You can hardly imagine a world without glass егэ ответы

Traditional methods of obtaining sheet glass and plate glass by flattening blown globes and cylinders and by drawing or rolling cylinders over rollers became obsolete in the 1980s. The principal method used today is the float-glass process.

The huge quantities of plate glass used for such industries as construction are produced, like the bulk of modern glass, by tank furnaces that manufacture glass 24 hours a day. These furnaces can easily produce continuous ribbons of glass up to 9 feet (3 meters) wide. In addition, up to about 1,000 feet (300 meters) of high-quality ground and polished plate glass can be made per hour. Tank furnaces are important pieces of equipment in the float-glass process. After processing in the tank furnace, a ribbon of glass flows between rollers and then through a slot onto the surface of a metal float bath in a closed chamber. The atmosphere in the chamber is carefully controlled to avoid oxidation of the metal, which if it occurred would mar the transparency of the glass.

In the float bath, the glass floats on a bath of molten metal, usually tin, until both surfaces are smooth. The glass is kept at a high temperature so that it is soft enough to flow and to smooth out any irregular spots. The molten glass is pulled across the surface of the bath and cooled to the temperature at which it is rigid while still floating on the molten metal. It then passes to a chamber called the annealing lehr, where it is cooled slowly to ensure that the glass remains free of strains. Float-glass surfaces are nearly as smooth as those of mechanically polished plate glass. The float process produces strong, smooth, flat glass, but it cannot produce glass of great thickness.

Bottles and other hollow objects—for example, beakers, Christmas tree ornaments, and coffee makers—are formed by blowing. In blowing, a bubble of molten glass is placed in a mold. Air pressure applied inside the bubble forces the glass against the sides of the mold, where it hardens to the desired shape. If the object being blown is round and without any projections, the glass or the mold may be rotated during blowing to produce a smoother surface. When this is done, the inside of the mold is coated with a carbonlike paste soaked in water. This process is called paste-mold blowing.

Objects that cannot be rotated are blown in an uncoated iron mold. Hot-iron-mold blowing, as this is called, cannot produce as smooth a surface as that produced by paste-mold blowing.

For many centuries the air pressure required for blowing was supplied by workers’ lungs. Today, in industrial glass production, nearly all blowing is done by machine. Several types of machines are used, each designed for a particular kind or size of product. By contrast, fine art glass is still produced by hand, not by machine, using blowing techniques developed long ago (see “History of Glassmaking” below).

Products such as ashtrays, pie plates, and floodlight lenses are formed by pressing. A gob of molten glass is loaded into a mold and then squeezed between the mold and a plunger. The glass flows and fills evenly the space between the mold and the plunger. When the glass is hard, the plunger is withdrawn.

Tubing for fluorescent lamps and industrial piping is formed by allowing molten glass to flow over a hollow, downward-pointing mandrel made of metal, at a rate that maintains an evenly thick coating. Air is passed through the mandrel so that as the glass is drawn off, a tube is formed. Air pressure from a hole in the mandrel keeps the tubing from collapsing before it has hardened.

Centrifugal, or spin, casting is used to form the conical section of television picture tubes called the funnel. A gob of glass is loaded in a rapidly rotating mold. Centrifugal force pushes the glass up the sides of the mold, where it cools and hardens.

Glass fibers are used for insulation and for reinforcement of the lightweight plastics that form boat hulls and car bodies. Such fiberglass is formed by drawing molten glass directly from a furnace through small holes in platinum bushings, or removable metal linings. About 400 strands of these very fine filaments are needed to make a single strand of usable fiber. As these filaments are drawn together, they are thinly coated with an adhesive to ensure that they stay securely connected. Some fibers for textiles are formed in continuous lengths of 1 million miles (1.6 million kilometers) or more of high quality filaments.

Foam glass is made by heating a mixture of ground glass and carbon in a mold. The mixture rises like a cake and fills the mold. When cooled and taken from the mold, the form is a mass of sealed glass bubbles, rigid but light enough to float on water. Foam glass is used for thermal insulation in building walls, storage tanks, and industrial equipment.

Optical glass, used in microscopes and cameras, is prepared more carefully than any other glass. It must be free of bubbles, ripples, or streaks. The two main classes of optical glass are crown glass, which has low refraction and dispersion, and flint glass, which has high refraction and dispersion. (For explanations of refraction and dispersion, see light, “Light and Matter.”)

Eyeglasses, or spectacles, were once regularly manufactured with optical glass, but in general this heavy glass has been replaced by lightweight plastics. Camera lenses, as well, are often made of lightweight plastics, such as styrene, instead of the traditional optical glass. Optical glass, however, may still be pressed into blanks for use as lenses; drawn into three- , four- , or five-sided bars from which prisms can be cut; or cast into slabs for use in special large windows.

Glass for fiber optics must also be extremely pure and transparent. The hair-thin glass fibers consist of a core with a lower refraction surrounded by an outer reflecting layer called the cladding with a higher refraction. (See also cable.)

Many different coatings can be applied to the surface of glass to prepare it for specific uses. Enamels, once used to decorate cookware and bottles and to coat scales on thermometers, have been largely replaced with more durable organic polymers. Producing the safety glass for automobile windows involves laminating glass. In this process, two sheets of glass are bonded together by a sheet of resin, such as polyvinyl butyral, and then exposed to high temperatures and pressures to bond the glass tightly to the resin.

