Glass

Glass, an amorphous substance made primarily of silica fused at high temperatures with borates or phosphates. Glass is also found in nature, as the volcanic material obsidian and as the enigmatic objects known as tektites (see Tektite). It is neither a solid nor a liquid but exists in a vitreous, or glassy, state in which molecular units have disordered arrangement but sufficient cohesion to produce mechanical rigidity. Glass is cooled to a rigid state without the occurrence of crystallization; heat can reconvert glass to a liquid form. Usually transparent, glass can also be translucent or opaque. Color varies with the ingredients of the batch.

Tiffany Vase American designer Louis Comfort Tiffany helped popularize the art nouveau style with its elongated, curving, plantlike forms, in the United States in the late 19th century. This vase is an example of Favrile glass, a silky, opalescent glass that Tiffany developed.Victoria & Albert Museum, London, UK/Bridgeman Art Library, London/New York

Molten glass is plastic and can be shaped by means of several techniques. When cold, glass can be carved. At low temperatures glass is brittle and breaks with a shell-like fracture on the broken face. Such natural materials as obsidian and tektites (from meteors) have compositions and properties similar to those of synthetic glass.


Glass was first made before 2000 bc and has since served humans in many ways. It has been used to make useful vessels as well as decorative and ornamental objects, including jewelry. Glass also has architectural and industrial applications.

II MATERIALS AND TECHNIQUES

The basic ingredient of glass compositions is silica, derived from sand, flint, or quartz.

A Composition and Properties

Silica can be melted at very high temperatures to form fused silica glass. Because this glass has a high melting point and does not shrink or expand greatly with changing temperatures, it is suitable for laboratory apparatus and for such objects subject to heat shock as telescope mirrors. Glass is a poor conductor of both heat and electricity and therefore useful for electrical and thermal insulation. For most glass, silica is combined with other raw materials in various proportions. Alkali fluxes, commonly the carbonates of sodium or potassium, lower the fusion temperature and viscosity of silica. Limestone or dolomite (calcium and magnesium carbonates) act as stabilizers for the batch. Other ingredients such as lead and borax give to glass certain physical properties.

A1 Water Glass and Soda-Lime Glass

Glass of high soda content can be dissolved in water as a syrupy fluid. Known as water glass, it is used commercially for fireproofing and as a sealant. Most manufactured glass is a soda-lime composition used to make bottles, tableware, lamp bulbs, and window and plate glass.

A2 Lead Glass

Sayings of Confucius The Sayings of Confucius (1956), a contemporary glass sculpture by Donald Pollard and Cho Chung-Yeng, was produced by the Steuben Glass Company. This sculpture echoes the shapes of natural crystal, as well as the shapes of Chinese fans. The Chinese characters are etched into the glass.John White/Cooper Hewitt/National Museum of Design/Smithsonian/Art Resource, NY

The fine-quality table glass known as crystal. is made from potassium-silicate formulas that include lead oxide. Lead glass is heavy and has an enhanced capacity to refract light, which makes it suitable for lenses and prisms, as well as for imitation jewels. Because lead absorbs high-energy radiation, lead glasses are used in shields to protect personnel in nuclear installations.

A3 Borosilicate Glass

Borosilicate glass contains borax as a major ingredient, along with silica and alkali. Noted for its durability and resistance to chemical attack and high temperatures, borosilicate glass is widely employed for cooking utensils, laboratory glassware, and chemical process equipment.

A4 Color

Impurities in the raw materials affect the color of glass. For a clear, colorless substance, glassmakers add manganese to counteract the effects of iron traces that produce greens and browns. Glass can be colored by dissolving in it metallic oxides, sulfides, or selenides. Other colorants may be dispersed as microscopic particles.

A5 Miscellaneous Ingredients

Typical glass formulas include broken waste glass of related composition (cullet), which promotes melting and homogenization of the batch. Fining agents such as arsenic or antimony are often added to cause the release of small bubbles during the melting.

A6 Physical Properties

Depending on the composition, some glass will melt at temperatures as low as 500° C (900° F); others melt only at 1650° C (3180° F). Tensile strength, normally between 280 and 560 kg per sq cm (4000 and 8000 lb per sq in), can exceed 7000 kg per sq cm (100,000 lb per sq in) if the glass is specially treated. Specific gravity ranges from 2 to 8, or from less than that of aluminum to more than that of steel. Similarly wide variations occur in optical and electrical properties.

