Pyrex glass is a borosilicate glass first produced by The Corning Glass Works company. It is made by heating raw materials like silica sand and boric oxide to extremely high temperatures for extended periods of time. The molten material is then processed into different types of glassware. First formulated during the early twentieth century, Pyrex has become an important material for a variety of applications that require heat and chemical resistance.
To understand how Pyrex is unique it is important to understand the nature of glass itself. Glass is a state of matter that has characteristics similar to both crystalline solids and liquids. On a macroscopic level glass appears to be like solids. It is rigid and remains in one piece when removed from a container. However, on a molecular level, glasses are more like liquids. In crystalline solids, molecules are arranged in an orderly fashion. In liquids they are randomly arranged. This random arrangement is also a characteristic of glass.
Glass is typically made by heating crystalline compounds to temperatures high enough to melt them. Melting breaks the ordered molecular structure, leaving them in a disordered state. When the melted material is cooled, the molecules become locked in place before they can reform in the ordered crystalline structure. The properties of a specific glass such as hardness, brittleness, clarity, and chemical and thermal resistance are dependent on its chemical composition.
When Pyrex was being developed, scientists were trying to create a glass composition that had a high thermal resistance. At some point it was discovered that glass compositions with boron could be heated to high temperatures without breaking. Boron, which is the fifth element on the periodic chart, has the unique ability to create a variety of chemical bonds. When bonded with oxygen it can create a three dimensional structure that is strong. In a glass composition, this extra strength gives it thermal and chemical resistance that makes it useful for cooking applications, thermometers, and laboratory equipment. Pyrex also has a low alkali content that gives it high corrosion resistance.
While the exact date people found that sand could be combined and melted with other materials to produce glass is not known, its discovery was likely accidental. Formal processes for glassmaking have been known for over 3,000 years. In Mesopotamia, archeologists have uncovered clay tablets that contain ancient "instructions" for making glass in furnaces. Throughout history, glass production technology became more sophisticated. People steadily discovered the best proportions to combine the raw materials and also learned manufacturing practices like glass blowing.
During the early twentieth century, kerosene lanterns were widely used for streetlights and railroad signaling devices. Unfortunately, the glass used for making these lanterns was sensitive to the heat of the flame and would often break. Scientists began searching for glass formulas that could withstand heat.
The first experiments led to the discovery that when boric acid was present in the raw materials, the glass was more heat resistant. These early formulas were chemically weak however, often breaking down in water. Work proceeded to find the right proportions of silica sand and boric oxide that would continue to be heat resistant and chemically stable. In 1912, an adequate formula was found. These glasses, called borosilicates, were then introduced into lantern production. One of the original types of borosilicate glass introduced by the Corning Glass Works Company was brand-named Nonex.
The potential for this product in the area of cooking was discovered in 1913 by Dr. Jesse T. Littleton who worked at Corning. He gave his wife a casserole dish made out of Nonex, the precursor to Pyrex. It worked as well as a ceramic cooking dish and a new era in cooking ware had begun. The Nonex glass formula was revised to remove lead, and the ovenware was given to the Philadelphia Cooking School for more testing. A series of successful tests there led to the introduction of Pyrex ovenware in 1915. This same year the Corning Glass Works Company patented the formula and gave it the trademarked name Pyrex. It has been suggested that the term Pyrex was either a derivative of the word "pie" (referring to its original use) or the Greek "pyra," which means hearth. In both cases, the "ex" suffix was used to give it brand-name similarity to Nonex.
When World War I broke out, scientists who relied on German glass products found that the new Pyrex material met their needs for beakers, test tubes, and other laboratory glassware. Borosilicate glass has steadily been made more chemical, heat, and shock resistant. It has also been applied to numerous products such eyeglasses, telescopes, and electronic components.
Three classes of materials are used in making Pyrex including formers, fluxes, and stabilizers. Formers are the main ingredients in all glassmaking. These are crystalline materials that, when heated high enough, can be melted and cooled to create glass. Fluxes are compounds that help lower the temperature required to get the formers to melt. Stabilizers are materials that help keep glass from crumbling, breaking, or falling apart. They are needed because fluxes typically destabilize glass compositions.
Eugene G. Sullivan established Corning Glass Works' research laboratory in 1908 and set out with William C, Taylor to make a heat-resistant glass for railroad lantern lenses. The problem was that flint glass (the kind in bottles and windows, made by melting silica sand, soda, and lime) has a fairly high thermal expansion but poor heat conductivity. Both causing the glass to break. Two solutions were possible: improve thermal conductivity or reduce thermal expansion. The formulation that Sullivan and Taylor devised was a borosilicate glass—a soda-lime glass with borax replacing the lime—with a small amount of alumina added. This gave the low thermal expansion needed and also had good acid-resistance, leading to use for the battery jars required for railway telegraph systems and other applications. The glass was marketed as "Nonex" (for nonexpansion glass).
Jesse T. Littleton joined Corning in 1913. A physicist, Littleton knew glass absorbs radiant energy well, while metal mostly reflects it. Littleton took a cut-off battery jar home and asked his wife to bake a cake in it. He took it to the laboratory the next day. Littleton developed variations on Nonex and the result was Pyrex, patented and trademarked in May of 1915.
Initial sale of Pyrex took place at the Jordan Marsh department store in Boston in 1915. By 1919 more than 4.5 million pieces were sold. In 1915, Pyrex was introduced into the laboratory. Laboratory glassware came from Germany but World War I cut off the supply. Corning filled the gap with Pyrex glassware, which worked so well that Pyrex replaced most other items. Today, Corning-style glassware is found in laboratories all over the world.
