Glass, Heat-Resistant

Glass, Heat-Resistant

During the twentieth century, heat-resistant borosilicate glass replaced more fragile flint or soda-lime glass. Although German optician Carl Zeiss (1816-1888) began experiments with heat-resistant glass for use in microscopes by adding boric acid to silicon, the stronger material evolved from the Corning Glass Works' invention of oven-proof Nonex and Pyrex in 1915, and Vycor, an essential material in stove heating elements. Later varieties include Pyroceram, Kimax, and Corning Ware, a multipurpose ceramic material used to make freezer-to-table dishes for freezing, cooking, baking, microwaving, and serving as well as for laboratory equipment.

Since the 1870s, the Corning Glass Works has developed and improved other heat-resistant glass products with practical applications, particularly globes for incandescent lighting, weatherproof lenses for railroad signal lanterns, and colorants for lens glass.

Developed by Eugene G. Sullivan and William C. Taylor for Corning Glass Works in 1915, Corning Ware evolved from an experiment by the wife of Jesse T. Littleton, Corning's chief physicist. When she baked puddings, Mrs. Littleton substituted the bottom portion of a battery jar for breakable casseroles. From her success came Pyrex or nonexpanding glass, made from borax, alumina, sodium, and soda and fired at over 2,500°F (1,371°C). The low thermal quality reduced the danger of breakage; its reduced sodium content lessened the chance of chemical interaction.

A second application of glass technology improved the usability of Pyrex glass for baking dishes that absorb heat rather than reflecting it, as do metal pans. Also, the creation of glass pie plates, cake pans, casseroles, and custard dishes decreased the carryover of flavors, which occurs in metal pans. The perfection of Pyrex led to a successful marketing of glass bakeware. Slightly higher in price because of the cost of high temperature fuels during manufacture, Pyrex holds its own in the market because it resists breakage where earlier types of tempered glass shatter. By 1919, over 4.5 million pieces of the ovenproof bakeware were in use. The surge of interest in heat-proof glassware proved propitious for industry during World War I, when hostilities halted the supply of laboratory beakers, funnels, tubing, culture dishes, thermometers, and flasks from Germany.

Scientific applications for Pyrex include stronger lenses for reflector telescopes, as blanks for mirrors, and as components in construction projects, such as hulls for small boats, thermal insulation, acoustical soundproofing, and fireproof fabrics. Pyrex is also an integral part of headlight lenses for automobiles and in pipelines that carry corrosive materials such as lye. Pyrex fibers, because of their resilience and nonabsorbancy, are clean and shatterproof and can be substituted for asbestos, which is harmful to the lungs of workers. Pyrex fibers are used primarily in construction, particularly as lining for ductwork and kitchen appliances, construction of naval vessels, rockets, launch pad insulation, gas turbine silencers and mufflers, high temperature gaskets and seals, solar panels, expansion joints, high temperature filtration, coverings for furnaces and steam generators, and ports for space vehicles. They are also valuable for decorative and safety purposes, as in soundproof, noncombustible ceiling panels.

The evolution of glass as a heat-resistant material has resulted in additional products. In 1957, Donald Stookey of Corning Glass Works discovered that the addition of certain nucleating agents to Fotoform glass (used in electronic components and fluidic control devices) produced glass objects that could be transformed into fine-grained ceramics by heat treatment. This new Pyroceram composition became the basis of Corning Ware, first marketed in 1958. Pyroceram is used in bakeware, hot plates, and stove tops. The thermal endurance and stability of this and other glass-ceramics, such as Schott Glaswerkes' Robax, have made them ideal for missile nose cones and astronomical telescope mirror substrates. Transparent glass-ceramics were first introduced in the 1970s.

For comparison, a purely crystalline silicate (which is not vitreous and therefore not a glass) known as cristobalite is made at over 3,000°F (1,649°C). The raw material is turned into vapor, then condensed. Its purity makes it the most heat-resistant of all types of silicates, although a major drawback in its production is its limited malleability. Because containers for this process must be made of tungsten or graphite, the cost of crystobalite objects, such as arc tubes for lamps, crucibles for melting semiconductors, covers for solar cells, envelopes for mercury vapor lamps, optical parts, and telescope mirror, is much higher than for Pyrex.