass, glass bricks), container glass, the glass used in technology (quartz glass, glass used in illuminating engineering, glass fiber), and the glass used in household glassware. Other types include radiation-absorbing glass, radiation-sensitive glass, photochromic glass, the glass used as a laser material, uviol glass, foam glass, and soluble glass. Soluble glass, which contains approximately 75 percent SiO2, 24 percent Na2O, and other components, forms a sticky liquid (liquid glass) with water. It is used as a thickening agent in the production of silicate paint and office glue and as a disperser and detergent. Soluble glass is also used to impregnate fabric and paper. The chemical composition of certain types of glass is given in Table 1.
Physicochemical properties. The properties of a particular type of glass depend on the components. The most characteristic property of glass is transparency; the percentage of light transmission of window glass is 83–90 percent, and of optical glass, up to 99.95 percent. Glass is typically brittle, being extremely sensitive to mechanical effects, especially shocks; however, its compressive strength is the same as that of cast iron.
To increase strength, glass is subjected to such hardening processes as tempering; ion exchange, in which sodium ions, for example, are replaced by lithium or potassium ions on the surface of the glass; chemical treatment; and thermochemical treatment. These processes weaken the action of surface microcracks (Griffith cracks) that arise on the surface of glass as a result of such environmental effects as temperature and moisture and that serve to concentrate stress. Hardening processes can increase the strength of glass by a factor of 4–50. Etching or compressing the surface layer is usually used to eliminate the influence of micro-cracks. With etching, the defective layer is dissolved by hydrofluoric acid and a protective film of, for example, polymers is applied to the exposed flawless layer. With tempering, the opening up of cracks is impeded by the compression of the surface layer.
|Table 1. Composition of certain types of glass and glass products produced industrially|
|Type||Chemical composition (in percent)|
|Container glass ...............||71.5||—||3.3||3.2||5.2||—||—||16||—||0.6||0.2|
|Laboratory glassware ...............||68.4||2.7||3.9||—||8.5||—||—||9.4||7.1||—||—|
|Optical glass ...............||41.4||—||—||—||—||—||53.2||—||5.4||—||—|
|Electric bulbs ...............||71.9||—||—||3.5||5.5||2||—||16.1||1||—||—|
|Heat-resistant glass ...............||57.6||—||25||8||7.4||—||—||—||2||—||—|
|Glass resistant to heat shock ...............||80.5||12||2||—||0.5||—||—||4||1||—||—|
|Thermometric glass ...............||57.1||10.1||20.6||4.6||7.6||—||—||—||—||—||—|
|Radiation-resistant glass ...............||48.2||4||0.65||—||0.15||29.5||—||1||7.5||—||—|
The density of glass is 2,200–8,000 kg/m3. The hardness on Mohs’ scale is 4.5–7.5, the microhardness is 4–10 giga-newtons/m2, and the modulus of elasticity is 50–85 giganewtons/ m2. The ultimate strength of glass is 0.5–2 giganewtons/m2 under compression, 30–90 meganewtons/m2 under bending, and 1.5–2 kilonewtons/m2 under shock bending. Glass has a heat capacity of 0.3–1 kilojoule/kg-°K and is resistant to heat shock in the range 80°-1000°C. The coefficient of thermal expansion is 0.56–12 × 10–6/°K. The thermal conductivity of glass is only slightly affected by chemical composition and is equal to 0.7–1.3 watts/m-°K. Glass’s refractive index is 1.4–2.2, electrical conductivity is 10–18–10–18 ohm1cm1, and permittivity is 3.8–16.
