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Glass

Glass is a brittle, inorganic solid, composed mostly of inorganic oxides. The main ingredient of most glasses is silicon dioxide, SiO2 ,or silica—found in nature as sand. Generally manufactured by heating sand, soda, lime, and other ingredients (and quickly cooling the molten mass), glass is a fundamental component of a variety of products, including tableware, windshields, thermometers, and telescope lenses. Given its durability and versatility, glass plays an important role in human culture. Glass blowing was first developed around 30 B.C.

Early peoples were likely to have discovered natural glass, which is created when lightning strikes sand, and were certain to have used obsidian-a dark volcanic glass-for weapons, ornaments, and money. The first manufactured glass probably took the form either of glass beads or ceramic glaze and appeared around 4000-5000 B.C. Surviving examples of Egyptian and Mesopotamian glass objects date to around 1550 B.C.

For centuries, glass, shaped by the use of molds, remained costly and difficult to produce. The invention of the blowpipe method of glass making (in which molten glass is puffed into shape with the use of a hollow tube) in about 30 B.C. made glass more commonplace. Typical uses at the time included windows as well as decorative objects.

The first four centuries after the birth of Christ are sometimes referred to as the First Golden Age of glass making, for during this period artisans produced a wide variety of artifacts that are now highly valued. After the decline of the Roman Empire, few developments took place in European glass making until the twelfth and thirteenth centuries, when stained glass windows (formed of pieces of colored glass outlined by lead strips and assembled into a narrative picture) began to appear in English and French churches. During the Crusades, Europeans were exposed to the accomplished glass making of the Near East, an influence evidenced by the growth of the craft in Italy, particularly Venice. Beginning around 1300, the Venetians ushered in the Second Golden Age of glass making; they became widely known for a particularly transparent, crystalline glass that was worked into a number of delicate objects.

In the late 1400s and 1500s the Germans and other northern Europeans were producing containers and drinking vessels that differed markedly in their utilitarian value from those produced by the Venetians. Nonetheless, Venetian glass was immensely popular during the reign of Queen Elizabeth I (1558-1603). In 1674, George Ravenscroft (1618-1681) brought fame to English glass making when he invented lead glass (now usually called lead crystal), an especially brilliant glass he produced accidentally when he added lead oxide to his mixture instead of lime. In colonial America, the glass made by this technique became known as flint glass, and was usually etched or cut into facets to lend it additional luster.

The first glass plant built in the United States was founded at Jamestown, Virginia, in 1608, but it survived for less than a year. Much later, in 1739, Caspar Wistar successfully launched the American glass industry with a plant in Salem City, New Jersey. Other prominent figures in early American glass making included Henry William "Baron" Stiegel (1729-1785) and John F. Amelung. The renowned Sandwich glass that is now much coveted by American collectors was made by the Boston and Sandwich Glass Company; the Bakewell Company of Pittsburgh was another famous glass manufacturer of the time.

The early 1800s saw a tremendous demand for glass windows, which were a symbol of affluence, particularly in the frontier communities of America. Window glass was originally made by spinning out a bubble of blown glass until it became flat; because of the bump or "crown" that was invariably left in its center, this was called crown glass. Around 1825, the cylinder process replaced the earlier method. Now the glass was blown into a cylinder shape that, when cooled, was cut down one side; when reheated, the cylinder flattened out to form a sheet. In 1842, John J. Adams invented a more sophisticated glass-flattening and tempering process that made not only plate glass but mirrors, showcases, and other products more widely available. During the last half of the nineteenth century, glass found wide use in medicinal containers, tableware, and kerosene lamps. Tempered glass (made exceptionally strong through a reheating process) was invented by François Royer de la Bastie in 1874, and wire glass (industrial sheet glass with metal mesh laminated into it) by Leon Appert in 1893. In 1895, Michael J. Owens (1859-1923) invented a bottle-making machine that allowed bottled drinks to be produced inexpensively.

The great technological advances of the twentieth century broadened the range of ingredients, shapes, uses, and manufacturing processes for glass. Natural gas replaced the wood and coal that had previously been used in the glass making process, and huge operations were established. One of the most common forms of glass now produced is flat glass, used for windows, doors, and furniture. Formed by flattening melted glass between rollers, annealing (heat treating) in an oven called a lehr, then cutting into sheets and grinding and polishing until smooth, this category includes sheet glass and the higher quality plate glass. The best quality of all is achieved in float glass, invented in 1952 by Alistair Pilkington. Float glass is made by floating a ribbon of liquefied glass on top of molten tin so that it forms a perfectly even layer; the result is glass with a brilliant finish that requires no grinding or polishing. In 1980, Pilkington invented kappa float glass, which features a special, energy-efficient glaze that traps thermal heat while allowing solar heat to filter through.

Other modern forms of glass include the laminated safety glass used for automobile windows, which is composed of sandwiched layers of plastic and glass; nonreflecting glass (invented by Katherine Burr Blodgett and others); structural glass, used in buildings; heat-resistant cookware such as Pyrex; and fiberglass.


Resources

Books

Doremus, R. H. Glass Science. New York: Wiley, 1990.

Zerwick, Chloë. A Short History of Glass. New York: Springer-Verlag, 1994.

Additional topics

Science EncyclopediaScience & Philosophy: Gastrula to Glow discharge