Glass: Properties, Uses, and Types - A Comprehensive Guide

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Introduction to Glass

Glass, a hard, brittle, and usually transparent material. As a scientific term, the word glass is often used to refer to any rigid substance that is amorphous (not made up of crystals), with the exception of plastics and certain other substances containing carbon. Glass is an extremely useful material. It is made into a large variety of familiar products, including windows, jars, mirrors, light bulbs, lenses, and tableware.

Glass is often classified according to its composition. Glass is also classified according to form, use, properties, or method of manufacture. For a short description of different kinds of glass, see, in this article, the section titled Some Kinds of Glass and Glass Products. Some kinds of glass, such as obsidian, occur naturally.

Composition

Glass is formed when certain substances are cooled rapidly and do not crystallize—that is, their atoms do not arrange themselves in the repeating, orderly pattern characteristic of most solids. Instead, the atoms become fixed in a disorganized pattern characteristic of the atoms of a liquid. For this reason, scientists often refer to glass as a liquid—a liquid with a very high viscosity (resistance to flowing). The viscosity of glass is so great that it is a rigid material.

The most common substance that can be cooled from the molten state without forming crystals is silica (silicon dioxide), the chief constituent of sand and sandstone. However, pure silica is not used for commercial glass products for two reasons: (1) it must be heated to a high temperature in order to melt, and (2) when molten it has a very high viscosity and is difficult to form.

Most glass is soda-lime glass. It is made from silica combined with soda ash (sodium carbonate) and lime. The soda ash lowers the temperature at which the mixture melts and reduces the mixture's viscosity; the lime makes the glass insoluble. (Silica and soda ash alone yield a type of glass called water glass, or sodium silicate, which will dissolve in water.)

Two other important types of glass are lead glass and borosilicate glass. Lead glass is made chiefly from silica, lead oxide, and soda ash or potash (potassium carbonate); borosilicate glass, from silica, boric oxide, and soda ash or potash.

Small amounts of arsenic oxide, antimony oxide, or other compounds are usually added to glass as fining agents, substances that help eliminate the gas bubbles formed during the melting process. Many other ingredients may be added to these basic materials to give the glass specific colors or other properties. For example, iron oxides can be added to give glass a green color.

Properties

Glass is a poor conductor of electricity and is thus used as an electrical insulator. It is also a poor conductor of heat. Glass, therefore, tends to shatter or crack when heated or chilled very quickly because the surface of the glass responds to a change in temperature (by expanding or contracting) before the interior does.

Light is refracted (bent) as it enters or leaves a piece of glass. Perfectly clear glass is transparent to all wavelengths of visible light. Different kinds of glass vary in the amount of ultraviolet radiation or other forms of electromagnetic radiation they transmit. Colored glass is opaque to some wavelengths of visible light and transparent to others.

Glass has a smooth, nonporous surface and is resistant to chemical attack. These properties make glass useful for food containers and laboratory apparatus.

Some Kinds of Glass and Glass Products

Art Glass

flint glass of high luster. It is hand- or machine-made in decorative shapes and may be cut, etched, or engraved.

Crown Glass

an optical glass with a low index of refraction. It is typically used for the converging element in a lens system.

Crystal, or Lead Crystal

flint glass that is used decoratively, chiefly for art objects or tableware. It is colorless and highly transparent.

Cut Glass

art glass with a design made by cutting or grinding the surface.

Fiberglass

glass made in the form of fibers used chiefly in making textiles, insulation, and reinforced plastics. Glass fibers used in fiber optics are usually called optical fibers.

Figured Glass

glass having a wavy or patterned surface.

Flat Glass

a general term for sheet glass, plate glass, float glass, and various forms of rolled glass. It is typically used in windows, doors and partitions.

Flint Glass

an optical glass containing a high proportion of lead oxide. It has a relatively high index of refraction and is usually used as the diverging element in a lens system.

Float Glass

a type of flat glass that has an especially smooth surface and is distortion-free.

Glass Blocks

solid or hollow blocks of glass used in construction to provide thermal and acoustic insulation.

Glass Fibers

See, in this list, FIBERGLASS; OPTICAL FIBER.

Glass-ceramic

a solid material, partly crystalline and partly glass, formed by the controlled crystallization of a glass.

Heat-absorbing Glass

glass containing chemical ingredients that absorb infrared waves, thus reducing heat flow between the interior and exterior of a building.

Heat-resisting Glass

glass able to withstand rapid temperature changes without cracking. An important use for heat-resisting glass is in making cooking utensils; one type, known by the trade name Pyrex, contains boron oxide in place of soda ash.