Glassmaking. Glassmaking encompasses processes of preparing the raw materials, mixing the batch, melting the batch, cooling the glass melt, and forming, annealing, and treating, thermally, chemically, or mechanically, the glass products. The chief components include natural formers, for example, SiO2, artificial formers, for example, Na2CO3, and substances containing basic (alkali and alkaline-earth) and acidic oxides. The main component of most of the glass produced industrially is silica (silicon dioxide), the content of which ranges from 40 to 80 percent by weight; in quartz and Vycor glass, the percentage ranges from 96 to 100 percent. Quartz sand, enriched if necessary, usually serves as the source of silica in glassmaking. Boric acid, sodium tetraborate, and other substances are the raw material containing boron oxide. Aluminum oxide is introduced with, among other substances, feldspars and nepheline, oxides of alkali metals are introduced with calcined soda and potash, and oxides of alkaline-earth elements are introduced with such substances as chalk and dolomite. Auxiliary components include compounds added to impart some property, such as color, or to accelerate the melting process. For example, compounds of manganese, cobalt, chromium, and nickel are used as dyes, compounds of cerium, neo-dymium. praseodymium, arsenic, and antimony are used as de-colorizers and oxidizing agents, and compounds of fluorine, phosphorus, tin, and zirconium are used as opacifiers— substances that cause intensive light diffusion. Such substances as sodium chloride, ammonium sulfate, and ammonium nitrate are used as fining agents. Prior to melting, all components are sifted, dried, pulverized if necessary, and mixed to a completely homogeneous powdery batch, which is then fed to a glass furnace.
The glassmaking process is usually divided into stages of silicate formation, glass formation, fining, homogenization, and cooling.
When the batch is heated, first the hygroscopic and chemically bound water evaporates. In the silicate-formation stage, thermal decomposition of the components occurs, as do reactions in the solid and liquid phases resulting in the formation of silicates. The silicates initially appear as a caked conglomerate containing even components that do not enter into the reaction. As the temperature increases, certain silicates fuse and, dissolving in each other, form an opaque melt containing particles of the batch components and large quantities of gases. The silicate-formation stage is completed at 1100°-1200°C.
In the glass-formation stage, the remaining components of the batch dissolve, and a foam separates. At this point, the melt becomes transparent. This stage coincides with the last part of silicate formation and occurs at a temperature of 1150°-1200°C. Glass formation proper is the process in which the residual quartz grains dissolve in the silicate melt, forming a relatively homogeneous glass melt. Ordinary soda-lime-silica glass contains approximately 25 percent silica not chemically bound into silicates, and it is only glass of this type that is suitable for practical use with regard to chemical stability. Glass formation occurs much more slowly than silicate formation, taking approximately 90 percent of the total time required for the complete melting of the batch and approximately 30 percent of the total time required for glass-making.
Chemically bound gases (CO2, SO2, O2) usually constitute approximately 18 percent of the glass batch. Most of the gas is driven out during melting, but some gas remains in the glass melt, forming various-size bubbles.
In the fining stage, while the temperature is held at 1500°-1600°C for a prolonged period, the melt becomes less supersaturated with gases; large bubbles rise to the surface, while small bubbles dissolve in the melt. To speed up fining, fining agents, which reduce the surface tension of the glass melt, are added to the batch. The melt is mixed either with special refractory stirrers or by the passage of compressed air or some other gas.
Homogenization—a process ensuring uniformity with respect to composition—proceeds simultaneously with fining. The inho-mogeneity of the glass melt usually results from poor mixing of the batch components, the high viscosity of the melt, and the slowness of diffusion processes. Homogenization is promoted by the separation of gas bubbles from the melt, which in their movement mix inhomogeneous regions and facilitate diffusion, thereby equalizing the concentration of the melt. Homogenization is best realized through mechanical mixing, a technique widely used in the production of optical glass.
The final stage of glassmaking is the cooling of the glass melt to the viscosity required for the forming of glass. This viscosity corresponds to a temperature range of 700°-1000°C. The chief requirement in cooling is a continuous slow decrease in temperature without a change in the composition and pressure of the gaseous medium; if the established gas equilibrium is upset, seeds (small bubbles) are formed.
Special features characterize the production processes of certain types of glass. For example, the melting of optical quartz glass in electric glass furnaces is begun in a vacuum, and the final stage is carried out in an atmosphere of inert gases under pressure. The production of each type of glass is governed by technological specifications.
