Glasses

Type of physical science: Condensed matter physics

Field of study: Liquids

Glass is produced through the fusion of certain inorganic substances that are then cooled into solid mass without crystallizing. Some glasses occur naturally, while others are man-made. Manufactured glass is used in a variety of practical applications.

Overview

Inorganic materials that are found in nature are often categorized as either crystalline or noncrystalline. Those materials that are crystalline are composed of atoms arranged in perfectly symmetrical sequences, or lattices, to form plane surfaces of varying angles to one another.

Despite its texture and appearance, however, glass is not crystalline. Yet, it is not noncrystalline either. This failure to fit either definition makes glass one of the more unusual materials found in nature.

Glass is composed of raw materials that are individually crystalline and that, after being heated to liquid and combined, would begin to crystallize if allowed to cool slowly.

Crystallization is thwarted in the manufacturing process by cooling the material swiftly, thus trapping atoms in solid suspension before they are able to recombine into lattices.

The temperature above which the raw materials in glass will not crystallize--and above which those materials that may be in crystal form will dissolve--is called the liquidous temperature. Each glass composite has its own unique liquidous temperature, and glass manufacturers have learned to avoid allowing the temperature of glass-in-process to linger near that temperature. At the same time, glass must be cooled in a way which keeps one part from cooling faster than another in order to avoid strain. This process is called annealing.

The process whereby crystals form in glass is called devitrification. When molten glass is allowed to cool slowly until it reaches a temperature below the liquidous temperature, devitrification can occur. Most glass, however, is engineered to specifications that discourage crystallization. In fact, glass thickens as it cools, a characteristic that puts the temperature at which it devitrifies just below the liquidous temperature.

Glass is a mixture of oxides that perform three different functions: glass formation, intermediation, and modification. Silica, or silicon dioxide, is by far the most heavily used glass former, the material that gives the composite its glassy structure. Other oxides, including phosphorus, vanadium, arsenic, boron, and germanium, can also be used as glass formers. Other materials, including aluminum oxide, antimony oxide, lead oxide, and zinc oxide, are called intermediates. They function as fluxes in some glasses and glass formers in others. Modifiers are used to bring the melting point of a glass mixture down, and these materials include calcium oxide, potassium oxide, barium oxide, sodium oxide, and lithium oxide. Sometimes, raw materials are not oxides, but rather those substances that will yield oxides during melting in reaction to the presence of other materials.

In the glass manufacturing process, all raw materials are broken down into small, granular grains and mixed with existing glass of similar size called cullet. The composite is heated to temperatures of up to 1,600 degrees Celsius until it forms a red-hot, syruplike mixture which can then be formed.

Natural glass is called obsidian. This dark, translucent material is found in many places around the world and is the product of volcanic activity. Obsidian has been used for more than seventy thousand years to produce tools and weapons.

History records that glass beads were manufactured about four thousand years ago, glassblowing began more than two thousand years ago, and lead glass appeared during the seventeenth century. Since then, glass technology has evolved significantly to satisfy demands that range over a broad spectrum of applications.

The fact that glass is neither crystal nor noncrystal makes it unusual, as does its perfect elasticity. Despite its apparent rigidity, glass will bend and stretch up to some point before breaking when stress is applied. When that stress is relieved, the glass always returns to exactly the shape and form that it had originally. Most other materials found in nature are not perfectly elastic.

Glass is also categorized according to its electrical properties. Because it is highly resistant to the flow of electrons, it is used heavily as an electrical insulating material. Some materials are good conductors of electromagnetic current while others, such as glass, are poor conductors. Glass insulators are used on electrical transmission poles and in other places where the inadvertent flow of electrons from one conducting medium to another is undesirable.

Glass is best known for its optical properties. While it is a poor conductor of electrical energy, it is an excellent conductor of light energy. Glass is almost invariably transparent, which means that most light energy falling on its surfaces is transmitted through it. All materials have unique characteristics which predict the way that they will behave in the presence of light, and transparency is a measure of how efficiently they will reflect, absorb, and transmit light.

Some glasses are designed to transmit light within one discrete range of the visible portion of the spectrum. The spectrum is a continuous band of electromagnetic waves of varying length, each of which is visible as one of the discrete colors demonstrated in the rainbow (red, yellow, green, blue, and violet). Such glasses transmit only one color, while light waves of all other lengths are either reflected or absorbed. Commercial glass that exhibits this behavior includes that found in green, amber, and red traffic lights and the turn-signal lamps on automobiles.

Some glass compositions are designed to absorb visible light and to transmit ultraviolet and infrared light, which are found adjacent to the visible spectrum. Others are designed to absorb ultraviolet and infrared light but to transmit visible light.

