Types of Glass

When people speak of glass, they ordinarily mean a transparent, shiny substance that breaks rather easily. They may think of the glass in windows and the glass used in eyeglasses as being the same material. Actually, they are not. There are many kinds of glass. Several important kinds of glass are discussed in this article.

Flat glass is used chiefly in windows. It is also used in mirrors, room dividers, and some kinds of furniture. All flat glass is made in the form of flat sheets. But some of it, such as that used in automobile windshields, is reheated and sagged (curved) over molds.

Glass containers are used for packaging food, beverages, medicines, chemicals, and cosmetics. Glass jars and bottles are made in a wide variety of shapes, sizes, and colors. Many are for common uses, such as soft-drink bottles or jars for home canning. Others are made from special glass formulas to make sure there will be no contamination or deterioration of blood plasma, serums, and chemicals stored in them. See .

Optical glass is used in eyeglasses, microscopes, telescopes, camera lenses, and many instruments for factories and laboratories. The raw materials must be pure so that the glass can be made almost flawless. The care required for producing optical glass makes it expensive compared with other kinds of glass.

Fiberglass consists of fine but solid rods of glass, each of which may be less than one-twentieth the width of a human hair. These tiny glass fibers can be loosely packed together in a woollike mass that can serve as heat insulation. They also can be used like wool or cotton fibers to make glass yarn, tape, cloth, and mats. Fiberglass has many other uses. It is used for electrical insulation, chemical filtration, and firefighters' suits. Combined with plastics, fiberglass can be used for airplane wings and bodies, automobile bodies, and boat hulls. Fiberglass is a popular curtain material because it is fire-resistant and washable.

Laminated safety glass is a “sandwich” made by combining alternate layers of flat glass and plastics. The outside layer of glass may break when struck by an object, but the plastic layer is elastic and so it stretches. The plastic holds the broken pieces of glass together and keeps them from flying in all directions. Laminated glass is used where broken glass might cause serious injuries, as in automobile windshields.

Bullet-resisting glass is thick, multilayer laminated glass. This glass can stop even heavy-caliber bullets at close range. Bullet-resisting glass is heavy enough to absorb the energy of the bullet, and the several plastic layers hold the shattered fragments together. Such glass is used in bank teller windows and in windshields for military tanks, aircraft, and special automobiles.

Tempered safety glass, unlike laminated glass, is a single piece that has been given a special heat treatment. It looks, feels, and weighs the same as ordinary glass. But it can be several times stronger. Tempered glass is used widely for all-glass doors in stores, side and rear windows of automobiles, and basketball backboards, and for other special purposes. It is hard to break even when hit with a hammer. When it does break, the whole piece of glass collapses into small, dull-edged fragments.

Colored structural glass is a heavy plate glass, available in many colors. It is used in buildings as an exterior facing, and for interior walls, partitions, and tabletops.

Opal glass has small particles in the body of the glass that disperse the light passing through it, making the glass appear milky. The ingredients necessary to produce opal glass include fluorides (chemical compounds containing fluorine). This glass is widely used in lighting fixtures and for tableware.

Foam glass, when it is cut, looks like a black honeycomb. It is filled with many tiny cells of gas. Each cell is surrounded and sealed off from the others by thin walls of glass. Foam glass is so light that it floats on water. It is widely used as a heat insulator in buildings, on steam pipes, and on chemical equipment. Foam glass can be cut into various shapes with a saw.

Glass building blocks are made from two hollow half-sections sealed together at a high temperature. Glass building blocks are good insulators against heat or cold because of the dead-air space inside. The blocks are laid like bricks to make walls and other structures.

Heat-resistant glass is high in silica and usually contains boric oxide. It expands little when heated, so it can withstand great temperature changes without cracking. This quality is necessary in cookware and other household equipment, and in many types of industrial gear.

Laboratory glassware includes beakers, flasks, test tubes, and special chemical apparatus. It is made from heat-resistant glass to withstand severe heat shock (rapid change in temperature). This glass is also much more resistant to chemical attack than ordinary glass.

