Glass is an amorphous substance made by heating a mixture of such materials as sand, soda (sodium carbonate), and limestone to a temperature of about 1,300 deg C (2,400 deg F). Temperatures only slightly lower than this can be achieved with a wood fire, which was the traditional fuel for glass melting. Today glass is made continuously in large tanks. Powdered raw materials are fed in at one end, and a molten strip or plate of glass emerges at the other end. The glass is heated by oil or gas flames over the melt, but since these fuels are becoming more expensive, the electrical glass-melting process, which employs metal electrodes in the melt to generate heat, is becoming more widely used. Windows, containers, light bulbs, optical instruments, and many specialty items of glass are an essential part of modern life. HISTORY OF GLASSMAKING Glass was used in Egypt for decorative objects, mainly as a colored glaze on stone or pottery beads, before 3000 BC. The art of making glass was perfected about 1500 BC in Egypt and the Near East. \Tglassblowing\t, which was probably discovered about 50 BC in Phoenicia, greatly extended the types of objects that could be made of glass. It also made them easier to fabricate and more transparent. The art of glassblowing spread rapidly throughout the Roman Empire, while special centers of glassmaking were established in Phoenicia, Rome, Egypt, the Rhineland, and the Rhone Valley. \Tglassware\t became common and relatively inexpensive. For many centuries after the fall of the Roman Empire, glassmaking decreased in importance in Western Europe, as did many other technologies and arts, and artistic glass almost disappeared. In Byzantium, however, Greek and Syrian glass centers continued to prosper. Beginning in the 11th century, several new centers of glassmaking arose in Western Europe. In Bohemia, ash from plants (potash, which is high in potassium) was used as a raw material to make a glass with a lower melting point. The most important European center of glassmaking developed near Venice, where new compositions, colors, forming techniques, and artistic skills were developed. The Venetians added manganese, in the form of the mineral pyrolucite, to oxidize iron impurities in glass, clarifying the glass and removing the green or brown tint caused by the reduced state of iron. By adding lead, borate, and more soda to glass they increased its working temperature range and were able to make more intricate shapes, thinner blown glass, and finer enamels. They also learned to color the glass with special additives. Although the Venetians tried to protect their proprietary knowledge of glassmaking by making it illegal for technicians to emigrate, many escaped anyway and spread the new techniques throughout Europe. Nevertheless Venetian glass was preeminent in Europe until the 18th century. So-called crystal was developed in England in the late 17th century to compete with Venetian cristallo glass. Purer raw materials, oxidation of iron, and addition of lead gave a more transparent glass; this transparency, together with the higher index of refraction resulting from the addition of lead, gave sparkle to faceted cut glass. London became an important center of glassmaking at this time. In the 19th century techniques of glassmaking advanced rapidly. The scientific community's growing need for improved optical glass stimulated the development of manufacturing processes that would strictly control bubbles, stria, refractive index, and color. Michael \TFaraday\t advanced scientific understanding of glass and characterized it as "a solution of different substances, rather than a strong chemical compound," a view that is still valid. Clay pots heated by a wood fire were used to melt glass until the end of the 18th century, when coal and then oil and gas became the preferred fuels. Pot or batch melting of glass is used today only for specialty, laboratory, and certain optical glasses. Production of Flat Glass for Windows Traditionally, window glass was made by hand by either the crown or cylinder process. In the crown process a gob of glass was blown out and one side of the resulting globe was flattened. A solid iron rod was attached to the flat part and the blowing pipe detached. The globe was then reheated and rotated until it formed a flat disc about one meter (3 feet) in diameter. Panes of glass were cut from the disc after it was slowly cooled. The part attached to the rod was the "bull's eye," which can still be seen in some older windows. In the cylinder process the blower made a large cylinder that was then split open and flattened. Cylinder glass was also made by machine. In the early part of the 20th century the Fourcault and Colburn processes for drawing sheet glass directly from the glass melt were developed. When used in