The forming of items from the glass melt is carried out mechanically through such processes as rolling, pressing, pressing and blowing, and blowing on glass-forming machines. After forming, the items are subjected to heat treatment (annealing).
As a result of annealing, that is holding the glass items at a temperature close to the softening point, and a subsequent slow cooling, the stresses that arise in the glass upon rapid cooling are removed. The controlled stress pattern introduced through another heat-treatment process, tempering, increases mechanical strength and resistance to heat shock and ensures that the glass will break in a certain (safe) way. Tempered glass is used in, for example, automobiles and railroad cars.
History. Glass occurs in nature in perlite and obsidian and in such cases is referred to as natural glass.
Glass other than that found in nature first appeared in connection with the development of pottery. During firing, a mixture of soda and sand might fall on the clay item, forming a glaze on the surface of the item. Glassmaking began in the fourth millennium B.C. (Egypt, Southwest Asia).
Opaque glass, used to imitate semiprecious stones (malachite, turquoise), was the first to be produced. The composition of glass gradually changed, and the quantity of oxides of alkali metals decreased from 30 percent (by weight) to 20 percent. Oxides of lead and tin were added, and compounds of manganese and cobalt were found to impart color. In the second millennium B.C., glass was melted in Egypt in clay crucibles having a capacity of approximately 0.25 liter.
Fundamental changes in glassmaking occurred at the beginning of the Common Era, when there were breakthroughs in two important areas—the production of transparent colorless glass and the forming of glass items by blowing. The production of transparent glass came with improvements in glass furnaces that made it possible to raise the temperature during the melting process and to accurately reproduce the conditions for good clarification of the melt. The blowpipe, invented in the first century B.C., was found to be a versatile tool that could be used to create simple inexpensive items of everyday use, for example, dishes. The book by the monk Antonio Neri, published in Florence in 1612, is regarded as the first scientific work on glassmaking. It included instructions on the use of oxides of lead, boron, and arsenic to clarify glass and gave compositions of stained glass. In the second half of the 17th century, the German alchemist J. Kunckel published his work Ars vitraria experimentalis (Experimental Art of Glassmaking). He also devised a method of producing ruby glass containing gold. In 1615 coal was first used to heat glass furnaces, thereby increasing the range of possible temperatures. A method of producing mirror plate by casting on copper sheets and then rolling was proposed in France in the early 17th century. Also at this time, a method of etching glass with a mixture of fluorspar and sulfuric acid was discovered, and the production of window glass and optical glass was mastered. Contributions to the scientific basis of glassmaking were made by the Russian scientists M. V. Lomonosov, E. G. Laksman, S. P. Petukhov, A. K. Chu-gunov, D. I. Mendeleev, and V. E. Tishchenko.
Manual labor predominated in glassmaking up to the end of the 19th century, and it has been only in the second half of the 20th century that large-scale production of some types of glass, for example, window glass and container glass, has been mechanized. Hand forming is now used only in producing artistic pieces and certain types of glassware used in the home.
N. M. PAVLUSHKIN
Art glass. Art glass includes stained glass, smalt mosaics, artistic vessels, architectural details, decorative compositions, sculpture (usually small shapes), lamps, and artificial gems. In antiquity, glassmaking was particularly developed in Egypt (Ptolemaic dynasty, fourth to first centuries B.C.), Syria, Phoenicia, and China. As a rule, in the art of the ancient world glass items (small vases, chalices, dishware, beads, earrings, amulets, seals) were made by pressing glass in open clay molds or by winding the glass melt onto a rod; the glass was usually opaque and green, pale blue, or turquoise in color. The invention of free glassblowing with a pipe and the elevation of the temperatures at which melting was carried out enabled Hellenistic and Roman craftsmen to make thin-walled items, sometimes with two layers, of relatively large dimensions and with greater transparency and uniformity of weight.
