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some glass facts
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Glass (art), an amorphous substance made primarily of silica fused with alkaline at a high temperature. The silica is generally obtained from sand, quartz or flint, and the alkali is generally soda-ash (obtained from seaweed) or potash (obtained from brushwood). To these fundamental materials, other ingredients are added to obtain different effects. The addition of lead, for example, produces glass of a distinctive clarity and brilliance. Glass can also be coloured by the addition of sulphides or metallic oxides.
Glass was first made before 2000 BC and has been used for a range of decorative as well as practical purposes. Its use as jewellery, vessels, and ornamental objects, and as window glass, will be considered here. For the industrial applications of glass, see Glass (industry).
Shaping
In its molten state, glass can be shaped by casting, blowing, pressing, drawing, and rolling.
Casting
In casting, known since ancient times, molten glass is simply poured into a mould and allowed to cool and solidify.
Glassblowing
The revolutionary discovery that glass could be blown 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 (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 moulds: part-size moulds pattern the gather, which is then removed and blown to the desired size. Full-size moulds 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 colour by dipping it into molten glass of contrasting colour. To make cased glass, a gather is placed within, and fused to, one or more layers of differently coloured 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.
Pressing
Some pressing was involved in the production of ancient cast wares to ensure that the glass had full contact with the mould. 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 United States, that led to the development of fully mechanical pressing. In this process, a gather of glass is dropped into a mould, and a plunger then squeezes the glass between itself and the outer mould and forms the final shape. Both the mould and the plunger may be patterned to impart decorative design to the object being made.
Drawing
Molten glass can be drawn directly from the furnace to make tubing, sheets, fibres, 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 centre of the cylinder.
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.
Lampworking
Lampworking consists of the reworking of preformed and annealed glass, generally to produce decorative toys and figures. Rods and cylinders are reheated by air-gas or oxygen-gas flames and refashioned by hand or machine.
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 creating new stresses.
Decoration
After annealing, glass can be embellished in various ways, the most common being engraving, etching, and painting. In wheel-engraving, the design is produced by holding the glass against rotating discs of various sizes. In diamond-point engraving, a freehand technique, the surface is hammered with a diamond-pointed stylus. Both diamond-point and wheel-engraving are suitable for producing pictorial designs and inscriptions. Stipple engraving, whereby the glass is pricked with tiny dots, produces a design that appears almost to be breathed onto the glass; it is a delicate technique particularly associated with 17th-century Dutch glassware. Designs on glass can also be produced by etching the surface with acid, or by sandblasting; this produces a frosted effect and is particularly suitable for large items such as window glass.
Glass can also be painted with enamels, which are then fused onto the surface in a low-temperature kiln. In gilding, gold leaf, gold paint, or gold dust is applied to the glass and sometimes left unfired; low-temperatire firing, however, is necessary to render it permanent.
History
Archaeological evidence indicates that glass was first made in the Middle East, sometime in the 3rd millennium BC.
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 (1504-1450 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 centres, 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.
Early Techniques
Before the invention of the blowpipe, several methods existed for shaping and embellishing objects of coloured 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 moulds to produce inlays, statuettes, and vessels such as jars and bowls. Preformed rods of glass could be heated and fused together in a mould to make "ribbon" glass. Patterns of great complexity were produced 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 mould to shape a vessel or plaque and then heated to fusion. "Gold band" glasses featured irregular bands of different-coloured glass, with gold leaf embedded in one translucent band.
Most pre-Roman glassware was fashioned by the core technique. A mixture of clay and dung was fixed to a metal rod and given the 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-coloured 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. When the object had 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 in this way. Later core-formed articles from the 6th century BC closely followed the forms of Greek pottery.
Roman Glass
Glassblowing, a less expensive and time-consuming method of manufacture, spread from Syria to Italy and other parts of the Roman Empire, gradually superseding older techniques. A new taste in glass styles developed: the earlier manufacturing processes emphasized colour and pattern; blowing enhanced the thin, translucent qualities of the material. Also, by the end of the 1st century AD, colourless glass had become more fashionable than coloured glass. 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 Roman 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 glassware was patterned in moulds, which enabled such novelty items as head-shaped flasks to be produced in quantity. 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 coloured glass were fused together and then carved so as to leave contrasting motifs in relief. The most famous example of Roman cameo glass is the Portland Vase (1st century AD, British Museum, London), which illustrates 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.
Western Glass
With the fall of the Roman Empire, the manufacture of household glass suffered a general decline in the West.
Medieval Glass
Under Frankish influence glassmakers in Britain and northern Europe continued to produce utilitarian vessels, some taking new, robust forms. The decoration of these objects was limited to simple moulded patterns, threading, and applied ornaments such as prunts (blobs of glass). The glass, mostly a greenish colour, 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; this produced potash-lime glass. Because the glasshouses were situated in the forests that provided fuel and ash, the glass made was called waldglas (forest glass). 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 the patronage of the Church, was mosaic glass in Mediterranean Europe and stained-glass windows in the north. 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 may have provided much of the flat glass for medieval cathedral windows. The glass was coloured, or flashed with colour, 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. This art declined in the late Renaissance but was revived in the 19th century.