B Mixing and Melting

After careful preparation and measurement, the raw materials are mixed and undergo initial fusion before being subjected to the full heat needed for vitrification. In the past, melting was done in clay pots heated in wood- or coal-burning furnaces. Pots of fireclay, holding from 0.5 to 1.5 metric tons of glass, are still used when relatively small amounts of glass are needed for handworking. In modern glass plants, most glass is melted in large tank furnaces, first introduced in 1872, that can hold more than 1000 metric tons of glass and are heated by gas, oil, or electricity. The glass batch is fed continuously into an opening (doghouse) at one end of the tank, and the melted, refined, and conditioned glass is drawn out the other end. In long forehearths, or holding chambers, the molten glass is brought to the correct working temperature, and the vitreous mass is then delivered to the forming machines.

C Shaping

When working glass in its plastic state, five basic methods are employed to produce an almost limitless variety of shapes: casting, blowing, pressing, drawing, and rolling.

C1 Casting

In this process, known to the ancients, molten glass is simply poured into a mold and allowed to cool and solidify. In modern times centrifugal casting processes have been developed in which the glass is forced against the sides of a rapidly rotating mold. Capable of forming precise, lightweight shapes, centrifugal casting is used for the production of television-tube funnels.

C2 Glassblowing

Creating Glass Vessels These men are using the method of glassblowing to create glass vessels. The man on the left is sitting in a chair with a support for his blowpipe. He has already created the initial rough shape and is now refining the molten glass, using a pair of tongs, or jacks. The process he is using is called necking, the technique by which the glassblower creates the neck of a bottle or vase.Roger A. Clark, Jr./Photo Researchers, Inc.

The revolutionary discovery that glass could be insufflated and expanded to any shape was made in the third quarter of the 1st century bc, in the Middle East along the Phoenician coast. Glassblowing soon spread and became the standard way of shaping glass vessels until the 19th century. The necessary tool is a hollow iron pipe about 1.2 m (about 4 ft) long with a mouthpiece at one end. The glassblower, or gaffer, collects a small amount of molten glass, called a gather, on the end of the blowpipe and rolls it against a paddle or metal plate to shape its exterior (marvering) and to cool it slightly. The gaffer then blows into the pipe, expanding the gather into a bubble, or parison. By constantly reheating at the furnace opening, by blowing and marvering, the gaffer controls the form and thickness. Simple hand tools such as shears, tongs (pucellas), and paddles are used to refine the form, often while the glassblower sits in the special “glassmaker's chair,” one with extended arms to support the blowpipe. Blown glass can also be shaped with molds: Part-size molds pattern the gather, which is then removed and blown to the desired size. Full-size molds into which the gather is entirely blown impart size, shape, and decoration. Additional gathers may be applied and manipulated to form stems, handles, and feet, or they may be trailed on and tooled for decoration. A shaped bubble can be “flashed” with color by dipping it into molten glass of contrasting color. To make cased glass, a gather is placed within, and fused to, one or more layers of differently colored glass. For finish work and fire polishing at the mouth of the furnace, the gather is transferred to a solid iron rod called a pontil, applied opposite the blowpipe, which is then removed. When the pontil is cracked off it leaves a “pontil mark” that may be later ground or polished away.

In 1903 a fully automatic blowing machine was perfected, thereby making mechanical glassblowing possible.

C3 Pressing

Some pressing was involved in the production of ancient cast wares to ensure that the glass had full contact with the mold. Islamic artisans used simple handpresses to form glass weights and seals. European manufacturers rediscovered the technique in the late 18th century, using it to make decanter stoppers and the bases of stemmed tableware. In the 1820s patents were taken out, particularly in the U.S., that led to the development of fully mechanical pressing. In this process, a gather of glass is dropped into a mold, and a plunger then squeezes the glass between itself and the outer mold and forms the final shape. Both the mold and the plunger may be patterned to impart decorative design to the object being made.

C4 Drawing

Molten glass can be drawn directly from the furnace to make tubing, sheets, fibers, and rods of glass that must have a uniform cross section. Tubing is made by drawing out a cylindrical mass of semifluid glass while a jet of air is blown down the center of the cylinder.

C5 Rolling

Sheet glass, and plate glass in particular, was originally produced by pouring molten glass on a flat surface and, with a roller, smoothing it out prior to polishing both its surfaces. Later it came to be made by continuous rolling between double rollers.

D Lampworking

Lampworking consists of the reworking of preformed and annealed glass, generally to produce scientific laboratory equipment and decorative toys and figures. Rods and cylinders are reheated by air-gas or oxygen-gas flames and refashioned by hand or machine.