The transmission of light through glass is never direct. When light is transmitted, it bends slightly because the speed of the light is slowed somewhat during the transmission, causing its appearance to alter slightly. This bending is called refraction, the process employed in the manufacture of lenses. Eyeglasses and other devices designed either to enlarge or to reduce the appearance of images to the human eye incorporate the concept of refraction to achieve their ends by focusing all incoming light on a specific spot, or focus point. Different glasses have different bending characteristics, which are reflected in a refractive index. The higher is the index, the greater is the angle that is created when light passes through.

Light transmitted through glass also exhibits a characteristic called dispersion. Some colors refract at sharper angles than others, making it difficult to focus all of the incoming waves on the same spot. Consequently, opticians and other lens designers sometimes engineer glass composites in order to absorb or reflect those problem colors. They may use several lenses in combination to capture and focus all incoming light on the same spot.

Glass does not corrode like many other materials. It can be exposed to a wide variety of materials that would attack less-resistant substances. Glass is often used in laboratories to mix and heat volatile mixtures without measurable effect, windows that are made of glass are often exposed to corrosive elements for years without being affected, and liquid products of all kinds are stored in glass for years without a dissolution of the glass into the mixture. There are, however, some acids and alkali solutions that will attack glasses that are made with silicates.

Obsidian is the only naturally occurring glass, but there is a wide variety of commercial glasses that are manufactured for a wide range of practical applications. The most common is soda-lime glass, which serves as typical window glass. It is made from the oxides of silicon, calcium, and sodium and is easily shaped. While soda-lime glass represents almost 90 percent of all glass production, it has drawbacks. Sudden temperature changes and heat can cause it to shatter, and it is not completely resistant to corrosion. Other kinds of glasses include lead-alkali (or lead glass), borosilicate glass, aluminosilicate glass, and fused silica. Soda-lime glass and lead-alkali soften at relatively low temperatures, while borosilicate and aluminosilicate soften at relatively high temperatures. For this reason, they are termed "soft" and "hard" glasses, respectively, as a characterization of their thermal properties.

Applications

Over the centuries, glass has found widespread application in many areas. The first use of glass--the naturally occurring volcanic composite, obsidian--came more than seventy thousand years ago when Neanderthals first learned how to make arrows, spear tips, and other cutting tools. During the second century B.C., the first manufactured glass appeared in the form of ornamental and decorative beads and glass fragments. In the seventeenth century, the British chemist George Ravenscroft invented lead-alkali glass, which quickly became popular with artists because of its brilliance and workability. The glass that is used to form ornamental crystal is actually lead-alkali glass, and not crystal in the chemical sense of the term.

Soda-lime glass is used primarily to manufacture windows and bottles, as it has been for centuries. The first type of manufactured glass, soda-lime glass is the easiest to make and the least costly. Borosilicate glass is highly resistant to sudden temperature changes and heat, which makes it particularly useful in the manufacture of lamp casings, headlights, laboratory equipment, cooking dishes, and dinnerware. For applications that require even greater resistance to high temperatures than borosilicate glass, aluminosilicate glass is used. This type of glass is used in electrical applications, particularly as resistors in circuitry. It is difficult to fabricate and expensive. Ninety-six percent silica glass is highly resistant to heat shock and can stand temperatures of up to 900 degrees Celsius without melting. For this reason, it is used to manufacture sight glasses for furnaces and windshields for spacecraft that must withstand high temperatures during reentry, as well as in other specialized equipment.

Coloring agents are often added to soda-lime glass and lead-alkali glass, which is the way that stained glass and other colored products are made. Opal glass is often used to manufacture cookingware, tableware, and lighting globes. Its soft, translucent appearance makes it ideal for those products. Sometimes, glass is ground into a fine powder and combined with other materials so that it can be pressed into intricate forms. The result is referred to as multiform glass, and it is created through the application of pressure and heat. The final result exhibits many of the same characteristics as the original glass before grinding.

Optical glass, which is used to manufacture lenses that are used in eyeglasses, telescopes, microscopes, cameras, and other devices, must achieve extremely high levels of purity. Most raw materials found in nature contain impurities that would interfere with the optical characteristics of glass, so great care must be taken to eliminate those impurities, as well as bubbles, ripples, lines, and other manufacturing defects.

By combining special agents with optical glass, photosensitive and photochromic glass can be obtained. Photochromic glass darkens when exposed to ultraviolet light but returns to its normal translucence when the ultraviolet light disappears. Photosensitive glass functions in a similar way, but it turns to opal when heated and does not return to its prior appearance. This type of glass is used to capture permanent images with a camera, as well as for doing elaborate photographic etching on glass.

Glass can also be drawn into extremely thin strands that can then be formed into pliable, lightweight compositions for use as thermal insulators. This material, often referred to as fiberglass, can be pressed into hard but flexible materials that are ideal for such products as skis, boats, and fishing poles.

Glass manufacturers have also learned how to temper glass in order to make it stronger than typical glass for use in high-stress applications. Glass can also be strengthened chemically for use in aircraft windows, among other products.

Researchers are continuing to develop glass materials for use in fiber-optic technology.