Renaissance to the 18th Century
Although glassmaking was practised in Venice from the 10th century onwards, 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. Colourless 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 from coloured 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 from opaque white glass and painted with enamels in the manner of Chinese porcelain. Novelties made of lampworked glass were made at Murano, but by the 17th century Nevers, France, became most famous for this type of ware. Particularly suited to soda glass was the practice of diamond-point engraving, a technique favoured in the 17th century by Dutch artisans. Hammering the glass with a diamond-pointed stylus produced a stippled effect, and ambitious pictorial designs were created.
Glass manufacturers throughout Europe tried to copy Venetian production methods, materials, and decorative vocabulary. Knowledge was spread through the glassware itself, through the Art of Glass (1612) by Antonio Neri, and through Venetian glassblowers working abroad. 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 glassware (as Venetian-style glassware had become known), as nationalistic preferences for certain forms or decorations modified the Venetian model.
Italy's supremacy 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 the Holy Roman Emperor, Rudolf II, in Prague, was largely responsible for the development of engraving in the early 1600s. Glasscutters and engravers in Nuremberg and Potsdam became famous for skilfully executed designs in the Baroque style. At the same time, the Germanic glasshouses continued their tradition of enamelled and cold-painted glass.
The other improvement in glass that served to diminish Europe's reliance on Venice was the lead-oxide glass formulated (c. 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 apogee in Anglo-Irish Neo-Classical glassware made between 1780-1830.
American Glass
Glassmaking was the first manufacturing industry to be established in America. A glasshouse was 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, window glass, 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 War of Independence was that of John Frederick Amelung in Frederick County, Maryland, which was in operation from 1784 to 1795.
19th and 20th Centuries
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 was a cheap, swift 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 mould. Simpler designs popular from the 1840s on, known as pressed pattern glass, were available in many forms. The more expensive cut glass declined in popularity because of the competition from pressed glass. Only in 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 Neo-Classical taste. Continental glass factories made a version of laminated gold-leaf glass, called zwischengoldglas. Cameolike effects were obtained through the use of encrusted sulphides, and from the mid-19th century actual cameo engraving and cutting were practised by artisans, 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. In the late 19th century Renaissance rock crystal inspired a technique of polished engraved glass.
Bohemia continued to excel in wheel-engraved decoration with the work of such artisans as Dominik Biemann. Other methods, such as those for producing cased glass, were practised in Bohemian factories and copied throughout Europe and the United States. Chemical advancements led to new opaque coloured glass such as lithyalin, which resembled semiprecious stones. Transparent enamels and stains were applied to vessels, paralleling the revival of stained-glass windows.
By 1880, inspired by the revivals of historical glassworking methods and spurred by the capabilities of improved chemical technology, glassmakers 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 United States, and Émile Gallé and the firm of Daum Frères (founded 1889) in France, were the leading exponents of the style. They produced glassware in naturalistic shapes, with sinuous lines, exotic colours, and unusual surface effects, such as Tiffany's iridescent Favrile glass.
After World War I new interests in texture and formalized decoration emerged, as seen in the designs of René Lalique and Maurice Marinot. From the 1930s, exquisitely clear, colourless 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 around the world.
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 these individual cultures; they, in turn, influenced Western forms.
Islamic Glass
The history of glass from the 8th to 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. Fine colourless glass with delicate wheel-engraved designs was also produced. The possibilities for decoration were expanded with the introduction of decoration in enamel colours and gilding, techniques for which the glasshouses at Aleppo and Damascus were famous. From Egypt came the discovery of lustre, which created rich 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 glassware, particularly in Venice and Spain.
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 enamelled in floral patterns. In the 18th century the British East India Company sold quantities of English glass to the Indian market, some of which was then wheel-engraved by Indian artisans.
Far Eastern Glass
Chinese-made glassware in the distinctive "eye bead" form, with inlays resembling eyes, has been excavated from Chou dynasty sites (c. 1027-256 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 jewellery and later for snuff bottles is a recurring theme in Chinese glass. Few glass vessels predating the establishment of the glassworks at the Beijing Imperial Palace in 1680 are known. 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 coloured objects with carved and enamelled 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 in Japan apparently ceased in the 13th century. The craft was revived about 1750.
Glass (industry), any glass prepared with commercially useful properties of transparency, or refractive index, or colour, and so on. Glass is 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 making the enigmatic objects known as tektites. Glass is called amorphous because 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; heating can reconvert glass to a liquid form. Usually transparent, glass can also be translucent or opaque. Colour varies with the ingredients of the batch from which the glass is made.
Molten glass is plastic and can be shaped by means of several techniques. When cold, it can be carved. At low temperatures it is brittle and breaks with a shell-like (conchoidal) fracture on the broken face.
Glass was first made before 2000 BC and has since served humanity in many ways. It has been used to make useful vessels as well as decorative and ornamental objects, including jewellery. The history of the art and technique of glassworking is discussed in Glass (art).