Beginning in the sixth century, Byzantium became a center for the production of art glass. Opaque colored glass was made for vessels and smalt items. During the Gothic period in Western Europe, the making of stained glass was an important branch of art, and it stimulated appreciation of art glass in general. Among the Islamic countries of the Middle East in the 12th to 14th centuries, Syria was renowned for its manufacture of glass pieces with enamel painting.
In the 15th and 16th centuries, Venetian glass became extremely important in European decorative and applied arts. With the invention of harder types of glass made with calcium in the 17th century and the development of engraving processes, the center of art glass manufacture shifted to Bohemia. In the 1770’s glass based on lead oxide (lead crystal, or flint glass) came into wider use, initially in England. Deep cutting, which would reveal the ability of lead crystal to refract or reflect light, was the chief method of finishing the glass. Beginning in the 18th century, the production of artificial gems underwent intensive development. At the turn of the 20th century, such masters of decorative and applied arts as E. Gallé, A. Daum, and E. Rousseau in France, J. Hoffmann in Austria, and L. C. Tiffany in the United States were turning to art glass. Features of art nouveau predominated in their works, which sought to evoke associations with natural, chiefly plant, forms. An extraordinary diversity of techniques and stylistic trends is characteristic of modern art glass. A fascination with refined, emphatically fanciful configurations and intricate surface designs coexists with an adherence to ascetically rigorous concepts that single out simplicity of form and transparency of the unadorned glass as the most important elements of the work.
In ancient Rus’, glassmaking (the making of ornaments, vessels, and smalt for mosaics) was well developed even before the Mongol conquests. Interrupted by the Tatar-Mongol invasion, the production of art glass was revived in the 17th century when the first glass factory in Russia was founded in 1635. M. V. Lomonosov, who in 1653 built a factory in Ust’-Ruditsy, made important contributions to the production of colored glass, chiefly that used in mosaics, bijouterie, and architectural facing. The Imperial Crystal and Glass Factory in St. Petersburg, which had been founded by Peter I in the early 18th century near Moscow and by the middle of the century had been transferred, together with the Iamburg factories, to St. Petersburg, played an extremely important role in the development of Russian glassmaking. The Gus’ Crystal Plant and the Diat’kovo Crystal Factory were also founded in the 18th century. Hütte glass (enameled and often having a blackish tint), made by free blowing and molding at small factories owned by merchants, and transparent glass, decorated chiefly by engraving, were typical of 18th-century Russian art glass. The latter type was made at the imperial factory and at large private enterprises; many items of opal glass were made at these same enterprises beginning in the mid-18th century. Details of lighting fixtures and furniture and decorative details of buildings were made (in the style of classicism) at the imperial factory according to the designs of such major architects as A. N. Voro-nikhin, C. Cameron, M. F. Kazakov, N. A. L’vov, K. I. Rossi, and J. Thomas de Thomon.
Beginning in the late 18th century, the casting of lead crystal and the cutting of diamond patterns was gradually mastered, and in the early 19th century a design imitating the facets of a brilliant (Russian stone) was typical. By the mid-19th century, a preference for large-size glass items had developed. Examples were to be seen in elaborate crystal chandeliers, in vases, and in glass pieces used in architecture. By the end of the century, an imitative trend, manifested in glass representations of stone, porcelain, wood, and metal, was developing, and the influence of art nouveau was spreading.