The telecommunications industry has installed light-sensitive wire to carry data and voice communication over long distances using light-pulse technology. These materials are designed for flexibility and strength, as well as for optimum optical characteristics.

Context

When one thinks of glass, the first image that comes to mind is often that of a window: a clear, transparent sheet of solid material that lets in the full spectrum of visible light, while at the same time acting as a barrier to the elements. While glass is valuable for its ability to transmit light, it has many other characteristics that have made it a popular manufacturing material for centuries. The earliest use for glass was as a cutting tool, taking advantage of its ability to provide extremely sharp edges that never dulled. Later, it was prized for its shiny appearance, which, when combined with coloring agents, could be enhanced for decorative purposes. During the twelfth century, its optical characteristics found application with the invention of the first pair of eyeglasses and, later, the telescope and microscope. More recently, glass has become prized for its electrical, chemical, and thermal properties, which make it ideal for a wide range of uses.

Another key factor in the widespread use of glass is the abundance of the raw materials that are required to manufacture it. Silicates, perhaps better known to the layperson in the form of the sand along any beach, are found in all corners of the globe and are easily extracted, transported, and processed. This availability helps to make common glass an economical product to produce and, in turn, to purchase. The fact that glass has so many attributes enhances its value across a spectrum of applications.

Indeed, the applications of glass have continued to emerge. The ability of glass to resist the flow of electricity has made it a useful tool in the electronic age. Glass provides transmission insulators that are inexpensive and dependable, and specially designed displays for high-technology equipment that are electronic windows into a new world of information, such as graphics, images, and colors that were unimaginable at the close of the nineteenth century.

As more and more is demanded of glass, it proves to be an amazingly resilient and adaptable material. The thermal properties of glass are useful in an age of exploration that is pushing the technical operating temperatures and pressures that are employed in manufacturing and other endeavors continually forward. New films and treatments have been developed that, when applied to glass, enhance its ability to insulate homes, resist temperature fluctuations, and otherwise remain stable under myriad hostile environmental conditions.

Perhaps no more hostile environmental conditions can be found than those that exist in outer space, where exploration requires a wide array of new materials that can function under those conditions. Much research has been aimed at applying glassmaking technology to space exploration. In fact, glass is known to be vulnerable to the effects of the high-energy radiation that is found in outer space. Because discoloration and physical damage result from exposure to γ rays and other types of radiation, efforts have been made to develop composites that can counter these effects.

It is difficult to imagine a world without glass. It has enhanced the quality of human life perhaps like no other material in history. It provides shelter, light, and warmth and acts as a barrier to darkness and cold. When used to manufacture lenses, it provides a tool which allows those with impaired vision to see with great clarity. Because it is flexible in its applicability, it can be found in every corner of the house, garage, office, and indeed, the world.

Principal terms

ANNEALING: the process whereby manufactured glass is cooled in such a way that no region cools at a faster or slower rate than any other

CRYSTALLINE: refers to materials whose atomic structures are arranged into symmetrical configurations which form planes that meet at varying angles

DEVITRIFICATION: the process of crystal formation that occurs when molten glass is cooled slowly

NONCRYSTALLINE: refers to materials whose atomic structures bond to one another in a random, asymmetrical order

OXIDES: binary chemical compounds that include oxygen

TRANSLUCENT: refers to any substance or material that transmits only part of the visible light spectrum

TRANSPARENT: refers to any substance or material that transmits the full range of visible light

Bibliography

Holloway, D. G. THE PHYSICAL PROPERTIES OF GLASS. London: Wykeham, 1973. Details the mechanical, chemical, thermal, and optical characteristics of glass. Emphasizes, in particular, the ways in which these properties benefited the products that have emerged over centuries of glassmaking.

Morey, George W. THE PROPERTIES OF GLASS. New York: Reinhold, 1954. Contains perhaps the best discussion of the basic chemical processes at play in the formation of glass and the processes that have been employed over the centuries to manufacture it. The approach is technical, but the format is organized and readable. Charts, graphs, and an index are provided.

Scholes, Samuel R., and Charles H. Greene. MODERN GLASS PRACTICE. Boston: Cahners Books, 1975. Details the techniques that are employed to produce glass and ceramic materials for use in sophisticated modern products. A good discussion of many of the materials that emerged during the twentieth century as a result of specialized research in the field of glassmaking. Presents charts, graphs, and an index.

Shand, Errol B. GLASS ENGINEERING HANDBOOK. New York: McGraw-Hill, 1958. A highly technical overview of the glass-manufacturing process for those wishing a deeper treatment of the subject. Contains a comprehensive look at the details of glassmaking in the modern age. An index is provided.

Volf, Milos B. TECHNICAL GLASSES. London: Pitman & Sons, 1961. Contains an overview of glasses and glass materials that are used for technical applications in manufacturing, electronics, and other industries that require specialized glass applications. Highly technical.

The Atomic Structure of Liquids

Reflection and Refraction