Glass is an amorphous solid made by fusing silica with a basic oxide. Although its atoms never arrange themselves in a crystalline order, atomic spacing in glass is tight. Glass is characterized by transparency, hardness at atmospheric temperatures, and excellent resistance to weathering and most chemicals except hydrofluoric acid.
Most glass is based on the silicate system and is made from three major constituents: silica (SiO), lime (CaCO3), and sodium carbonate (NaCO3). Various oxides are added to tailor the properties of glass to meet specific requirements.
Nonsilicate-system glasses, which account for only about 5% of all glass produced (excluding glass ceramics, described in the chapter, Engineering ceramics), include phosphate glasses (that resist HF acid), rare-earth borate glasses (for high refractive index), heat-absorbing glasses (made with FeO), and systems based on oxides of aluminum, vanadium, germanium, and other metals. Nearly all glasses can be categorized into one of six basic types, based on chemical composition. Within each type, except for fused silica, are several distinct compositions.
Soda-lime glass, the most common type, is the glass of bottles, windows, light bulbs and drinking glasses. Its composition is similar to that of the earliest man-made glass -- a mixture of the oxides of silicon, calcium, and sodium. Approximately 90% of all glass melted today is soda-lime (or simply "lime" as it is commonly called). This inexpensive glass is readily fabricated to a wide variety of shapes. Resistance to high temperatures and to sudden temperature changes is poor, and resistance to attack by chemicals is only fair.
Borosilicate glass, the oldest type of glass to have appreciable resistance to thermal shock and higher temperatures, also has excellent resistance to chemical attack. In this glass structure, the first to carry the Pyrex trademark, some of the SiO″ is replaced by boric oxide.
Borosilicate glass has a low coefficient of thermal expansion and is, thus, suited for telescope mirrors and other precision parts. Also, because this glass can withstand thermal shock, it is used for oven and laboratory ware, headlamp lenses, and boiler gage glasses. Most borosilicate glasses have better resistance to acids than do soda-lime glasses, but poor resistance to alkalis. Glass fibers used in reinforcing plastic compounds are a modified borosilicate glass.
Lead-alkali glass, or lead glass, contains lead monoxide, PbO, to increase its index of refraction. This glass is a better electrical insulator than soda-lime or borosilicate glasses. Lead glass is used for optical applications such as prisms and lenses and as a shield against atomic radiation. It is easy to work and is well suited for slow, manual operations. Because of its natural luster, lead glass is used for fine crystal tableware. Like lime glass, lead glass has poor resistance to high temperatures and to thermal shock.
Aluminosilicate glass (in which some alumina, Al2O3, replaces silica) is another thermal-shock-resisting glass similar to borosilicate but able to withstand higher operating temperatures. These glasses also resist chemical attack and are good electrical insulators. Aluminosilicate glasses are suited for high-performance applications such as high-temperature thermometers, space-vehicle windows, and ignition tubes. Coated with an electrically conductive film, they are used as resistors in critical electronic circuitry. Aluminosilicates cost about three times more than borosilicates and are appreciably more difficult to fabricate.
96% silica glass is a highly heat-resistant glass made from borosilicate glass by a proprietary (Corning Glass Works) process. This glass can be formed more readily and into more shapes than can fused silica. Its properties are so close to those of fused silica that it is sometimes used as a substitute in optical components and spacecraft windows, where it must withstand the heat of reentry into the earth's atmosphere. It is also used as a heat-resisting coating such as on the exterior of NASA's space-shuttle vehicles. Other uses include laboratory ware and lighting components such as arc tubes in halogen lamps.
Fused silica is the only one of the six categories that contains a single composition. This glass consists simply of silica (silicon dioxide) in the noncrystalline, or amorphous, state. Fused silica, most expensive of all glasses, offers the maximum resistance to thermal shock as well as the highest permissible operating temperature (900°C for extended periods, to 1,200°C for short periods). It also has maximum transmission in the ultraviolet range and the highest resistance to chemical attack of any glass. Fused silica is used in applications where requirements are extremely strict, such as mirror blanks for astronomical telescopes, ultrasonic delay lines, optical communications waveguides, and crucibles for growing crystals. Fabrication of fused silica is difficult, and the number of available shapes is, therefore, sharply limited.
These six types of glass can be grouped in three pairs. Soda-lime and lead-alkali are termed soft glasses because they soften or fuse at relatively low temperatures. Borosilicate and aluminosilicate are called hard glasses because they soften or fuse at relatively higher temperatures. And 96% silica and fused silica are the hardest of all.
The oldest of the glasses is soda-lime, which was known some 4,000 years ago. Lead-alkali was developed in 1676, borosilicate in 1912, aluminosilicate in 1936, 96% silica in 1939, and fused silica in 1952.
Today, many glass products are made from composites, made up of several glasses of differing composition. High-strength tableware is made of a sandwich of a low-expansion glass and a high-expansion glass core. Optical communications fibers (waveguides) are drawn from a boule built up from glass having a controlled variation in composition. Aerospace-vehicle windows are composed of multiple panes of glass, each pane with a unique property; the outermost panes are heat resistant, the innermost panes are mechanically strong.
Light-sensitive glasses, although not considered a basic type, are available in three grades. Photochromic glass darkens when exposed to ultraviolet radiation and fades when the ultraviolet stimulus is removed or when the glass is heated. Some photochromic compositions remain darkened for a week or longer. Others fade within a few minutes after ultraviolet is removed. A chief use for the faster-fading compositions is in eyeglass lenses that automatically darken and fade when exposed to or removed from sunlight.
Photosensitive glass also responds to light, but in a different manner from photochromic glass. When exposed to ultraviolet energy and then heated, photosensitive glass changes from clear to opal. When the UV exposure is made through a mask, the pattern of the mask is reproduced in the glass. The image developed is permanent and will not fade, as would a similar image in a photochromic glass. The exposed, opalized photosensitive glass is much more soluble in hydrofluidic acid than the unexposed glass. Immersion in this acid produces shapes, depressions, or holes by etching away of those exposed and developed areas.
Photochromatic glasses are full-color photosensitive glasses. Developed in 1978 by Corning Glass Works laboratories, their characteristics imply applications such as information storage, decorative objects, windows, or other transparencies, and containers. Photochromic glasses have true color permanence.