Hollowware

bottles, jars, vials, and other containers for holding a liquid or gas.

Insulating Glass

a glass product containing a layer of air sealed between two panes of glass. The use of such glass, sometimes called double glazing, provides thermal insulation. Thermopane is a well-known trade name for this type of glass.

Laminated Glass

a glass product consisting of two panes of glass separated by and bonded to a layer of clear plastic. When laminated glass is fractured, the plastic helps hold pieces of broken glass in place, reducing the danger of injury.

Lead Glass

glass containing a substantial proportion of lead oxide. It has a high refractive index and is readily decorated by grinding. It is used for such articles as vases, ashtrays, and drinking glasses.

Lime Glass

See SODA-LIME GLASS, in this list.

Optical Fiber

glass fiber through which light can be transmitted and from which the escape of light is prevented or minimized.

Optical Glass

glass of high quality made to have specific optical properties. Flint glass and crown glass are two important types of optical glass.

Photochromic Glass

glass containing silver halide compounds that darken when exposed to ultraviolet or visible light and regain their clarity when the exposure ends. It is used mainly in eyeglasses.

Plate Glass

a type of thick rolled glass usually ground and polished on both sides and used in mirrors, display cases, and windows.

Pressed Glass

glass formed by pressure between a mold and a plunger.

Safety Glass

flat glass made in a way that makes it relatively resistant to breakage and hinders the formation of sharp, dangerous fragments if the glass is broken. Safety glass includes laminated glass, tempered glass, and wire glass.

Sheet Glass

also called Window Glass, flat glass made in thin panes, usually 1/4 inch (0.6 cm) thick or less.

Soda-lime Glass

glass composed of silica, soda ash, and lime; the oldest and most widely used kind of glass.

Stained Glass

colored glass used in art objects.

Tempered Glass

a pane of specially treated plate, sheet, or float glass. When broken at any point, the entire pane immediately breaks into small, granular pieces without jagged edges. It is used in doors and in automobile windows.

Thermopane

See INSULATING GLASS, in this list.

Water Glass

glass made of silica and soda ash. It is soluble in water.

Window Glass

See SHEET GLASS, in this list.

Wire Glass

a pane of glass with a layer of meshed wire completely embedded in it.

Manufacture

Glassmaking involves two main steps: (1) heating and mixing raw materials to produce molten glass, and (2) forming the molten glass into the desired shape. Most glass then receives further treatment to produce the final product.

Making Molten Glass

Glass manufacture begins with the careful selection and measurement of raw materials. The most important raw material is sand. Other raw materials used depend on the type of glass being made.

Broken glass, called cullet, is usually added to the raw materials. It promotes the melting of the raw materials as they are heated. Most cullet is waste from glass-forming operations; some is obtained from recycled glass products.

Melting is done by batches or as a continuous process. With the continuous-tank furnace, the most common type of furnace used for melting, raw materials are fed into one end and molten glass is withdrawn continuously from the other end.

The raw materials and cullet are heated until they have melted into a spongy mass full of bubbles. The temperature of the melt is then increased to make it more fluid, allowing the bubbles to rise to the surface and escape. The glass at this stage is clear and homogeneous. Soda-lime glass, the most common type of glass, is initially heated to about 2,550 F. (1,400 C.) and then rapidly heated to about 2,800 F. (1,540 C.). Most other kinds of glass must be heated to higher temperatures.

Forming

When the glass is withdrawn from the furnace its viscosity is too low for it to hold any form. As the glass is cooled, it flows less easily and can be formed. Any of several processes can be used:

Blowing. In machine blowing, blasts of compressed air are used to force gobs or ribbons of glass into molds. Bottles are made by machine blowing.

In hand blowing, a mass of molten glass is gathered on the end of a four- to five-foot (120- to 150-cm) pipe called a blowpipe or blowing iron. The glassmaker blows through the pipe, giving the material a hollow, balloon-shaped form. The glass is further shaped with various metal tools. It is usually reheated several times to keep it pliable while it is being shaped.

Drawing. In producing flat drawn glass, a horizontal wire called a bait is lowered into the molten glass and then raised. Glass adheres to the wire and is drawn upward as a continuous sheet. Once hardened, the glass is cut into sheets for use in windows and inexpensive mirrors. Flat drawn glass has a fire-finished surface that forms naturally as the molten glass cools. Drawn glass tubing is made by forcing molten glass along a ceramic cone, called a mandrel, that forms the glass into tubing.

