Glass (redirected from people in glass houses shouldn't throw stones)

glass, hard substance, usually brittle and transparent, composed chiefly of silicates and an alkali fused at high temperature.

Composition and Properties of Glass

Most glass is a mixture of silica obtained from beds of fine sand or from pulverized sandstone; an alkali to lower the melting point, usually a form of soda or, for finer glass, potash; lime as a stabilizer; and cullet (waste glass) to assist in melting the mixture. The properties of glass are varied by adding other substances, commonly in the form of oxides, e.g., lead, for brilliance and weight; boron, for thermal and electrical resistance; barium, to increase the refractive index, as in optical glass; cerium, to absorb infrared rays; metallic oxides, to impart color; and manganese, for decolorizing. The term "crystal glass," derived from rock crystal, was at first applied to clear, highly refractive glass; it has come to denote in the trade a high-grade, colorless glass and is sometimes applied to any fine hand-blown glass.

The Process of Glassmaking

The processes of glassmaking have remained essentially the same since ancient times. The materials are fused at high temperatures in seasoned fireclay containers, boiled down, skimmed, and cooled several hundred degrees; then the molten glass (called metal) is ladled or poured into molds and pressed, or is blown (sometimes into molds), or is drawn. The shaped glass is annealed to relieve stresses caused by manipulation, then is slowly cooled. The glass, formerly annealed on shelves in a melting furnace, is now usually carried on rollers through annealing ovens (lehrs).

Although today most hollow vessels such as light bulbs or containers are machine blown, fine ornamental hollow ware is still made by gathering a mass of glass at the end of a long, iron blowpipe, blowing it into a pear-shaped bulb, which is rolled on an oiled slab (marver), shaped with tools, and then reblown, often into a mold; the glass is reheated periodically in a small furnace (glory hole). It is finally transferred to an iron rod (punty) attached to the base of the vessel, and the lip is shaped and smoothed. Methods of decoration include cutting, copper-wheel engraving, etching with hydrofluoric acid, enameling, gilding, and painting.

Development of the Glass Industry

Humans have used glass since prehistoric times, at first fashioning small objects from natural glass such as obsidian, a volcanic glass, or from rock crystal, a colorless, transparent quartz whose brilliance and clarity are emulated in manufactured glass.

Ancient Glassmaking

The place and date of origin of manufactured glass are not known. The oldest known specimens of glass are from Egypt (c.2000 B.C.), where the industry was well established c.1500 B.C. Many varieties of glass were known during Roman times, including cameo glass, such as the Portland vase, and millefiore glass, produced from fused and molded bundles of thin glass rods of many colors. Glass was also used for window panes, mirrors, prisms, and magnifying glasses. Except for the work done in Constantinople, little is now known of the methods of glassmaking used in Europe from the fall of Rome until the 10th cent., when stained glass came into use.

Early European Glassmaking

Venice was the leader in making fine glassware for almost four centuries after the Crusades and attempted to monopolize the industry by strict control at Murano of glassworkers, who were severely penalized for betraying the secrets of the art. After the invention (c.1688) of a process for casting glass, France was for many years supreme in the manufacture of plate glass such as that used to line the Galerie des Glaces at Versailles. Late in the 17th cent. England began to make flint glass, whose lead oxide content imparted a brilliance and softness that made it suitable for cut glass.

Glassmaking in Colonial America

The first glass factory in America was built in 1608, and glass was carried in the first cargo exported to England. Although other glasshouses were operated in the colonies, especially in New Amsterdam, the first successful and enduring large-scale glasshouse was set up by the German-born manufacturer Caspar Wistar in New Jersey in 1739. Some of the finest colonial glassware was produced in the Pennsylvania glasshouses of the German-born manufacturer H. W. Stiegel.

Beginnings of the Modern Era

The invention of a glass-pressing machine (c.1827), used by the American manufacturer Deming Jarves in his Boston and Sandwich Glass Company (1825–88), permitted the manufacturing of inexpensive and mass-produced glass articles. Nevertheless, in the 19th and 20th cent., there has remained a sense of pride in individual craftsmanship. The American artist Louis C. Tiffany was responsible for the design and manufacture of an extraordinary iridescent glass used in a variety of objects in the late 1800s. Exceptionally fine blown glassware has been designed by such artists as René Lalique and Maurice Marinot in France, Edvard Hald and Simon Gate in Sweden, as well as Sidney Waugh in the United States.

