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A Historical Look at Glass

The History, It's Nature, and it's Recipe
Courtesy of PPG Industries, Inc.

 

History of Glass

The mysterious physical, optical and aesthetic properties of glass have always intrigued man. Even the most sophisticated 20th century man is amazed and bemused by this solid, which he has been told is really a rigid uncrystallized liquid. The product and the process used to manufacture it seem to smack of alchemy, for glass is nothing but coarse sand and soda ash transformed into smooth transparent forms.

According to the Roman historian Pliny, who wrote in Naturalis Historica in 77 A.D., man first produced glass by accident about the year 5000 B.C. Phoenician sailors feasting on a beach near Belus in Asia Minor, could find no stones on which to place their cooking pots; therefore, they set them on blocks of soda carried by their ship as cargo. As the fire's heat increased, the sand and soda turned to molten glass.

Pliny's anecdote now is considered apocryphal, but it contains an accurate recipe for producing glass: heat plus silica and soda ash.

Ornamental glass beads dating from 2500 B.C. have been found in Egypt, and glass rods from even earlier have been uncovered in Babylon. The first useful glass objects date to Egypt's 18th dynasty, about 1500 B.C. Egyptians attached metal rods to silica paste cores, which they dipped repeatedly into molten glass to produce small bottles. The cores later were removed. The goblet of Thutmose III, made about 1490 B.C. and now at New York's Metropolitan Museum of Art, was produced in this manner.

Glassblowing, a Babylonian discovery, probably came about when glassmakers using the core-dipped method switched to hollow metal rods to hold silica paste cores and then discovered that molten glass could be blown into shapes. After this discovery, which dates to about 250 B.C., glass vessels suddenly became easy and inexpensive to produce. Romans imported Syrian and Babylonian glassmakers, and small bowls and bottles were selling for only a Roman penny in 200 B.C. Pliny the Elder noted in 79 B.C. that fine glass cups were replacing cups of precious metals as a status symbol among the Roman rich.

Glass, however, did not replace shutters at the windows of Roman homes. The Romans tried but failed to cast transparent flat glass to enclose or ornament their homes. Slabs 1/2" thick have been excavated - including a 32 by 44-inch piece at Pompeii - but Romans did not discover the art of grinding and polishing cast glass to make it transparent. Instead of glass, the rich used thin, translucent sheets of alabaster to enclose wall openings.

With the breakdown of the Roman Empire, glassmaking technology stagnated in Europe; in fact, it almost disappeared. True, Gothic cathedrals of the late 12th century and later featured brilliant bits of colored glasses, complex designs and rate and were prohibitively expensive. Even the rich still shuttered their windows, and the Middle English word for windows - "wind eyes" - underlined the fact that wall openings enclosed in glass were, for all practical purposes, nonexistent.

During the 13th and 14th centuries, glassmaking was revived in Venice as a result of that Italian state's trade contacts with Byzantium. Soda-Lime was developed by glassmakers of the island or Murano in about 1450, and Venetians termed this clear, thin glass cristallo. Despite attempts to keep their technology secret, it soon spread north over the Alps to Germany, France, Belgium and England.

In England, where deforestation was a problem as early as the 15th century, glassmakers were required after 1615 to use coal instead of wood in the glassmaking process. About 1675, the English learned to add lead oxide to the basic glass formula, and the resulting solid, heavy and durable vessels progressively replaced the fragile glasses of Venice.

Flat glass for windows was still rare during much of the 17th and 18th centuries. Small panes were made by blowing a large glob of glass, removing it from the blowing iron and then rotating the glass quickly so it would spread and flatten. Such glass had a dimple in its center, many air bubbles and a pattern of concentric circles, but it was transparent and effective in keeping out the weather. At the end of the 17th century, the French learned how to grind and polish cast glass to produce plate glass, but only the rich could afford it.

During the 1800s, glass technology improved rapidly. A hand-operated split mold developed in 1821 that ended the age of blowing individual bottles, glasses and flasks. A semi-automatic bottle machine perfected 50 years later mass-produced bottles and turned them into the everyday miracle they are today.

Great strides were made in the manufacture of flat glass during the 19th century. Compressed air technology led to flatter, better glass panes. Controlled amounts of air were used to blow a large glass cylinder, which was slit lengthwise, reheated and allowed to flatten under its own weight. Large, relatively inexpensive lites of glass were produced in this manner. As a result of such technological advances, window areas that required 18 to 24 panes to enclose in 1730 could be increased dramatically and glass prices dropped by the 1860s, glass-enclosed "wind eyes" were commonplace in the humblest homes.

Plate glass, that wickedly expensive French product, also became commonplace by the end of the 19th century. Water power, then steam and then electricity made the grinding and polishing of heavy glass plates faster and easier. By the 1860s, smart stores and office buildings in Europe and North America glistened with plate glass. France, Belgium and Germany monopolized the production of the product until 1883, when the Pittsburgh Plate Glass Company became the first successful manufacturer of the product in the United States. By 1895, the company could produce 20 million square feet of plate glass a year, and imports from Europe fell sharply.