Floating. Molten glass is allowed to flow onto a bath of molten tin and is then allowed to cool. The tin has a low melting point and remains liquid at temperatures at which the glass hardens. The surface of the metal leaves the glass with a very smooth surface. Float glass is used in windows and other flat glass products.

Pressing. A measured amount of molten glass is placed, by machine, into a mold, and a metal plunger presses the glass outward to fill the mold. When the glass has cooled and is firm enough to hold its shape, the mold is removed. Pressing techniques are well-suited for rapid mass production. Many of the glass objects made for everyday use, such as drinking glasses and heat-resistant glassware, are produced by pressing.

Rolling. As the molten glass leaves the furnace, it flows between sets of large, heavy rollers. This method is typically used for flat glass that does not require a fine finish, such as glass made with a figured pattern on its surface.

Further Processing

After forming, glass is usually heat-treated, either by annealing or by tempering. It may also be decorated.

Annealing. Stresses build up in glass objects as they are cooled from forming temperatures to room temperature. These stresses greatly weaken the objects and may cause them to fracture spontaneously. In annealing, glass products are reheated in ovens called lehrs, and then allowed to cool slowly under controlled conditions, so that the stresses do not recur.

Tempering. Tempered glass is tougher (more resistant to breakage) than ordinary glass. In tempering, controlled, uniform stresses are deliberately set up in the surface of a glass object. The glass is heated to just below the temperature at which it would begin to soften and is then quenched (rapidly chilled) to stress its entire surface uniformly. Tempered glass cannot be cut or drilled. If a piece of tempered glass is pierced in any way, the uniformity of the stress is destroyed and the whole piece shatters into small pieces.

Decorating. Glass objects can be decorated in many ways. Some glass objects are painted or glazed. Others are etched with hydrofluoric acid, either to produce an artistic design or to frost the entire surface. Designs can also be cut or ground into the surface.

History

Glassmaking has been practiced for more than 4,000 years. Among the earliest known examples of glassmaking are ancient Egyptian beads with a glass coating. Around 1500 B.C. a method of making bottles and jars was developed in which a core made from sand and other materials was covered with softened glass. After the glass hardened, the core was scraped out. Blown glass was introduced shortly before the Christian Era, probably in the eastern Mediterranean region.

Glassmaking reached great excellence in ancient Rome. Some objects were transparent, but many were opaque. One Roman technique involved dipping a glass object into molten glass of another color. The coating, when cool, was partially carved away, leaving a cameolike design.

After the fall of Rome, the quality of glassmaking declined, and in many areas glassmaking ceased. Glassmaking continued in the Near East, where it already had a 2,000-year history. Islamic glassmakers perfected carving, enameling, and gilding techniques. Knowledge of glassmaking brought by returning Crusaders led to the revival of glassmaking in western Europe during the 11th and 12th centuries.

By the late 15th century, Venice had become the world's glassmaking center. Strong guilds were established and laws were passed that made spreading information about glassmaking an offense punishable by death. Nevertheless, the information was spread, and by the early 17th century, quality glass was being made throughout Europe. Tumblers, goblets, bottles, and other objects were made in great quantity. The glass used was soda-lime and potash-lime glass with various impurities that gave it different colors, usually green or brown. Glass of many other colors, as well as clear glass, was also produced.

About 1675, George Ravenscroft had produced the first vessels of lead glass, which later became known as crystal. This type of glassware had a much greater clarity and brilliance than the other types of glass manufactured at that time.

Flat glass was produced up until the 19th century by blowing. In one method, a blob of glass was gathered on a blowpipe and blown into a large hollow globe. The blowpipe was then detached from the glass, creating an opening in the globe. An iron rod called a pontil was then attached to the glass at a point opposite the opening, and the rod was spun to spread the glass into a disc up to about five feet (1.5 m) in diameter. Small, nearly flat panes could then be cut from near the outside of the disc.

Another method of flat-glass manufacture consisted of blowing a large cylinder of glass and then cutting the cylinder open and flattening it out to produce a flat sheet.

A twofold revolution in glassmaking methods took place after 1900. The industry became almost completely mechanized. Also, scientific knowledge of the physics and chemistry of glass made it possible to create special glasses for countless new purposes. Many of these developments were pioneered in the United States. Some of the landmarks were: the first fully automatic bottle machine (1903); the Colburn continuous method of drawing sheet glass (1904); quantity production of Pyrex heat-resistant glass (1915); laminated safety glass (1928); fiberglass (1931); and float glass (1959).

By the mid-1970's the float method had become one of the major methods of making flat glass. Research and development of new glasses, particularly glasses required for specific applications, is continuing.