E Annealing

After being formed, glass objects are annealed to relieve stresses built up within the glass as it cools (see Annealing). In an oven called a lehr, the glass is reheated to a temperature high enough to relieve internal stresses and then slowly cooled to avoid introducing new stresses. Stresses can be added intentionally to impart strength to a glass article. Because glass breaks as a result of tensile stresses that originate across an infinitesimal surface scratch, compressing the surface increases the amount of tensile stress that can be endured before breakage occurs. A method called thermal tempering introduces surface compression by heating the glass almost to the softening point and then cooling it rapidly with an air blast or by plunging it into a liquid bath. The surface hardens quickly; the subsequent contraction of the slower-cooling interior portions of the glass pulls the surface into compression. Surface compressions approaching 2460 kg per sq cm (35,000 lb per sq in) can be obtained in thick pieces by this method. Chemical strengthening methods also have been developed in which, through an ion-exchange process, the composition or structure of the glass surface is altered and surface compression introduced. Strengths exceeding 7000 kg per sq cm (100,000 lb per sq in) can be attained by chemical strengthening.

F Decoration

Decorating Glass There are many different techniques for decorating glass. Some of these, such as cutting or engraving, are more often used on three-dimensional pieces. Painting or etching are usually used for flat glass, such as decorative windows or plates. Other techniques include gilding and sandblasting. The pieces here are an etched glass plate (left), a faceted cut glass vase (center), and a painted glass roundel (right).Victoria & Albert Museum, London;Beniaminson/Art Resource, NY;Aldo Tutino/Art Resource, NY

After annealing, a glass object may be embellished in a number of ways. Some of them are as follows:

In cutting, to produce cut glass, facets, grooves, and depressions are ground into the surface with rotating disks of various materials, sizes, and shapes and a stream of water with an abrasive. The steps are marking the pattern, rough cutting, smoothing, and polishing.

Designs are engraved by means of a diamond point or a metal needle, or with rotating wheels, generally of copper.

In the etching process intaglio decoration is achieved with acid, the results varying from a rough to mat finish.

In sandblasting, fine grains of sand, crushed flint, or powdered iron are projected at high speed onto the glass surface, leaving a design in mat finish.

In cold painting, lacquer colors or oil paints are applied to glass but are not affixed by firing.

In enamel painting, enamel colors are painted and then fused onto the surface in a low-temperature firing.

In gilding, gold leaf, gold paint, or gold dust is applied to glassware and sometimes left unfired; low-temperature firing, however, is necessary for permanency.

III GLASS AS AN ART FORM

Archaeological evidence indicates that glass was first made in the Middle East, sometime in the 3rd millennium bc.

A Ancient Glass

The earliest glass objects were beads; hollow vessels do not occur before about 1500 bc. Asian artisans may have established the glass industry in Egypt, where the first vessels date from the reign (1479-1425 bc) of Thutmose III. Glass production flourished in Egypt and Mesopotamia until about 1200 bc, then virtually ceased for several hundred years. In the 9th century bc, Syria and Mesopotamia emerged as glassmaking centers, and the industry spread throughout the Mediterranean region. In the Hellenistic era, Egypt, because of the glassworks at Alexandria, assumed a leading role in supplying royal courts with luxury glass. It was on the Phoenician coast, however, that the important discovery of glassblowing occurred in the 1st century bc. In the Roman period glassmaking was undertaken in many areas of the empire, from Rome to Cologne.

A1 Early Techniques

Phoenician Glass Flask This Phoenician glass unguent flask from the 5th century bc was made using the core technique, which preceded glassblowing. A mixture of clay and dung was attached to a rod and formed into the shape of the flask. Hot threads of glass were wound around the form, smoothed, reheated, then wound with more glass. This process was repeated until the vessel was finished, and handles, a foot, and a lip were added. The rod, along with the core material, was then removed.Bridgeman Art Library, London/New York

Before the invention of the blowpipe, several methods existed for shaping and embellishing objects of colored glass, both translucent and opaque. Some articles were carved from solid blocks of glass. From potters and metalworkers glassmakers adapted casting processes, pouring molten glass into molds to produce inlays, statuettes, and open vessels such as jars and bowls. Preformed rods of glass could be heated and fused together in a mold for a “ribbon” glass. Patterns of great complexity were achieved by a mosaic technique, in which elements, fused in a rod, together made a design in cross section. Slices of such rods could be arranged in a mold to shape a vessel or plaque and then heated to fusion. “Gold band” glasses featured irregular bands of different colored glass, with gold leaf embedded in one translucent band.

