Glass and Glassmaking

Summary

Glassmaking is a diverse field with applications ranging from optics to art. Most commercially produced glass is used to make windows, lenses, and food containers, such as bottles and jars. Glass is also a key component in products, such as fiber-optic equipment and medical devices. Many types of glass are recyclable, making them relatively environmentally friendly. Most careers in glassmaking require significant hands-on experience as well as formal education.

Definition and Basic Principles

Glass is one of the most widely used materials in residential and commercial buildings, vehicles, and many devices. Glass windows provide natural light while protecting indoor environments from changes in the outside temperature and humidity. The transparency of glass also makes it a good choice for lightbulbs, light fixtures, and lenses for items, such as eyeglasses. A nonporous and nonreactive substance, glass is ideal for food packaging and preparation. Most types of glass can be recycled with no loss in purity or quality, which has increased its appeal.

Although glass resembles crystalline substances found in nature, it does not have the chemical properties of crystals. Glass is formed through a fusion process involving inorganic chemical compounds, such as silica, sodium carbonate, and calcium oxide (also known as lime). The compounds are heated to form a liquid. When the liquid is cooled rapidly, it becomes a solid but retains certain physical characteristics of a liquid, a process known as glass transformation.

Background and History

The earliest known glass artifacts come from Egypt and eastern Mesopotamia, where craftspeople began making objects, such as beads, more than five thousand years ago. Around 1500 Before the Common Era (BCE), people began dipping metal rods into molten sand to create bottles. In the third century BCE, craftspeople in Babylon discovered that blowing air into molten glass (glassblowing) was a rapid, inexpensive way to make hollow shapes.

Glassmaking techniques quickly spread throughout Europe with the expansion of the Roman Empire in the first through fourth centuries. The region later known as Italy, led by the city of Venice, dominated the glass trade in Europe and the Americas for several hundred years with wares that became known as Venetian glass. Another notable tradition was Bohemian glass, a decorative style produced in Bohemia and surrounding regions beginning in the thirteenth century.

With the development of the split mold in 1821, individual glass objects no longer needed to be blown and shaped by hand. The automation of glass manufacturing allowed American tinsmith John L. Mason to introduce the Mason jar in 1858. Over the next few decades, further innovations in glassmaking led to a wide range of glass applications at increasingly lower costs. In 1915, the Corning Glass Works trademarked Pyrex, a heat-resistant type of glass that became widely used in cookware.

One of the most noteworthy advancements in glass use came in the 1960s and 1970s with the design and rollout of fiber-optic cable for long-range communications.

How It Works

Most glasses have translucent properties, which means that certain frequencies, or colors, of light can pass through them. Transparent glass can transmit all frequencies of visible light. Other types of glass act as filters for certain colors of light so that when objects are viewed through them, the objects appear to be tinted a specific shade. Some types of glass transmit light but scatter its rays so that objects on the other side are not visible to the human eye. Glass is often smooth to the touch because surface tension, a feature similar to water, binds its molecules together during the cooling process. Unless combined with certain other compounds, glass is brittle in texture.

There is a widespread but incorrect belief that glass is a liquid. Many types of glass are made by heating a mixture to a liquid state and then allowing it to reach a supercooled state where it cannot flow. The process, known in glassmaking as the glass transition, causes the molecules to organize themselves into a form that does not follow an extended pattern. In this state, known as the vitreous or amorphous state, glass behaves like a solid because of its hardness and tendency to break under force.

Chemical Characteristics. The most common type of glass is known as soda-lime glass, made primarily of silica (60 to 75 percent). Sodium carbonate, or soda ash, is added to the silica to lower its melting point. Because soda ash makes it possible for water to dissolve glass, a third compound, such as calcium carbonate, or limestone, must be added to increase insolubility and hardness. For some types of glass, compounds, such as lead oxide or boric oxide, are added to enhance properties like brilliance and resistance to heat.

Manufacturing Processes. The melting and cooling of a mixture to a liquid, then supercooled state, is the oldest form of glassmaking. Most glass products, including soda-lime, are made using this process. As the liquid mixture reaches the supercooled state, it is often treated to remove stresses that could weaken the glass item in its final form. This process is known as annealing. The item's surface is smoothed and polished through a stream of pressurized nitrogen. Once the glass transformation is complete, the item can be coated or laminated to increase traits, such as strength, electrical conductivity, or chemical durability.

Specialized glass can be made by vapor deposition or sol-gel (solution-gel) processes. Under vapor deposition, chemicals are combined without being melted. This approach allows for the creation of thin films that can be used in industrial settings. Glass created through a sol-gel process is made by combining a chemical solution, often a metal oxide, with a compound that causes the oxide to convert to a gel. High-precision lenses and mirrors are examples of sol-gel glass.

Applications and Products

Glass is one of the most widely used materials in the world. Some of the most common uses of glass include its incorporation into buildings and other structures, vehicles such as automobiles, fiberglass packaging materials, consumer and industrial optics, and fiber optics.

Construction. The home building industry uses glass to design and install windows, external doors, skylights, sunrooms, porches, mirrors, bathroom and shower doors, shelving, and display cases. Many office buildings have floor-to-ceiling windows or are paneled with glass. Glass windows control internal temperatures and humidity levels while still allowing natural light to enter a room. Light fixtures frequently include glass components, particularly bulbs and shades, because of the translucent quality of glass and its low manufacturing costs.