conjunction with a continuous glass-melting furnace these processes are capable of producing large quantities of flat glass of reasonable quality. Until recently, plate glass of the highest quality was made by flowing the glass from the furnace through rollers. The rough-surfaced glass is then ground and polished by large automatic machines. This process requires a large capital investment but is economical since it produces large quantities of glass continuously. The ground and polished plate glass is very flat but is more expensive than the sheet glass that is drawn directly from the melt. The sheet glass surface has a fine fire-polished finish, but shows some surface distortion because of variations in processing conditions as the glass is drawn from the melt. In the 1950s an ingenious new method of making relatively inexpensive flat glass of high quality was developed in England by Alistair Pilkington, of the Pilkington Glass Co. In this float process a continuous strip of glass from the melting furnace floats onto the surface of a molten metal, usually tin, at a carefully controlled temperature. The flat surface of the molten metal gives the glass a smooth, undistorted surface as it cools. After sufficient cooling the glass becomes rigid and can be handled on rollers without damaging the surface finish. The glass can be formed at high speeds and is much less expensive to produce than similar quality glass made by grinding and polishing. As a result, many glass manufacturers have converted to the float process, and today most flat glass is made by this process. Production of Glass for Other Uses Glass containers such as \Lbottle\ls and jars are made by blowing hot glass into a mold on a continuous machine. Light bulbs are also made by blowing hot glass into a mold; but in this case the glass is fed from the melting furnace as a ribbon, rather than an individual gob of glass, and a special nozzle blows glass from the ribbon into a mold. This spectacular high-speed ribbon machine produces light bulbs at a rate of more than one every two seconds. Certain glass objects such as plates, tumblers, and vases can be made inexpensively by pressing hot glass in a mold. This pressed glass was especially popular in the United States in the 19th century because it was much cheaper than cut crystal glass imported from Europe. In this method a gob of hot glass is placed in a metallic mold, and a metallic plunger is forced into the mold to form the glass into the desired shape. Patterns in the mold surface are thus pressed onto the glass. Pressed glass can also be made continuously using automatic feeders for molds on a rotating bed. TYPES OF GLASS The most important commercial glasses are the silicates, in which silica sand is a major constituent. A variety of types of silicate glasses are produced for different uses. Soda Lime Because soda-lime glass can be melted at a relatively low temperature, is easy to form, has good chemical durability, and is inexpensive, it accounts for about 90% of all glass produced. A typical commercial soda-lime glass is composed of 72% silica, 15% soda, 5% lime, 4% magnesia, 2% alumina, and 1% boric oxide. Also present are impurities in the raw materials and small amounts of special additives, such as antimony oxide to help remove bubbles from the glass melt. Soda-lime glass has a relatively high thermal expansion coefficient of 9.2 X 10 to the power of -6 per degree Celsius and is thus easily subject to thermal shock. It has a softening temperature of about 700 deg C (1,292 deg F). The softening temperature is technically defined as the temperature at which a rod 0.3 m (1 ft) long and 0.76 mm (0.03 in) in diameter elongates 0.04 in (1 cm) per minute. More simply, it is technically defined as the temperature above which the glass can be readily formed and worked. Soda-lime glass is often called soft glass because of this relatively low softening temperature. Soda-lime glass is used for windows, mirrors, and flat glass of all kinds; for containers such as bottles, jars, and tumblers; for light bulbs and envelopes of many kinds; for decorative objects; and for many other purposes. Pyrex Pyrex (the most common brand of borosilicate glass) was developed at Corning Glass Works to provide thermal and chemical properties superior to those of soda-lime glass while retaining reasonably low melting temperatures (1,300-1,400 deg C/2,370-2,550 deg F). This kind of glass contains about 81% silica, 13% boric oxide, 4% soda, and 2% alumina. It has a low thermal expansion coefficient of 3.3 X 10 to the power of -6 per degree Celsius, giving it good resistance to thermal shock. It is more resistant to chemical attack than soda-lime glass because it contains less alkali (sodium). Pyrex glass has a softening point of about 820 deg C (1,500 deg