Glass for electrical uses. Glass has properties that make it useful in electrical applications: ability to resist heat, resistance to the flow of electric current, and ability to seal tightly to metals without cracking. Because of these properties, glass is used in electric light bulbs and for picture tubes in television sets.

Glass optical fibers are glass fibers used to transmit information as pulses of light. Thin, extremely pure optical fibers are used to carry telephone and television signals and digital (numeric) data over long distances. Glass optical fibers are also used in control board displays and in medical instruments.

Glass tubing is used to make fluorescent lights, neon signs, glass piping, and chemical apparatus. Glass tubing is made from many kinds of glass and in many sizes.

Glass-ceramics are strong materials made by heating glass to rearrange some of its atoms into regular patterns. These partially crystalline materials can withstand high temperatures, sudden changes in temperature, and chemical attacks better than ordinary glass can. They are used in a variety of products, including heat-resistant cookware, turbine engines, electronic equipment, and nose cones of guided missiles. Glass-ceramics have such trade names as Pyroceram, Cervit, and Hercuvit.

Radiation-absorbing and radiation-transmitting glass can transmit, modify, or block heat, light, X rays, and other types of radiant energy. For example, ultraviolet glass absorbs the ultraviolet rays of the sun but transmits visible light. Other glass transmits heat rays freely but passes little visible light. Polarized glass cuts out the glare of brilliant light. One-way glass is specially coated so that a person can look through a window without being seen from the other side.

Laser glass is an optical glass containing small amounts of substances that enable the glass to generate laser beams efficiently. Such glass is used as the active medium in solid-state lasers, a type of laser that sends light out through crystals or glass (One substance commonly used in laser glass is the element neodymium. Researchers are using glass lasers in an attempt to harness nuclear fusion (the joining of atomic nuclei) as a source of commercially useful amounts of energy. In their experiments, powerful glass lasers heat hydrogen atoms until hydrogen nuclei fuse, releasing large amounts of energy.

"Invisible glass" is used principally for coated camera lenses and eyeglasses. The coating is a chemical film that decreases the normal loss of light by reflection. This allows more light to pass through the glass.

Photochromic glass darkens when exposed to ultraviolet rays and clears up when the rays are removed. Photochromic glass is used for windows, sunglasses, and instrument controls.

Photosensitive glass can be exposed to ultraviolet light and to heat so that any pattern or photograph can be reproduced within the body of the glass itself. Because the photographic print then becomes an actual part of the glass, it will last as long as the glass itself.

Photochemical glass is a special composition of photosensitive glass that can be cut by acid. Any design can be reproduced on the glass from a photographic film. Then when the glass is dipped in acid, the exposed areas are eaten away, leaving the design in the glass in three dimensions. By this means, lacelike glass patterns can be made.

Heavy metal fluoride glass is an extremely transparent glass being developed for use in optical fibers that transmit infrared rays. Infrared rays are much like light waves but are invisible to the human eye. In optical fibers, infrared light transmits better over distance than visible light does.

Chalcogenide glass is made up of elements from the chalcogen group, including selenium, sulfur, and tellurium. The glass is transparent to infrared light and is useful as a semiconductor in some electronic devices. Chalcogenide glass fibers are a component of devices used to perform laser surgery.

Sol-Gel glass can be used as a protective coating on certain solar collectors or as an insulating material. It is also used to make short, thick tubes that are drawn into optical fibers. To make Sol-Gel glass, workers dissolve the ingredients in a liquid. They then heat the liquid. The liquid evaporates, leaving behind small particles of glass. Heating these particles fuses (joins) them to form a solid piece of glass. The temperatures involved in Sol-Gel processes are often lower than those needed to make ordinary glass.

• Steve W. Martin, Ph.D., Professor of Materials Science and Engineering, Iowa State University.

Steve W. Martin, "Glass," Discovery Channel School, original content provided by World Book Online, http://www.discoveryschool.com/homeworkhelp/
, August 2001.

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