Automotive Glass. Windshields and windows in automobiles, trucks, and other vehicles are nearly always made of glass. Safety glass is used for windshields. Most safety glass consists of transparent glass layered with thin sheets of a nonglass substance, such as polyvinyl butyral. The nonglass layer, or laminate, keeps the windshield from shattering if something strikes it and protects drivers and passengers from injury in an accident. A car's side and back windows generally are not made of safety glass, but rather they are tempered glass treated to be heat resistant and block frequencies of light, such as ultraviolet rays.

Fiberglass. Russell Games Slayter, an employee of Owens-Corning, invented a glass-fiber product in 1938, which the company named Fiberglas. The product was originally sold as thermal insulation for buildings and helped phase out the use of asbestos, a carcinogen. The tiny pockets of air created by the material's fabriclike glass filaments are the source of its insulating properties. Fiberglass is also used in manufacturing vehicle bodies, boat hulls, and sporting goods, such as fishing poles and archery bows, because of its light weight and durability. Because recycled glass makes up a significant portion of many fiberglass products, the material is considered environmentally friendly.

Packaging. Processed foods and beverages are often sold in glass bottles and jars. Glass's clarity allows consumers to see the product and judge its quality. Glass is a nonporous, nonreactive material that does not affect the flavor, aroma, or consistency of the product it contains. Some consumers also prefer glass because it can be recycled. A disadvantage of glass packaging is the ease with which containers can be shattered. Glass is also heavier than competing packaging materials, such as plastic, aluminum, and paper.

Consumer Optics. Glass lenses can be found in a wide range of consumer-oriented optical products, from eyeglasses to telescopes. The use of glass in eyeglasses has diminished some as lenses are also made from specialized plastics. Cameras, however, rarely use plastic lenses, as they create lower-quality images and are at greater risk of being scratched. Binoculars, microscopes, and telescopes designed for consumer use are likely to contain a series of glass lenses. These lenses are often treated with coatings that minimize glare and improve the quality of the image being viewed.

Industrial Optics. For the same reasons that glass is preferred in consumer optics, it is the material of choice for industrial applications. Many specialized mirrors and lenses used for telescopes, microscopes, and lasers are asymmetrical or aspherical in shape. The manufacture of aspherical lenses for high-precision equipment was difficult and expensive until the 1990s when glass engineers developed new techniques based on the processing of preformed glass shapes at relatively low temperatures. These techniques left the surface of the glass smooth and made it more cost-effective to manufacture highly customized lenses.

Fiber Optics. The optical fibers in fiber-optic communications infrastructure are made from glass. The fibers carry data in the form of light pulses from one end of a glass strand to the other. The light pulses then jump through a spliced connection to the next glass fiber. Fiber-optic cables can transmit information more quickly and over a longer distance than electrical wires or cables. Glass fibers can carry more than one channel of data when multiple light frequencies are used. They are also not subject to electromagnetic interference, such as lightning strikes, which is advantageous when fiber-optic cable is used to wire offices in skyscrapers.

Electronics. Glass is an essential component of cellphones, computer monitors, televisions, and other electronic devices. The proliferation of mobile devices, including smartphones, tablets, and laptops, has led to the development of specialized glass that resists breaking. Glass powder is used in manufacturing electronic components, while glass seals are used to protect some devices.

Careers and Course Work

The skills required to work in the glassmaking industry depend on the nature of the glass products being made. Manufacturing and installing glass windows in buildings is a career within the construction industry while designing glass homeware involves the consumer products industry. Making lenses for eyeglasses is a career track that differs significantly from the production of specialty lenses for precision equipment, such as telescopes.

Glaziers in construction gain much of their knowledge from on-the-job training. Many glaziers enter apprenticeships of about three years with a contractor. The apprentice glazier learns skills, such as glass cutting, by first practicing on discarded glass. Tasks increase in difficulty and responsibility as the apprentice's skills grow. This work is supplemented by classroom instruction on topics, such as mathematics and the design of construction blueprints. Similarly, makers of stemware and high-end glass products used in homes learn many skills through hands-on work with experienced designers.

Careers in optical glass and lens design generally require more formal education than other areas of glass production. Several universities offer course concentrations in glass science, optics, or ceramics within undergraduate engineering programs. Graduate programs can focus on fields as narrow as electro-optics.

Students planning careers in optical glass take general classes, such as calculus, physics, and chemistry. Coursework on topics, such as thermodynamics, microscopy, spectroscopy, and computer-aided design, are also standard parts of an optical engineer's education.

Social Context and Future Prospects

Demand for glaziers in the construction industry rises and falls with the rate of new buildings being built. Other societal trends can also impact the glassmaking industry. For example, in the early twenty-first century, the development of window glass with energy-efficient features led to the upgrading of windows in office buildings and homes, which increased the need for skilled glaziers. As climate change increasingly became a serious concern as the twenty-first century progressed, innovations continued to be made in glass glazing and design to allow for more low-energy structures.

Manufacturers have developed a wide range of optical glass types for specialized applications, such as lasers. By the end of the second decade of the twenty-first century, the need for increasingly precise lenses had grown steadily, with developments in fields ranging from microsurgery to astronomy. Innovation continued as smart glass and electrochromic technologies allowed for glass that changed from transparent to opaque depending on conditions. New glass coatings also increased the strength and flexibility of glass. Finally, the glass manufacturing industry focused on sustainability measures and energy efficiency. 

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