Contemporary Applications of Glass

Glass has become invaluable in modern architecture, illumination, electrical transmission, instruments for scientific research, optical instruments, household utensils, and even fabrics. New forms of glass, new applications, and new methods of production have revolutionized the industry. Recently developed forms of glass include safety glass, which is usually constructed of two pieces of plate glass bonded together with a plastic that prevents the glass from scattering when broken; fiberglass, which is made from molten glass formed into continuous filaments and used for fabrics or for electrical insulation; and foam glass, which is made by trapping gas bubbles in glass to yield a spongy material for insulating purposes. Certain uses of glass are now being superseded by newly developed plastics.

See also window.

Bibliography

See G. O. Jones, Glass (2d ed. 1971); L. D. Pyle et al., Introduction to Glass Science (1972); R. H. Doremus, Glass Science (1973); I. Fanderlik, Optical Properties of Glass (1983); P. Bansal, Handbook of Glass Properties (1986).

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glass

Solid material, typically a mix of inorganic compounds, usually transparent or translucent, hard, brittle, and impervious to the natural elements (“vitreous properties”). It is made by cooling molten ingredients fast enough so no visible crystals form. A poor conductor of heat and electricity, glass takes on colours when certain metal oxides are included in the mix. Most glass breaks easily. Obsidian is a naturally occurring glass. Everyday glass (soda-lime or soda-lime-silica) is made of silica (silicon dioxide), soda (sodium carbonate), and limestone (calcium carbonate), with magnesia (magnesium oxide) for sheet glass or alumina (aluminum oxide) for bottle glass. Fused silica is an excellent glass but expensive because of pure silica's very high melting point. Borosilicate glass (e.g., Pyrex) is used for cookware and laboratory glassware because it expands very little when heated. Lead crystal is used for fine tableware. It has a heavy feel because of its lead oxide content and a sparkle due to its high refraction index. Even more specialized glasses include optical, photosensitive, metallic, and fibre-optic. Since glass has no sharp melting point, most types can be shaped while hot and plastic by many techniques, mostly blowing or molding. See also volcanic glass.

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Glass

Materials made by cooling certain molten materials in such a manner that they do not crystallize but remain in an amorphous state, their viscosity increasing to such high values that, for all practical purposes, they are solid. Materials having this ability to cool without crystallizing are relatively rare, silica, SiO₂, being the most common example. Although glasses can be made without silica, most commercially important glasses are based on it. The most important properties are viscosity; strength; index of refraction; dispersion; light transmission (both total and as a function of wavelength); corrosion resistance; and electrical properties.

Chemically, most glasses are silicates. Silica by itself makes a good glass (fused silica), but its high melting point (1723°C or 3133°F) and its high viscosity in the liquid state make it difficult to melt and work. To lower the melting temperature of silica to a more convenient level, soda, Na₂O, is added in the form of sodium carbonate or nitrate, for example. This has the desired effect, but unfortunately the resulting glass has no chemical durability and is soluble even in water (water glass). To overcome this problem, lime, CaO, is added to the glass to form the basic soda-lime-silica glass composition which is used for the bulk of common glass articles, such as bottles and sheet (window) glass. Although these are the main ingredients, commercial glass contains other oxides (aluminum and magnesium oxides) and ingredients to help in oxidizing, fining, or decolorizing the glass batch.

Special kinds of glass have other oxides as major ingredients. For example, boron oxide is added to silicate glass to make a low-thermal-expansion glass for chemical glassware which must stand rapid temperature changes, for example, Pyrex glass. Also, lead oxide is used in optical glass because it gives a high index of refraction.

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1.		GLASS - General LAnguage for System Semantics. An Esprit project at the University of Nijmegen. ftp://phoibos.cs.kun.nl/pub/GLASS.
2.		glass - (IBM) silicon.