Materials and Techniques
The basic ingredient of glass is silica, derived from sand, flint, or quartz.
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 objects subject to heat shock (deformation due to relatively rapid temperature change), such 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 certain physical properties to glass.
Water Glass and Soda-Lime Glass
Glass of high soda content can be dissolved in water to form 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.
Lead Glass
The fine-quality table glass known as crystal glass 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.
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.
Colour
Impurities in the raw materials affect the colour of glass. For a clear, colourless substance, glassmakers add manganese to counteract the effects of iron traces that produce greens and browns. Glass can be coloured by dissolving metallic oxides, sulphides, or selenides in it. Other colourants may be dispersed as microscopic particles.
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.
Physical Properties
Depending on the composition, some glass will melt at temperatures as low as 500° C (900° F); others melt only at 1,650° C (3,180° F). Tensile strength, normally between 2,745 and 5,500 N/sq cm (4,000 and 8,000 lbwt/sq in), can exceed 68,650 N/sq cm (100,000 lbwt/sq in) if the glass is specially treated. Specific gravity (density relative to water) ranges from 2 to 8, or from less than that of aluminium to more than that of steel. Similarly wide variations occur in optical and electrical properties.
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 1,000 metric tons of glass and are heated by gas, oil, or electricity. The glass batch is fed continuously into an opening at one end of the tank, and the melted, refined, and conditioned glass is drawn out at 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.
Shaping
The basic methods employed to shape glass are casting, blowing, pressing, drawing, and rolling. These are all ancient processes (see Glass (Art)), but they have been modified in various ways to produce glass for industrial purposes. For example, centrifugal casting processes have been developed in which the glass is forced against the sides of a rapidly rotating mould. Centrifugal casting is capable of forming precise, lightweight shapes such as television-tube funnels and fully automatic blowing machines make mechanical glassblowing possible.
Stressing Glass
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 of the glass pulls the surface into compression. Surface compressions approaching 24,100 N/sq cm (35,000 lbwt/sq in) can be obtained in thick pieces by this method. Chemical strengthening methods have also 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 70,000 N/sq cm (100,000 lbwt/sq in) can be attained by chemical strengthening. See also Annealing.
Types of Commercial Glass
The wide range of uses of glass has resulted in the development of a number of different types.
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, and the blowpipe removed (see Glass (Art); glass blowing). The reheated crown was spun on the rod, the hole left by the blowpipe enlarged, and eventually the disc, 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.
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 St Gobain, in 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. When 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 so high that the surface imperfections are 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, resists shattering when struck. Safety glass, for such items as car windscreens, is made by sandwiching 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.
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 blob of molten glass is dropped into a narrow, inverted mould and forced down by an air blast into the lower portion of the mould, which corresponds to the neck of the finished bottle. A baffle then drops over the top of the mould, 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 mould, 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 mould by a plunger before being blown in a finishing mould. Shallow jars, such as those used for cosmetics, are merely pressed.
Optical Glass
Most lenses used in spectacles, microscopes, telescopes, cameras, and certain other optical instruments are made from optical glass (See 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 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 the 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.
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 behaviour 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 and act as sunglasses when in the sun and lighten again when removed from sunlight. This kind of glass also finds uses in electronics.
Glass Ceramics
Glass containing certain metals will form a localized crystallization when exposed to ultraviolet radiation. If heated to high temperatures, these glasses 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 cooking utensils, rocket nosecones, and heat-resistant tiles to clad space shuttles. Other glasses containing metals or alloys can be magnetized, are strong and flexible, and prove very useful in high-efficiency electrical transformers.
Fibreglass
It is possible to produce glass fibres that can be woven or felted like textile fibres by drawing out molten glass to diameters of less than a hundredth of a millimetre (a few ten-thousandths of an inch). Both long, continuous multifilament yarns and short-staple fibres 25 to 30 cm (10 to 12 in) long may be produced.
Woven into textile fabrics, glass fibres 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 fibres with plastics, a composite fibreglass is formed that combines the strength and inertness of glass with the impact resistance of the plastic. The most remarkable types of glass fibre are those made to carry optical signals for computer and telephone communication, in the rapidly expanding new technology of fibre optics.
Miscellaneous Types of Glass
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 fibres were developed that have many uses in science, medicine, and industry. High-refractive-index glass fibres, laid parallel to one another and separated by thin layers of low-refractive-index glass, can transmit images. Fibrescopes incorporating such bundles can convey an image through acute angles, thus easing the examination of normally inaccessible sites. Such solid fibre-optics applications as magnifiers, minifiers, and faceplates also improve viewing. When used in conjunction with lasers, optical fibres are now central to long-distance telephony and computer communication.
Laser glass consists of glass "doped" with several per cent of neodymium oxide and is capable of emitting laser light if the glass is assembled in the proper device and "pumped" with ordinary light. It is considered a good laser source because of the relative ease with which large, homogeneous specimens of the glass can be obtained.
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 mist over in damp 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.