In the USSR, production of art glass began in earnest in the late 1930’s. The sculptor V. I. Mukhina played a leading role in the development of this type of glassmaking. In the 1950’s and 1960’s, artistic workshops were set up at nearly all large modern factories producing household glassware. Prominent masters of decorative and applied arts working at factories in the USSR in the 1960’s and 1970’s included G. A. Antonova, A. A. Astvatsa-tur’ian, A. G. Balabin, S. M. Beskinskaia, M.-T. V. Grabar’, O. I. Gushchin, Iu. V. Zhul’ev, A. D. Zel’dich, Kh. Kyrge, L. M. Mitiaeva, V. S. Muratov, V. S. Murakhver, M. A. Pavlovskii, S. Raudvee, E. I. Rogov, B. A. Smirnov, V. A. Filatov, V. Ia. Shevchenko, L. O. Iurgen, and E. V. Ianovskaia. Trends in contemporary art glass include the Leningrad school (uncol-ored and colored crystal of spare form with diamond facets), the Vladimir school (continuing the traditions of Russian Hütte glass), the Ukrainian school (traditions of Ukrainian Hütte glass, bright polychromy), the Baltic school (delicately colored pressed glass with delicate engraving). The 1960’s and 1970’s have witnessed a continuing development of the art of working with stained glass, a wider use of crystal fountains and glass decoration in architecture, and the production of glass items, including tapestries of glass fabric, for decorating interiors.
N. V. VORONOV
REFERENCESPetukhov, S. P. Steklodelie. St. Petersburg, 1898.
Bezborodov, M. A. Ocherkipo istorii russkogo steklodeliia. Moscow, 1952.
Evstrop’ev, K. S., and N. A. Toropov. Khimiia kremniia i fizicheskaia khimiia silikatov. Moscow, 1950.
Kachalov, N. Steklo. Moscow, 1959.
Batanova, E. I., and N. V. Voronov. Sovetskoe khudozhestvennoe steklo. [Moscow, 1964.]
Bartenev, G. M. Stroenie i mekhanicheskiesvoistva neorganicheskikh stekol. Moscow, 1966.
Tekhnologiia stekla, 4th ed. Moscow, 1967.
Shelkovnikov, B. Russkoe khudozhestvennoe steklo. Leningrad, 1969.
Appen, A. A. Khimiia stekla, 2nd ed. Leningrad, 1974.
Rawson, H. Neorganicheskie stekloobrazuiushchie sistemy. Moscow, 1970. (Translated from English.)
Rozhankovskii, V. F. Steklo i khudozhnik. Moscow, 1971.
Voronov, N. V., and E. G. Rachuk. Sovetskoe steklo. [Leningrad] 1973.
Journal of Glass Studies. (Published since 1959.)
Grover, R., and L. Grover. Contemporary Art Glass. New York .
What does it mean when you dream about a glass?
Glass (in the sense of glass windows rather than a drinking glass) frequently represents the invisible social or emotional barriers we erect between ourselves and others. A dream in which glass breaks can thus mean a breaking down of barriers. (See also Window).
Materials made by cooling certain molten materials in such a manner that they do not crystallize but remain in an amorphous state, their viscosity increasing to such high values that, for all practical purposes, they are solid. Materials having this ability to cool without crystallizing are relatively rare, silica, SiO2, being the most common example. Although glasses can be made without silica, most commercially important glasses are based on it. The most important properties are viscosity; strength; index of refraction; dispersion; light transmission (both total and as a function of wavelength); corrosion resistance; and electrical properties.
Chemically, most glasses are silicates. Silica by itself makes a good glass (fused silica), but its high melting point (1723°C or 3133°F) and its high viscosity in the liquid state make it difficult to melt and work. To lower the melting temperature of silica to a more convenient level, soda, Na2O, is added in the form of sodium carbonate or nitrate, for example. This has the desired effect, but unfortunately the resulting glass has no chemical durability and is soluble even in water (water glass). To overcome this problem, lime, CaO, is added to the glass to form the basic soda-lime-silica glass composition which is used for the bulk of common glass articles, such as bottles and sheet (window) glass. Although these are the main ingredients, commercial glass contains other oxides (aluminum and magnesium oxides) and ingredients to help in oxidizing, fining, or decolorizing the glass batch.
Special kinds of glass have other oxides as major ingredients. For example, boron oxide is added to silicate glass to make a low-thermal-expansion glass for chemical glassware which must stand rapid temperature changes, for example, Pyrex glass. Also, lead oxide is used in optical glass because it gives a high index of refraction.
An Esprit project at the University of Nijmegen.