F) and is sometimes termed hard glass because its softening point is considerably higher than that of soda-lime glass. Pyrex is somewhat more expensive to produce than soda-lime glass because it must be melted at a higher temperature and the borate raw material is relatively expensive. Pyrex borosilicate is used in cooking and laboratory ware, in automobile headlights, and in other applications requiring superior resistance to thermal shock and greater chemical durability. Pyrex was used for the 200-in (5-m) mirror in the Mount Palomar telescope because of its low thermal expansion coefficient, although it is still necessary to correct minute distortions of the mirror surface caused by temperature differences. Lead Silica A variety of lead glasses are important as low-melting sealing and solder glasses and for use in lead crystal glassware. A typical sealing glass is composed of 77% silica, 9% soda, 5% potassium oxide, 8% lead oxide, and 1% lime, giving a thermal expansion coefficient of 9.3 X 10 to the power of -6 per degree Celsius and a softening point of 630 deg C (1,166 deg F). Lead glass for fine crystal contains much more lead. Fused Silica A glass of special interest is fused silica. Because the highest melting point of crystalline silica, cristobalite, is 1,710 deg C (3,110 deg F), fused silica must be melted at a higher temperature. Few materials are suitable as containers at such high temperatures; graphite and refractory metals such as tungsten, which are used commercially for this purpose, are expensive. Although fused silica is quite costly to produce, it is widely used because of its great purity, excellent optical transparency, high temperature and chemical durability, and extreme resistance to thermal shock--it has a thermal expansion coefficient of 0.5 X 10 to the power of -6 per degree Celsius. These properties make fused silica especially appropriate for use as arc tubes in lamps, as crucibles for melting high-purity semiconductors, as optical parts, as containers for high-temperature processing, and, recently, as telescope mirrors. Two mirrors of fused silica in the 5-m (200-in) range have been made; the fabrication of these mirrors in one solid piece at temperatures above 1,800 deg C (3,272 deg F) was a remarkable technological feat. PROPERTIES OF GLASS Structure Glass technology has traditionally defined glass as an inorganic product of fusion which has been cooled to a rigid condition without crystallization (see \Tcrystal\t). But it is also possible to make glass without cooling it from the molten state; for example, it may be deposited from vapor or from a liquid solution such as sodium silicate in water (water glass). Moreover, organic materials such as polymers and even alcohol can be cooled to a glassy state. Thus it seems better to define glass as an amorphous solid, where solid is defined as a rigid material that does not flow when subjected to moderate forces. A material is amorphous when its molecules have no regularity in their arrangement on a scale larger than a few times the size of the molecular constituents. To form glass a liquid must be cooled rapidly enough to prevent its crystallization. Thus viscous liquids, which crystallize slowly, are more likely to form glasses than fluid liquids. If cooled fast enough, materials not normally thought of as glassy, such as some metals and \Lalloy\ls, can be made into glasses. Such amorphous metals were first made by "splat cooling," in which a globule of liquid metal is thrown against a rotating metal plate. The liquid is spread into a thin film that cools very rapidly. Amorphous metals have unusual properties and are being studied intensively for possible applications. The basic structural element of silicate glasses on the atomic level is a three-dimensional network of silicon-oxygen bonds. The atomic level is a three-dimensional network of silicon-oxygen bonds. The bonds are arranged regularly on a short-range scale (a few atomic distances), but at a longer distance there is no regular pattern to the arrangement of atoms. This random network structure contrasts sharply with the regular long-range arrangement of atoms in crystals (see \Tsilica\t AND \Tsilicate minerals\t). As modifying oxides, such as those of sodium and calcium (soda and lime), are added to pure silica, the silicon-oxygen network is progressively broken up, leading to the lower viscosities and lower melting and softening temperatures of glasses containing these oxides. Even in these glasses the silicon-oxygen network is still the basic structural element, providing coherence and strength. Strength Although the network structure of silicate glass is extremely strong, glass itself is very brittle. Brittle materials fracture more readily than their structure leads one to expect because of tiny flaws