Other coatings are used to reflect heat or light. The windows of the crane cabs used in steel mills are coated with a thin film of metallic oxide that lets in light and reflects heat. Some coatings applied to windows make it possible to see through only one side of a window. A thin film of silver is sometimes applied to glass surfaces to make mirrors.

Thin film coatings that conduct electricity are applied to glass rods or tubes to make resistors for electronic circuits. The same films may be applied to sheets of glass to make electric heater panels and to the windshields of aircraft to keep frost from forming.

In Europe there was a revival of glassmaking by the early 12th century, with the development of stained-glass windows for cathedrals and monasteries. These windows consisted of small pieces of glass of different colors held together by strips of lead that often outlined the main design. Secondary details, such as facial features and drapery folds, were painted on the windows with metallic oxides mixed with powdered glass and fired on prior to leading. In this way the decoration became permanently bonded to the glass.

A flourishing glass industry did not develop in Europe until the end of the 13th century, when Venice became a major glassmaking center. The Venetians may have acquired their glassmaking techniques through their contacts with the Middle East during the Crusades. The Venetians provided the link between the ancient and modern glassmaking arts. They developed a clear glass similar to crystal, called cristallo, that provided the Venetians with a thriving export trade that spread throughout Europe. By the 16th century Venice had attained a dominant position in Europe’s glass industry. Venetian glass was noted for its brilliance and for its light, imaginative forms.

From the 16th century onward, two glassmaking traditions evolved in Europe. One was the delicate use of colorful glass practiced by the Venetians and their imitators. The other was the development of solid, practical, and elaborately enameled and cut or engraved forms in northern Europe.

However, the use of glass for ordinary windows remained largely restricted to churches and castles for many centuries. By the 17th century the country of Bohemia, in what is now the Czech Republic, became the second most productive glassmaking center in Europe.

Late in the 17th century the Englishman George Ravenscroft found that by adding lead oxide to the glass composition a far more brilliant, sparkling glass could be produced than had ever been made before. Lead glass, as it came to be known, was also more solid and heavier than any glass previously made. It became a favorite type of glass for tableware among Europeans. The ability of lead glass to bend light rays at different angles made high-quality microscopes and telescopes possible. Lead glass was also well suited to decorative purposes, and its massive, brilliant forms, either blown or cut, are still valued.

America’s first glass factory was built in 1608 at the Jamestown Colony in Virginia. However, it was not until the establishment of Caspar Wistar’s glassworks in southern New Jersey in 1739 that America’s glass industry achieved its first successes. Wistar’s factory produced mainly window glass and bottles, though his workers also made other types of containers of some elegance.

Henry William Stiegel, who established a glass factory in Pennsylvania in 1763, made the first American glassware that was good enough to compete with European imports. Another pioneer in American glassmaking was John Frederick Amelung, who founded the New Bremen Glassmanufactory in Maryland in 1785. He produced the most sophisticated glass that had been made in America up to that time.

In the early 19th century, as new glass factories were founded in the East and the Midwest, a number of technical improvements were introduced. One important advance was the development of hinged metal molds, used to make bottles and fine tableware. Designs for the molds were copied or adapted from more expensive glassware. By 1825 the invention of the pressing machine by the Bakewell, Page, and Bakewell Company of Pittsburgh, Pa., had made the mass production of glass possible. The pressing machine revolutionized the American glass industry.

In the middle of the 19th century the Pittsburgh area became the most important glassmaking center in the United States. There coal and sand, the raw materials needed in glassmaking, were available in large quantities. Transportation on the Ohio and Mississippi rivers made possible the efficient distribution of finished products. Among the glass factories on the East coast, those at Sandwich and Cambridge, Mass., were also important at this time.

During the latter part of the 19th century constant improvements were made in the mechanical methods used to form and decorate glass. In 1903 Michael J. Owens invented the bottle-making machine. Soon after, techniques for making window glass were perfected in both Europe and the United States.

In 1903 J.H. Lubbers invented a machine for blowing glass mechanically. Two years later Irving W. Colburn patented a process for the production of continuous sheets of glass drawn directly from huge tanks that could be fired for long periods of time. In the same year Émile Fourcault, a Belgian, perfected the vertical draw method, introduced in the United States in 1923. Later improvements included the twin-grinding machine for the continuous manufacture of plate glass. This machine began to be replaced in 1959 by the float-glass process still in use today.

In the 1960s laser glass, chemically toughened glass for use in making dinnerware and springs, laminated glass, and photochromatic glass were developed. Photochromatic glass, which turns darker when exposed to light and clears up when the light source is dimmed, is used in eyeglasses that become sunglasses in the appropriate setting. Titanized glass was developed in the early 1970s to lengthen the life of returnable glass bottles and other containers that could be reused continuously. Titanizing glass involves spraying bottles with a titanium compound while the bottles are still red-hot. The durable coating that forms on the bottles reduces friction and lessens tiny cracks that develop on bottles that are transported regularly and passed through cleaning and filling plants innumerable times.

By 1990 fiber-optic glass pipes that could maintain the brightness and intensity of light and transmit it over long distances in laser communication systems had been improved. Glass capable of storing radioactive wastes safely for long periods of time had also been developed.