The primary formers used for making Pyrex include silica sand and boric acid. Silica sand is also known as silicon dioxide. It is a crystalline material and was probably the major component of the first glass used by humans. In a typical Pyrex glass composition, silicon dioxide makes up about 60-80% by weight.
Pyrex has a droplet in matrix phase structure. The silicon dioxide creates the basic matrix. The borate material creates the droplets within that structure. The borate former can come from a material like sodium tetraborate. Prior to manufacture, this compound is chemically reduced with sulfuric acid to create boric acid. When boric acid is mixed with silicon dioxide and heated, it oxidizes into boric oxide. Boric oxide is responsible for the unique Pyrex molecular structure. Boric oxide makes up anywhere from 5% to 20% of Pyrex glass.
Secondary ingredients used in glass production include fluxes, stabilizers, and colorants. Fluxes are included in glass mixtures because they reduce the melting temperature of the borosilicate glass. Fluxes that can be used in manufacture include soda ash, potash, and lithium carbonate. They make up about 5% of a Pyrex glass composition.
Unfortunately, fluxes also cause the glass to be more chemically unstable. For this reason stabilizers such as barium carbonate and zinc oxide are included. In Pyrex manufacture, about 2% aluminum oxide is added to make the glass stiffer when it is molten. Finally, to produce glass with different colors, silver compounds can be added.
The manufacturing process can be broken down into two phases. First, a large batch of molten glass composition is made. Next, the glass is fed into shaping machines to create different types of glassware. The process moves at tremendous speeds and is quite efficient.
- 1 Large batches of Pyrex glass are produced in a specified compounding area of the production plant. Here, glassmakers follow formula instructions and add the required raw materials in the correct proportions into large tanks. Prior to use, the raw materials are pulverized and granulated to a uniform particle size. They are stored in batch towers. The materials are mixed together and heated to temperatures over 2,912°F (1,600°C). This high temperature melts the ingredients and allows them to thoroughly mix to create molten glass. However, the mixture typically requires longer heating—up to 24 hours—to remove excess bubbles that can lead to a weaker structure.
- 2 The batch tanks are designed so that the molten glass will flow slowly toward the working end of the tank. This end of the tank is connected to continuous feed forming machines. As the glass moves from the tank it looks like a thick, red-orange syrup. The forming machines work the material quickly because as it cools it becomes rigid and unworkable. Typical glass processing machines blow, press, draw, and roll it into various shapes.
- 3 The forming process used depends on the final product. Glass blowing is used to create thin-walled products like bottles. A bubble of the molten glass is put inside a two-piece mold. Air is forced into the mold, which presses the glass against its sides. The glass cools inside the mold and conforms to the shape. Glass pressing is used to create thicker pieces of glass. The molten glass is put into a mold and a plunger is lowered which forces the glass to spread and fill the mold. Drawing is used to create tubing or rods. In this process molten glass is drawn down over a hollow cone called a mandrel. Air is blown through it to keep the tube from collapsing until the glass becomes rigid. For glass sheets, like windows, a rolling process is used.
- 4 After the product is formed, it is cooled and polished. It may then be decorated with various printing or markings and fitted with plastic pieces if necessary. The glass product is then checked for imperfections, put in protective boxes and shipped out to customers. Depending on the size of the batching tank, as much as 700,000 lb (317,520 kg) of glass product can be produced in one year.
Since the quality of the glass depends on the purity of the raw materials, manufacturers employ quality control chemists to test them. Physical characteristics are checked to make sure they adhere to previously determined specifications. For example, particle
Since Pyrex is made from compounds that become oxides when heated, air pollution is a potential problem. A variety of byproducts may be released during manufacture including nitrates, sulfates, and chlorine. These chemicals can react with water to form acids. Acid rain has been shown to cause significant damage to manmade structures as well as natural ecosystems. One method glassmakers use to reduce pollution is by making glass compositions that have lower melting temperatures. Lower temperatures reduce the amount of volatilization thereby reducing the amount of gaseous pollutants. Another pollution control is the use of precipitators that are installed in chimneys. These devices help reduce air pollution by filtering out solids that persist in smoke and vapor created by the melting process. Waste-disposal drains are monitored to ensure that only allowable amounts of factory waste are released into the environment. This helps prevent water pollution.
An additional method of pollution control is the use of ventilators. These devices are also called regenerators because they help recover and recycle heat energy consumed during manufacture. This has the double effect of reducing air pollution and lowering production costs. Other cost reducing and environmentally sound techniques employed include the use of electric heat instead of gas heat, and the incorporation of broken recycled glass during the production of new glass.
In the future, borosilicate glass manufacturers will concentrate on increasing sales and improving the production process. To increase sales, glass manufacturers will be involved in finding and promoting new applications for their products. This could require new glass formulations that have a range of characteristics from clarity, melt point, and shatter resistance. From a production stand-point, future improvements will focus on increasing manufacturing speeds, minimizing chemical waste, and reducing overall costs.
Where to Learn More
Bansal, N. P., and R. H. Doremus. Handbook of Glass Properties. New York: Academic Press, Inc., 1986.
Kirk-Othmer Encyclopedia of Chemical Technology. Vol. 12. New York: John Wiley & Sons, 1994.
Mazurin, 0. V. Handbook of Glass Data. New York: Elsevier Science Publishing Co., 1991.
Rogove, S. T., and M. B. Steinhauer. Pyrex by Corning: A Collector's Guide. New York: Antique Publications, 1993.
Corning Museum of Glass Web Page. 1 October 2001. < http://www.cmog.org >.
United States Patent 4,075,024. Colored Glasses and Method. 1976.