or cracks in their surfaces. When glass is loaded with a tensile (pulling) force, the force at the tips of these flaws is multiplied many times, and the flaw or crack grows until it breaks. Glass is also subject to fatigue; that is, it becomes weaker when it is loaded for a period of time, as a result of the reaction of water vapor with the glass. In spite of its brittle nature, glass can be strengthened by a process called tempering. Traditionally this has been done by cooling the surface of the glass more rapidly than the interior. The surface becomes rigid first, and when the interior cools and contracts it pulls on the surface, causing a residual compressive stress. This can be done relatively easily and inexpensively during processing, but it is hard to control and often results in nonuniform surface stress. It is commonly used for strengthening glass windows and doors. Chemical tempering is another way to strengthen glass by developing a compressive stress in its surface. In this process smaller sodium ions in the glass are replaced by larger potassium ions. This "stuffing" of a larger ion into the glass causes a compressive stress in the glass surface, and is accomplished by treating the glass in a fused potassium salt, such as potassium nitrate, at about 400 deg C (752 deg F). This process is more expensive than ordinary tempering by rapid surface cooling, but is more easily controlled and leads to a more uniform stress. It is now used for strengthening eyeglass lenses and will probably be extended to other forms of glass as its cost decreases. Electrical Conductivity The electrical conductivity of glass results from the motion of alkali ions, usually sodium, in the glass. These ions are the most mobile because they are the most loosely bonded in the glass structure. Even pure fused silica, which has a concentration of alkali ions of less than one part per million, conducts electricity through the transport of sodium and lithium ions. At room temperature, however, these ions are not very mobile, so glass is a good insulator. At this temperature any electrical conductivity of bulk glass results from surface conductivity. How this surface conductivity is produced remains uncertain, but it probably results from the motions of ions such as sodium in a layer on the glass surface. The surface conductivity is strongly influenced by ambient humidity; in dry air it is low, but in moist air it can be high, especially in glasses containing sodium, such as soda-lime glass (see \Tconduction, electric\t). The electrical (or dielectric) breakdown of glass is related to two factors: surface conductivity and strength. Thus a strong glass with low surface conductivity has the highest breakdown strength and makes the best electrical insulator. Color Glass can be colored by adding particular compounds: chromates for green, copper and cobalt for blue, and manganese for purple. The common green of bottles results from the addition of oxidized iron, and brown is made by adding a combination of iron and sulfur. Very small metal particles in glass can color it deeply; for instance, the addition of gold produces ruby-colored glass. Red glass can also be made by adding copper or selenium. Fluorescent glasses have recently been used in lasers and as optical elements for amplifying laser light; one type of fusion reactor being tested involves many fluorescent glass lenses. The potential optical clarity of fused silica has led to its application as a transmission medium for optical signals in \Tfiber optics\t. Glass used for this purpose must have low optical absorption for lengths up to a mile, and fused silica fibers meeting these optical requirements are now being mass-produced. Impurity and defect levels in such fibers must be very low to prevent optical absorption. These fibers are produced by combining glasses of different refractive indexes (see \Trefraction\t), carefully controlled by special additions; one glass is used as a core and another as a surface cladding. Optical fibers will soon begin to replace metallic cables for long-distance signal transmission. Although glass has found wide technological applications, it is still a valuable artistic and decorative medium, just as it was in ancient Egypt. Technical developments in glassblowing and glass forming, in coloring, in clarity, and in flexibility of working temperature have provided fresh possibilities for expression. Robert H. Doremus Bibliography: Bansal, N.P., and Doremus, R.H., Handbook of Glass Properties (1986); Doremus, Robert H., Glass Science (1973; repr. 1983); Douglas, R. W., and Frank, S., A History of Glassmaking (1972); Izumitani, T.S., Optical Glass (1984); Uhlmann, D.R., and Kreidel, N.J., eds. Glass Science and Technology, 5 vols. (1983-90); Zerwick, C.A., Short History of Glass (1980).