With the 20th century came an era of revolutionary technology. Machines were developed, improved and perfected to produce endless ribbons of sheet (window) glass, to produce plate glass polished and ground simultaneously on both sides and to produce float glass on a bed of molten tin. Also developed were processes to strengthen glass through thermal and chemical tempering, to add tints to glass for reduced heat transmission and glare and to coat glass with transparent metal and metal oxide films that reflected heat or conducted electricity. And products marrying these processes and developments were created to help make life more convenient, more comfortable, safer and more beautiful.

In retrospect, the romance of glass is not an Egyptian producing a bottle for a Pharoah or window glass being made from a cylinder, a pane at a time, in a one-man glass house. The true romance of glass is the story of the reasonable cost for use in architecture, transportation, industry, science and the home. Billions of people now benefit because technology has made glass a versatile, easy-to-use miracle.

The Nature of Glass
Glass is not easily described.

Its physical structure does not conform to liquid, solid or gas. Glass actually is more of a liquid than the solid it appears to be. Its complex nature has intrigued man from ancient times.

The American Society for Testing and Materials defines glass as "an inorganic product of fusion which has cooled to a rigid condition without crystallizing". Glass can be considered, then, an unusual material which has the random atomic arrangement of liquid but which somehow has been "frozen" in place so that it is a solid and permanent substance. Glass can be transparent, translucent or opaque. It is non-porous, non-absorptive and impervious to the common elements and many harsh chemicals and liquids. It is exceptionally resistant to abrasion and surface scratches. It is one of the best electrical insulation materials, yet can be treated to conduct electricity. Glass has lower head conductivity than most metals and can possess a very low, zero or even negative coefficient of expansion. Because it contains a large proportion of silica and is produced by the action of heat upon that silica, it is generally categorized as a ceramic. Glass, however, stands in a class by itself, quite distinct from other ceramics. Most ceramic materials are shaped cold and then fired to produce the desired result; glass is shaped at extremely high temperatures and then allowed to cool. It again may be made semi-plastic, plastic or even molten by the further application of heat. For this reason, glass also is considered a thermoplastic material, which softens when heated and hardens when cooled.

The Flat Glass Recipe

Glass goes back millenniums formed by nature as obsidian, or black glass, a hard noncorrosive, semiopaque substance fused by volcanic eruptions and enduring centuries of erosion.

This natural glass is composed of three elements of the earth-sand, soda and lime. These same elements in varying forms also make up the basic composition of manufactured glass products ranging from containers and glassware to windshields and windows for high-rise commercial buildings. About 50 other chemical elements are used in modern glassmaking, in major and minor ways, to affect color, viscosity or durability, or to impart some desired physical property. But nature's original ingredients are still basic elements in the formulation of glass.

Glass largely is an open chain of silicon atoms with atoms of various oxides occupying the spaces between. It is this loose structure that permits transparency. Silica, or sand, is the most important ingredient in glassmaking since it is the source of, and provides the structure for, transparency. But sand requires soda and lime for practical glassmaking.

Today, an average batch mix used to manufacture flat glass products contains about 70 percent silica sand, 13 percent lime, 12 percent soda and small amounts of other materials. About one-quarter of the batch is in the form of cullet, or cleaned and crushed glass recovered from previous glassmaking operations.

Silica or silicon dioxide, which is converted into glass by the action of heat is very difficult to fuse, requiring extremely high temperatures. Ancient scientists discovered that other materials such as soda, when melted in close contact with sand, would permit the melting of silica at much lower temperatures. Such materials are known as fluxes, and soda was probably the first flux.

The primary forms of soda used in glassmaking are soda ash (sodium carbonate) or caustic soda (sodium hydroxide). When a mixture of sand and soda dissolves in the molten soda, forming sodium silicate. Depending on the proportions of sand and soda, this sodium silicate is more or less soluble in water and is known as water glass. To overcome water solubility of glass, another element, lime, is required.

Lime (calcium oxide) usually is introduced into the glass batch mix in the form of limestone. Its use in correct proportion causes formulation of a soda-lime-silicate composition that is virtually unaffected by moisture or acids. Lime also renders the glass more viscous at the working temperature, shortens the setting time and improves weathering properties.

Because of its low melting range, the soda-lime-silicate composition undoubtedly was the type used by ancients to produce the earliest known vessels, vases, semi-precious glass stones and beads, and, much later, the earliest form of window glass. Today, soda-lime-silicate is the basis for float glass, and of course, products fabricated from it.

Other materials are added to produce different properties in the basic flat glass product or to replace one of the basic elements to produce different types of commercial glasses. Lead, for example, in the form of lead oxide, may be used to replace lime, and is introduced to increase brilliance, density and index of refraction. Lead glasses included optical and ophthalmic glasses and the finest stemware and art objects. Boron, substituted in whole or in part for the silica, increases the refractive index, deepens the color produced by various other coloring materials, and greatly reduces the coefficient of thermal expansion. Borosilicate glasses are used for such high heat resistant products as ovenware, laboratory glasses and range surfaces. Metallic oxides are added to produce tinted or colored glasses.

 

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