The majority of pre-Roman glasswares were fashioned by the core technique. A mixture of clay and dung was fixed to a metal rod and given the internal shape of the desired vessel. It was dipped into a crucible of molten glass or was wound with threads of glass. The object was constantly reheated and smoothed on a flat stone. Threads of different colored glass were trailed on and combed, creating striking feather patterns, as seen on Egyptian glass of the 18th and 19th dynasties. Handles, feet, and the neck were added and the object cooled. The rod was withdrawn and the core material picked out. Only vessels of limited size, such as cosmetic containers and small vases, could be made this way. Later core-formed articles from the 6th century bc closely followed the forms of Greek pottery (see Pottery).

A2 Roman Glass

Roman Glassware These examples of early Roman glassware date from the 1st and 2nd centuries, when colorless glass had become more popular than opaque and colored styles. The four pieces on the left are burial pieces; the piece on the right was probably used as a jug for water or wine.Bridgeman Art Library, London/New York

Glassblowing, a less expensive and time-consuming method of manufacture, spread from Syria to Italy and other parts of the Roman Empire, gradually superseding the old techniques. A new taste in glass styles developed: The earlier manufacturing processes emphasized color and pattern; blowing enhanced the thin, translucent qualities of the material. Also, by the end of the 1st century ad, colorless glass supplanted colored glass as the most fashionable sort. Glassblowing made large-scale production possible and changed the status of glassware to an everyday material, used for windows, drinking vessels, and containers of all kinds.

The structure of the empire doubtless fostered the extraordinary developments in glassmaking that occurred in this period. Most of the known decorative techniques were invented by artisans of the Roman era. Blown glasswares were patterned in part and full-size molds. Such molds enabled novelty items such as head-shaped flasks to be produced in quantity. A delicately patterned ewer (1st century ad) in the Corning Museum of Glass, Corning, New York, is one of a remarkable group of mold-blown objects that bear the names of their makers. Some Roman glass has elaborately threaded and tooled decoration. Glasswares could be painted with religious and historical scenes, or could feature designs in gold leaf pressed between two layers of clear glass. Ancient glassmakers adapted lapidary skills to make lathe-cut, carved, and engraved glass of considerable beauty. In cameo glass, layers of different colored glass were fused together and then carved so as to leave contrasting motifs in relief. Best known of Roman cameo glass is the Portland Vase (1st century ad, British Museum, London), which depicts the myth of Peleus and Thetis. Delicate effects were achieved in the diatreta, or caged cups, in which great portions of the outer surface were cut away, leaving an intricate openwork frame that appears to stand almost free of the vessel within. The famous Lycurgus Cup (4th century ad, British Museum) epitomizes this practice.

B Western Glass

The manufacture of household glass suffered a general decline in the West with the fall of the Roman Empire.

B1 Medieval Glass

Rose Window, Notre Dame The north rose window of the Cathedral of Notre Dame, Paris, was built by Jean de Chelles from 1240 to 1250. It is 129 m (43 ft) in diameter and consists of brilliantly colored pieces of glass with lead around each piece, held in an iron framework. The details in the religious scenes are painted on the glass.Bridgeman Art Library, London/New York

Under Frankish influence glassmakers in northern Europe and Britain continued to produce utilitarian vessels, some of new, robust forms. The decoration of these objects was limited to simple molded patterns, threading, and applied ornaments such as prunts (blobs of glass). Mostly green in color, the glass was at first a soda-glass composition made with ashes of marine plants imported from the Mediterranean, as they had been during Roman times. By the late Middle Ages, however, soda was no longer available, and northern glassmakers turned to the wood ash from their own wood-fired furnaces as a flux, for a potash-lime glass. Because the glasshouses were situated in the forests that provided fuel and ash, the glass made was called forest glass, waldglas. Common glass in the waldglas style continued to be made in the lesser European factories until modern times.

The glory of Western glassmaking in the medieval period, through patronage of the church, was mosaic glass in Mediterranean Europe and stained-glass windows in the north (see Mosaics; Stained Glass). Mosaics were made of small glass cubes, or tesserae, embedded in cement. The tesserae, cut from solid cakes of glass, could be extremely elaborate, with gold and silver lead inlaid. Little is known of the production of mosaic glass before the 14th century.

Glass windows in churches are mentioned in documents as early as the 6th century, but the earliest extant examples date from the 11th century. The finest windows are considered those from the 13th and 14th centuries, primarily in France and England. Glasshouses in Lorraine and Normandy (Normandie) may have provided much of the flat glass for medieval cathedral windows. The glass was colored, or flashed with color, and then cut into the shapes required by the design. Details were painted into the glass, often with a brownish enamel. The pieces were fitted into lead strips and set in an iron framework. The art declined in the late Renaissance but was revived in the 19th century.

