History of Glass
Glass, amorphous substance made primarily of silica fused at high temperatures with borates or phosphates. Glass is neither a solid nor a liquid but exists in a vitreous state, in which molecular units have disordered arrangement but sufficient cohesion to produce mechanical rigidity. Molten glass is cooled to this rigid state; heat can reconvert it to a liquid form. Glass can be transparent, translucent, or opaque; its color varies with its ingredients. Molten glass can be shaped by means of several techniques. Glass is also found in nature, as tektite and the volcanic material obsidian.
Materials and Techniques
For most glass, silica—derived from sand, flint, or quartz—is combined with other raw materials in various proportions. Most manufactured glass is a soda-lime composition. The fine-quality table glass known as crystal is made from potassium-silicate formulas that include lead oxide. Typical glass formulas include broken waste glass of related composition, which promotes melting and homogenization of the batch. Other agents are often added to cause the release of small bubbles during the melting. Depending on the composition, the melting point, specific gravity, tensile strength, as well as optical and electrical properties of glass will vary.
In the past glass was melted in clay pots heated in wood- or coal-burning furnaces, which still are used for handworking. In modern glass plants, most glass is melted in large tank furnaces.
When working molten glass, five basic methods are employed to produce an almost limitless variety of shapes: casting, blowing, pressing, drawing, and rolling. In casting, an ancient process, molten glass is poured into a mold and allowed to solidify. Glass blowing was the standard way of shaping glass vessels from the 1st century BC to the 19th century AD. Using a hollow iron pipe with a mouthpiece at one end, the glassblower, or gaffer, collects a small amount of molten glass on the end of the pipe and rolls it against a paddle or metal plate to shape its exterior and cool it slightly. The gaffer then blows into the pipe, expanding the glass into a bubble. By constantly reheating it at the furnace opening, blowing, and rolling, the gaffer controls the form and thickness of the glass. Blown glass also can be shaped with molds. Since 1903 mechanical glassblowing has been possible.
Some glass pressing was involved in the production of ancient cast wares. European manufacturers rediscovered the technique in the late 18th century. In the 1820s fully mechanical pressing was developed, in which molten glass is dropped into a mold, and a plunger squeezes the glass between itself and the outer mold to form the final shape. Molten glass also can be drawn directly from the furnace to make tubing, sheets, fibers, and rods of glass. Plate glass was originally made by rolling molten glass on a flat surface; later, it was made by continuous rolling between double rollers.
Glass may be decorated in a number of ways, including cutting, using facets, grooves, and depressions; engraving, using a diamond point, metal needle, or rotating wheels; etching, using acid; sandblasting, using sand, flint, or powdered iron; cold painting, wherein paint is applied to glass but not affixed by firing; enamel painting, wherein enamel colors are painted and then affixed through firing; and gilding, wherein gold leaf, gold paint, or gold dust is applied.
Archaeological evidence indicates that glass was made first in the Middle East, sometime during the 3rd millennium BC. Glass production flourished in Egypt and Mesopotamia until about 1200 BC. In the 9th century BC Syria and Mesopotamia emerged as glassmaking centers, and the industry spread throughout the Mediterranean region. Glassmaking was undertaken in many areas of the Roman empire. Most known decorative techniques were invented by artisans of the Roman era.
Glassblowing was discovered in the 1st century BC on the Phoenician coast. It later spread from Syria to Italy and other parts of the Roman Empire. The process made large-scale glass production possible and changed the status of glassware from an elite item to an everyday material, used for windows as well as containers.
The manufacture of household glass suffered a decline in the West with the fall of the Roman Empire. The medieval period produced mosaic glass in Mediterranean Europe and stained-glass windows in the north. Glass windows in churches are mentioned in documents as early as the 6th century AD, but the earliest examples existing today date from the 11th century.
Although glassmaking was practiced in Venice (now in Italy) from the 10th century on, the earliest known Venetian glassware dates from the 15th century. The Venetian industry dominated the European market until 1700; its major contribution was the development of a highly refined, hard-soda glass, known as cristallo. Glass manufacturers throughout Europe tried to copy the Venetians, and each country developed its own variation on the Venetian model.
In the 17th century Germany's potash-lime glass was thicker and harder than cristallo. German glass cutters and engravers became famous for skillfully executed glass designs in the baroque manner (see Baroque Art and Architecture). Another improvement in glass was lead-oxide glass, developed in England about 1676. More brilliant and durable than cristallo, English lead glass was considered the finest glass of the 18th century. Lead glass reached its full potential in the neoclassical pieces of the Anglo-Irish period (1780-1830). Glassmaking was the first manufacture undertaken in America, with a glasshouse built at Jamestown, Virginia, in 1608.
In the 19th century glassmaking was influenced by rapid advances in technology and the rediscovery and adaptation of older methods. Mechanical pressing, introduced in the United States, was a cheap, fast means of production that greatly expanded the role of glass in the home and in industry. Chemical advancements led to new opaque colored glass that resembled semiprecious stones. Transparent enamels and stains were applied to vessels, paralleling the revival of stained-glass windows. By 1880 glassmakers were creating new styles of handworked glass, generally called art glass. Between 1890 and 1910 styles reflected the international art nouveau movement. Louis Comfort Tiffany in the United States was one of the leading proponents of the art nouveau style. After World War I (1914-1918) the designs of French glassmakers René Lalique and Maurice Marinot were popular. A new era in glassmaking began in the early 1960s with the studio glass movement.
The history of glass from the 8th century through the 14th century is focused on the Islamic world of the Middle East. Muslim artisans made high-relief cut vessels, many with animal subjects. Quality colorless glass, fired-on enamel colors, and gilding techniques were developed. Egypt introduced lustrous metallic effects on both pottery and glass. Glass was made in India as early as the 5th century BC, but the industry was not established until the Mughal period. Indian glass was often gilded or enameled in floral patterns. Chinese-made glassware from the Zhou dynasty (1027?-256 BC) has been excavated. Early glass objects were small and carved in close imitation of gemstones. No evidence exists of glass made in Japan before 200 BC. Some glass Buddhist relic bottles and urns are believed to date from the Asuka and Nara periods (AD 552-784).
Types of Commercial Glass
Window glass, in use since the 1st century AD, was made originally by casting or blowing. Today nearly all window glass is made mechanically by drawing glass upward from a molten pool fed from a tank furnace, pouring it onto an iron table, rolling it continuously between double rollers, and floating the sheet on a bath of molten tin to create smooth and even surfaces. Unpolished rolled glass, often with figured surfaces produced by designs cut into the rolls, is used architecturally. Safety glass, for automobile windshields, is made by laminating a sheet of transparent polyvinyl butyral plastic between two sheets of thin plate glass.
Bottles are mostly produced by an automated process that combines pressing (to form the open end of the bottle) and blowing (to form the hollow body). Glass lenses used in optical instruments, such as eyeglasses and microscopes, are made from optical glass, which requires pure ingredients (see Lens; Optics). Photosensitive glass contains gold or silver ions that respond to the action of light. This glass is used in printing and reproduction processes.
Glass containing certain metals will, if heated to high temperatures, convert to crystalline ceramics with mechanical strength and electrical insulating properties greater than that of ordinary glass. Other types of glass include glass fibers, hollow glass bricks, glass optical fibers (see Fiber Optics), and laser glass.