Glass

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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.

Glass is generally considered an amorphous (non-crystalline) solid that is hard, usually transparent, and easily shattered. It is formed mostly of silica (sand) that is fused with such substances as phosphates or borates while at high temperatures. Compared with crystals, the structure of glass is an irregular arrangement of atoms. It is non-crystalline yet almost completely undeformable. This characteristic is called the vitreous state. Vitreous substances, when heated, will transform slowly through stages of decreasing viscosity. As a sample of glass is heated, it becomes more and more deformable, eventually reaching a point where it resembles a very viscous liquid. (Ice, on the other hand, does not go through these changes as it is heated. It changes directly from a solid to a liquid. Ice, therefore, is not a vitreous substance.) Glasses are only very slightly deformable. Glass tends to bend and elongate under their own weight, especially when formed into rods, plates, or sheets. Glass can be either organic or inorganic materials.

Many definitions of glass include the idea that they do not crystallize as they solidify. Since the definition of solidification is the act of crystallization, this idea of glass as a non-crystalline solid may not be entirely correct. Crystallization occurs when the molecules of a substance arrange themselves in a systematic, periodic fashion. The atoms or molecules of glass do not exhibit this periodicity. This has led to the idea of glass as an extremely viscous, or supercooled liquid.

Glass is often referred to as an amorphous solid. An amorphous solid has a definite shape without the geometric regularity of crystalline solids. Glass can be molded into any shape. If glass is shattered, the resulting pieces are irregularly shaped. A crystalline solid would exhibit regular geometrical shapes when shattered. Amorphous solids tend to hold their shape, but they also tend to flow very slowly. If left undisturbed for a long period, glass will very slowly crystallize. Once it crystallizes, it is no longer considered glass. At this point, it has devitrified. This crystallization process is extremely slow and in many cases may never occur.

The chemical make-up of standard window glass is quite similar to the mineral quartz. An x-ray crys-tallographic picture of quartz would show atoms arranged in an orderly, periodic sequence. X-ray crystallography studies of glass show no such arrangement. However, the atoms in glass are disordered and they show no periodic structure. This irregular arrangement of atoms not only defines a substance as glass, but also determines several of its properties.

The most common glasses are silicon based. Most glasses are 75% silicate. These glasses are based on the SiO₂ molecule. This molecule creates an asymmetric, aperiodic structure. Some of the oxygen atoms are not bridged together, creating ions that need to be neutralized by metal cations. These metal cations are randomly scattered throughout the glass structure, adding to the asymmetry. The oxides of elements other than silicon can also form glasses. These other oxides include Al₂O₃, B₂O₃, P₂O₅, and As₂O₅.

The bonds between the molecules or ions in glass are of varying length, which is why they show no symmetry or periodic structure. Because the bonds are not symmetrical, glass is isotropic and has no definite melting point. The melting of glass instead takes place over a wide temperature range. Changing the state of a substance with asymmetric bonds requires more energy than a crystalline structure would need to change its state. The tendency of glass to devitrify is a result of the atoms moving from a higher to a lower energy state.

Given its durability and versatility, glass plays an important role in human culture. Glass blowing was first developed around 30 BC. Early people 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 from around 4000 to 5000 BC. Surviving examples of Egyptian and Mesopotamian glass objects date to around 1550 BC.

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 BC made glass more commonplace. Typical uses at the time included windows as well as decorative objects.

The first four centuries AD 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 one year. Much later, in 1739, Caspar Wistar (1761–1818) 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 the United States. 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 Francois 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.