B2 Renaissance to the 18th Century

Venetian Glass Ewer This fanciful boat-shaped ewer is an example of Venetian glass from the mid-16th century. It is made of cristallo glass, which gets its name from its resemblance to rock crystal. The design of this piece is attributed to Armenia Vivarini and combines blue glass with stamped designs and gold work.Bridgeman Art Library, London/New York

Although glassmaking was practiced in Venice from the 10th century on, the earliest known Venetian glassware dates from the 15th century. Concentrated on the island of Murano, the Venetian industry dominated the European market until 1700. The major contribution of the Venetians was the development of a highly refined, hard-soda glass of great ductility. Colorless and highly transparent, the glass resembled rock crystal and was known as cristallo.

The first cristallo wares were simple forms, often embellished with jewel-like enamel designs. Objects were also blown of colored and opaque glass. By the late 16th century, forms became lighter and more delicate. The blowers exploited the workable nature of their material to produce fanciful tours de force. A type of filigree glass was developed in Venice and widely imitated. With lacelike effect, opaque white threads were incorporated in the glass and worked into intricate patterns. Some vessels were blown entirely of opaque white glass and painted with enamels in the manner of Chinese porcelain. Novelties made of lampworked glass were made at Murano, but Nevers, France, became most famous for this type of ware by the 17th century. Particularly suited to soda glass was the practice of diamond-point engraving, a technique favored in the 17th century by Dutch artisans. By hammering the diamond-point stylus for a stippled effect, they created ambitious pictorial designs.

Glass manufacturers throughout Europe tried to copy the Venetians in their production methods, materials, and decorative vocabulary. Knowledge was spread through the glasswares themselves, through the Art of Glass (1612) by Antonio Neri, and through Venetian glassblowers. Although forbidden by law to leave Venice and to divulge the secrets of their craft, many Murano glassmakers left Italy to set up glasshouses elsewhere in Europe. Each country developed its own façon de Venise, as nationalistic preferences for certain forms or decorations tempered the Venetian model. Italy's influence was ultimately weakened in the 17th century by the development of new glass recipes in Germany and England.

Germany's potash-lime glass, thicker and harder than cristallo, was well suited to wheel-engraved decoration. Caspar Lehmann, at the court of Holy Roman Emperor Rudolf II in Prague, was largely responsible for the development of engraving in the early 1600s. Glass cutters and engravers in Nürnberg and Potsdam became famous for skillfully executed designs in the baroque manner. At the same time, the Germanic glasshouses continued their tradition of enameled and cold-painted glass.

The other improvement in glass that served to diminish Europe's reliance on Venice was the lead-oxide glass formulated (circa 1676) by George Ravenscroft in England. Softer, more brilliant, and more durable than the brittle cristallo, English lead glass was considered the finest glass of the 18th century. English table glass dominated the European and colonial markets and became a model for Continental production. English innovations of the mid-18th century were glasses with air or opaque-enamel twists encased in the stems. Among the most prestigious forms of the period was the English cut-glass chandelier. Lead glass, especially suited to cutting, reached its full potential in the neoclassical wares of the Anglo-Irish period (1780-1830).

B3 American Glass

Glassmaking was the first manufacture undertaken in America, with a glasshouse built at Jamestown, Virginia, in 1608. The first commercially successful glassworks was that of Caspar Wistar in Salem County, New Jersey, between 1739 and 1777. Immigrant German artisans there and at other factories produced bottles, windowpanes, and some table glass in Germanic styles. Henry William Stiegel sought to imitate English imported lead glass at his factory in Lancaster County, Pennsylvania, from 1763 to 1774. The most important glassworks built after the American Revolution was that of John Frederick Amelung in Frederick County, Maryland, which was in operation from 1784 to 1795.

B4 19th and 20th Centuries

Art Glass by Dale Chihuly Dale Chihuly, a leader in the art-glass movement in the United States, created these decorative glass pieces: Niijima Floats: Garnet Black and Mint Green Float with Dimple (1991); Niijima Floats: Snow White and Gold Leaf (1991); and Niijima Floats: Mottled Blue Black Float with Silver Leaf (1992). All are in the Smithsonian American Art Museum, Smithsonian Institution, in Washington, D.C.Smithsonian American Art Museum/Art Resource, NY

The stylistic history of glass in the 19th century is dominated by rapid advances in glass technology and by the rediscovery and adaptation of older methods.

Mechanical pressing, introduced in the U.S., was a cheap, fast means of production that greatly expanded the role of glass in the home and in industry. Before 1850, wares were pressed in intricate lacy designs that offset a cloudiness in the glass caused by contact with the cooler mold. Simpler designs popular from the 1840s on, known as pressed pattern glass, were available in many forms. The more expensive cut glass declined in favor because of the competition from pressed glass. Only about 1880 did cut glass regain some of its earlier popularity with the elaborate “brilliant” patterns, examples of great technical virtuosity that exploited the refractive properties of quality glass.

Beginning in the late 18th century, a number of Roman glassmaking techniques were revived and modified to suit neoclassical taste. Continental glass factories made a version of laminated gold-leaf glass, called zwischengoldglas. Cameolike effects were attained with encrusted sulphides, and actual cameo engraving and cutting were practiced by artisans beginning in midcentury, culminating in the work of Thomas Webb and Sons (founded 1837), a glasshouse in Stourbridge, England. Paperweights, popular from about 1845, were often made in a millefiori (thousand flowers) design recalling the mosaic glass of ancient times. Renaissance rock crystal inspired a technique of polished engraved glass in the late 19th century.

Bohemia continued to excel in wheel-engraved decoration with the work of such artisans as Dominik Biemann. Other methods, such as cased glass, were practiced in Bohemian factories and copied throughout Europe and the U.S. Chemical advancements led to new opaque colored glass such as lithyalin, which resembled semiprecious stones. Transparent enamels and stains were applied to vessels, paralleling the revival of stained-glass windows.

Inspired by the revivals of historical glassworking methods and spurred by the capabilities of improved chemical technology, glassmakers by 1880 were creating new styles of handworked glass, generally called art glass. These were mostly decorative and novelty forms, made in reaction to mass-produced wares. Between 1890 and 1910 the most fashionable styles reflected the international art nouveau movement. Louis Comfort Tiffany in the U.S., and Émile Gallé and the firm of Daum Frères (founded 1889) in France, were the leading proponents of the style. They produced glasses of naturalistic shapes, sinuous lines, exotic colors, and unusual surface effects, such as Tiffany's iridescent Favrile glass.

After World War I new interests in texture and formalized decoration emerged, seen in the designs of René Lalique and Maurice Marinot. Beginning in the 1930s, exquisitely clear, colorless lead glass, often engraved, was popularized by several Scandinavian and American firms.

A new era in glassmaking began in the early 1960s with the studio glass movement, led by the Americans Harvey Littleton and Dominick Labino. With small tank furnaces in studio settings, artisans explore glass as an artistic medium. Innovative sculptural forms and decorative techniques are being developed at workshops in the U.S. and around the world.

C Non-Western Glass

Glassmaking was not as strong a tradition in Islamic and Far Eastern countries as it was in the West. Forms and techniques developed that closely reflected their individual cultures; these, in turn, influenced Western forms.

C1 Islamic Glass

Mosque Lamp This mosque lamp from the early 14th century is an example of the kind of glasswork that had been produced in Islamic countries for 500 years. This piece features enameled decoration and is inscribed with three quotations from the Koran. It is dedicated to Beybars II.

The history of glass from the 8th through the 14th century focuses on the Islamic world of the Middle East. Earlier Sassanian traditions of carved glass were continued by Muslim artisans, who made high-relief cut (hochschnitt) vessels, many with animal subjects. Quality colorless glass with fine wheel-engraved designs was also produced. The possibilities for decoration were expanded with the introduction of fired-on enamel colors and gilding, techniques for which the glasshouses at ?alab (Aleppo) and Damascus were famous. From Egypt came the discovery of luster stains, which created lustrous metallic effects in browns, yellows, and reds on both pottery and glass. Mosque lamps, bowls, beakers, and bottles were painted in the rhythmic, geometric patterns of Islam. Their shapes and decorations influenced later Western production, particularly in Venice and Spain.

C2 Indian Glass

Glass was made in India as early as the 5th century bc, but the industry was not established until the Mughal period, and particularly in the 17th century. Forms included hookah bases, sprinklers, and dishes, usually gilded or enameled in floral patterns. In the 18th century the English East India Company sold quantities of English glass to the Indian market, some of which was then wheel-engraved by Indian artisans.

C3 Far Eastern Glass

Giant Carved Bottle, Ch'ien Lung Dynasty This bottle from the Ch’ien Lung Dynasty (1736-95) is typical of the style being produced at that time in China. It features a simple shape, with intricate scenes of Chinese life carved in red glass on top of the white glass surface. The elaborate design is a reaction to the simplicity of the Sung style that preceded it.Bridgeman Art Library, London/New York

Chinese-made glassware in the distinctive “eye bead” form, with inlays resembling eyes, has been excavated from Zhou (Chou) dynasty sites (1045?-256 bc). Early glass objects, often melted from imported preformed glass cakes, were small and were carved in close imitation of gemstones. The use of glass to simulate semiprecious stones for jewelry and later for snuff bottles is a recurring theme in Chinese glass. Few vessels of glass are known before the glassworks at the Beijing Imperial Palace was erected in 1680. Under the influence of the Jesuits at the Beijing court, blown glass vessels in Western European styles were produced. Glass in the Chinese idiom dominated 18th- and 19th-century production, however, featuring richly colored objects with carved and enameled decoration. The Chinese mastered the art of cameo cutting in glass. Chinese glass vessels are characteristically of simple, porcelain-inspired shapes, with thick, often multilayered walls and a waxy surface sheen.

No evidence exists of glass made in Japan before 200 bc. Some glass vessels in the forms of Buddhist relic bottles and cinerary urns are believed to date from the Asuka/Nara periods (ad 552-784), but glassmaking apparently ceased in the 13th century. The craft was revived about 1750.

IV TYPES OF COMMERCIAL GLASS

The wide range of uses of the material has resulted in the development of a number of different types of glass.

A Window Glass

Window glass, in use since the 1st century ad, was originally made by casting, or by blowing hollow cylinders that were slit and flattened into sheets. The crown process was a later technique, in which a gather of glass was blown and shaped into a flattened globe or crown. The pontil rod was attached to the flat side, the blowpipe removed. By spinning the reheated crown on the rod, the hole left by the blowpipe enlarged, and eventually the disk, through centrifugal force, flapped out in a large circular sheet. The pontil rod was cracked off, leaving a scar, or bull's-eye. Today, nearly all window glass is made mechanically by drawing glass upward from a molten pool fed from a tank furnace. In the Fourcault process the glass sheet is drawn through a slotted refractory block submerged in the surface of the glass pool, into a vertical annealing furnace from which it emerges to be cut into sheets.

B Plate Glass

Ordinary drawn window glass is not entirely uniform in thickness because of the nature of the process by which it is made. The variations in thickness distort the appearance of objects viewed through panes of the glass.

The traditional method of overcoming such defects has been the use of ground and polished plate glass. Plate glass was first produced at Saint Gobain, France, in 1668, by pouring glass into an iron table and rolling it flat with a roller. After annealing, the plate was ground and polished on both sides. Plate glass is now made by rolling the glass continuously between double rollers located at the end of a forehearth. After the rough sheet has been annealed, both sides of it are finished continuously and simultaneously.

Grinding and polishing are now being supplanted by the cheaper float-glass process. In this process flat surfaces are formed on both sides by floating a continuous sheet of glass on a bath of molten tin. The temperature is high enough to allow the surface imperfections to be removed by fluid flow of the glass. The temperature is gradually lowered as the glass moves along the tin bath, and the glass passes through a long annealing oven at the end.

Unpolished rolled glass, often with figured surfaces produced by designs incised in the rolls, is used architecturally. Wire glass, made by introducing wire mesh into the molten glass before it passes between the rollers, is used to prevent the glass from shattering if it is struck. Safety glass, for automobile windshields, is made by laminating a sheet of transparent polyvinyl butyral plastic between two sheets of thin plate glass. The plastic adheres tightly to the glass and holds the broken shards in place even after hard blows.

C Bottles and Containers

Bottles, cosmetic jars, and other glass containers are produced by an automatic process that combines pressing (to form the open end of the container) and blowing (to form the hollow body of the container). In a typical automatic bottle-blowing machine, a gob of molten glass is dropped in a narrow, inverted mold and forced down by an air blast into the lower portion of the mold, which corresponds to the neck of the finished bottle. A baffle then drops over the top of the mold, and a blast from the bottom, up through the neck, partly forms the bottle. The half-formed bottle, called a parison, is held by the neck, inverted, and then lowered into a second finishing mold, in which another air blast blows it out to its finished dimensions. In another type of machine, used for large-mouthed containers, the parison is simply pressed in a mold by a plunger before being blown in a finishing mold. Shallow jars, such as those used for cosmetics, are merely pressed.

D Optical Glass

Most lenses used in eyeglasses, microscopes, telescopes, cameras, and certain other optical instruments are made from optical glass (see Lens; Optics). Optical glass differs from other glass in the way in which it bends, or refracts, light. The manufacture of optical glass is a delicate and exacting operation. The raw materials must be of the highest purity, and great care must be taken so that no imperfections are introduced in the manufacturing process. Small air bubbles and inclusions of unvitrified matter will cause distortion on the surface of the lens. Striae, the streaks caused by incomplete chemical homogeneity in the glass, will also cause serious distortion, and strains in glass caused by improper annealing will further impair optical qualities.

Optical glass was originally melted in pots for prolonged periods, during which it was constantly stirred by a refractory rod. After a lengthy annealing, the glass was broken into pieces. The best fragments were further reduced, reheated, and pressed into the desired forms. In recent years a method has been adopted for the continuous manufacture of glass in platinum-lined tanks, using platinum-lined stirrers in the cylindrical end chambers (or homogenizers). This process produces greater quantities of optical glass that are cheaper and superior to glass produced by the earlier method. (Plastics are increasingly used in place of optical glass for simple lenses: Although not as durable and scratch resistant as glass, they are strong and lightweight and can absorb dyes.)

E Photosensitive Glass

Photosensitive glass is similar to photographic film in that gold or silver ions in the material will respond to the action of light. This glass is used in printing and reproducing processes. Heat treatment following an exposure to light produces permanent changes in photosensitive glass.

Photochromic glass darkens when exposed to light but fades to its original clear state when the light is removed. This behavior is achieved by the action of light on extremely small silver chloride or silver bromide crystals distributed throughout the glass. Photochromic glass finds a natural use in spectacle lenses that darken into sunglasses when in the sun and lighten again when removed from sunlight. The field of electronics also finds uses for this kind of glass.

F Glass Ceramics

Glass containing certain metals will form a localized crystallization when exposed to ultraviolet radiation. If heated to high temperatures, the glass will convert to crystalline ceramics with mechanical strength and electrical insulating properties greater than that of ordinary glass. Such ceramics are now made for such uses as cookware, rocket nosecones, and space-shuttle tiles. Other metallic glasses—including alloys of pure metals—can be magnetized, are strong and flexible, and prove very useful in high-efficiency electrical transformers.

G Glass Fibers

It is possible to produce fibers that can be woven or felted like textile fiber by drawing out molten glass to diameters of a few ten-thousandths of an inch. Both long, continuous multifilament yarns and short-staple fibers 25 to 30 cm (10 to 12 in) long may be produced.

Woven into textile fabrics, glass fibers make excellent drapery and upholstery materials because of their chemical stability, strength, and resistance to fire and water. Glass fabrics alone, or in combination with resins, make excellent electrical insulation. By impregnating glass fibers with plastics, a composite fiberglass is formed that combines the strength and inertness of glass with the impact resistance of the plastic.

H Miscellaneous Types of Glass

Fiber Optic Strands A strand of fiber optic cable reflects the light that passes through it back into the fiber, so light cannot escape the strand. Fiber optic cables carry more information, suffer less interference, and require fewer signal repeaters over long distances than wires.James L. Amos/Corbis

Glass bricks are hollow construction blocks with ribbed or patterned sides that can be laid in mortar and used for exterior walls or interior partitions.

Foam glass, used in floats or as insulation, is made by adding a foaming agent to finely ground glass and heating the mixture to the softening point. At that point the foaming agent releases a gas that produces a multitude of small bubbles within the glass.

In the 1950s glass optical fibers (see Fiber Optics) were developed that have many uses in science, medicine, and industry. High-refractive-index glass fibers, laid parallel to one another and separated by thin layers of low-refractive-index glass, can be optically worked as a lens. Fiberscopes incorporating such bundles can transmit an image through acute angles, thus easing the examination of normally inaccessible sites. Such solid fiber-optics applications as magnifiers, minifiers, and faceplates also improve viewing. When used in conjunction with lasers, optical fibers are also proving important in the development of various communications systems (see Telephone). A new kind of glass called halide glass, discovered in the 1970s, may prove especially useful for this application. It is made of a halide, such as fluorine, combined with a heavy metal, such as zirconium, barium, or hafnium.

Laser glass is doped, or mixed, with several percent of neodymium oxide and is capable of emitting laser light if the glass is pumped and assembled in the proper device. It is considered a good laser source because of the relative ease with which large, homogenous specimens of the glass can be obtained for extremely high-powered generation.

Double-glazing cells are units in which two sheets of plate or window glass are sealed together at their edges, leaving an air space between. Various types of seals and spacing materials may be used in their construction. As windows they provide superior heat insulation and will not cloud over in moist air.

A method for making large glass structures without using high temperatures was developed in the 1980s at the University of Florida. Called the sol-gel technique, it mixes water with a chemical such as tetramethoxysilane to produce a silicon oxide polymer; a chemical additive slows down the condensation process and allows the polymer to build up uniformly. The method may prove useful for making large, complex shapes with specific properties.

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