raymond723
07-01-2009, 00:29
Since the nineteenth century, many centuriesold glass-forming methods have been mechanized, greatly increasing the rate of production of glass objects, and lowering the prices of these objects. For example, the "ribbon machine," developed in the 1920s for the automatic glassblowing of lightbulbs, is a milestone of mechanical glass forming. In the ribbon machine, puffs of air blow glass bubbles from a rapidly moving ribbon of molten glass into a moving stream of molds that give the bulbs their shape and then release them. Small lightbulb blanks can be made at the rate of 1,000 per minute.
With so many millions of windows in buildings and vehicles everywhere, we tend to take sheets of flat glass for granted. Throughout most of human history, however, there were no sheets of flat, transparent glass. Even as recently as the eighteenth century, glass windows were quite uncommon.
In a very limited way glass windows did start to appear in the Roman world during the third century, but they were generally small glass fragments set in bronze or wooden frames. In that era most windows were not glass, but were thin sheets of translucent horn or marble, or perhaps panes of mica (isinglass). Around 600 C.E., during the Byzantine period, glass windows (usually made of small pieces of colored glass) began to appear in the large churches, but glass windows in houses and other secular buildings remained quite rare until the end of the eighteenth century.
The principal method for making flat glass during the 1700s called for blowing a hot glass bubble, securing an iron rod to the bubble's other side, and then cutting the bubble free from the blowing pipe. The tulip-shaped hot glass was then rotated rapidly around the iron rod axis until the centrifugal force forced the glass tulip to open up and form a disk. The rod was then removed from the glass (leaving a spot in the middle of the glass disk that looked rather like a bull's eye). This method was the source of the old "bull's-eye" windows that can still sometimes be found in English pubs. The windows were limited in size and poor in optical quality (besides having a bull's-eye at their centers).
The chief method for making flat glass during the 1800s was the cylinder method. The first step was to blow a large glass bubble (compressed air was often used); it would then be swung back and forth until the bubble became elongated and acquired a cylindrical shape; finally the cylinder was split lengthwise, reheated, and allowed to flatten on an iron table. The resulting pane of glass was not really flat, and it had a lot of optical distortion, but the method was used widely to make sheet glass. For example, it was used to produce the 300,000 panes of glass that were used to build London's Crystal Palace, the huge greenhouse constructed for the London World's Fair Exhibition of 1851.
By the twentieth century these methods were replaced by an innovative technique invented by a Belgian named Foucault, who had learned how to draw up continuous sheets from a tank filled with molten glass. Even this glass was of nonuniform thickness and had some roughness at its surface, therefore, for high quality flat glass, it had to be ground and polished.
Then, in 1959, the Pilkington Glass Works in Britain introduced the "float glass" process. In the float process, molten glass is allowed to flow continuously onto a mirrorlike surface of molten tin at 1,000oC (1,832oF). At this temperature the glass spreads out and becomes a layer that is about 6 millimeter (1/4 inch) thick. If the layer is stretched as it cools, a thickness of 2 millimeter (0.08 inch) can be achieved. The glass is allowed to advance on the hot liquid tin until, at 600oC (1,112oF), it becomes solid enough to be lifted off the molten tin surface. It is then annealed (heated to relieve any strain) before being cut into desired sheet lengths. The float glass method rapidly replaced the Foucault drawing process, and today it is the standard method for making flat glass. A large modern float glass plant can produce 5,000 tons of glass sheet per week, and it can be operated 24 hours a day, 365 days a year, for several years before serious repairs are apt to be needed. Float glass has uniform thickness and bright fire-polished surfaces that need no grinding or polishing.
The drawing of glass fibers had long been of interest, but glass fibers found little use until the twentieth century. Articles such as wedding gowns made from glass fiber cloth were largely curiosities, made for show rather than use. In the 1930s glass researchers learned to feed molten glass into platinum bushings having hundreds of tiny holes. Fine glass filaments of 10 to 50 microns were rapidly drawn downward and assembled as bundles or strands of glass fiber. Today a major use of glass cloth or filaments is to strengthen the plastics used to make fiberglass-reinforced composites. These composites are widely used in making boats, from canoes to yachts, and bodies for cars, such as the Corvette.
An even larger poundage market is that of glass wool insulation. In a process much like that used to make cotton candy, fine glass fibers are spun, sprayed with an organic bonding agent, and then heat-cured and cut into mats of various sizes, to be used for insulating buildings and appliances.
Surely the most significant glass fiber development in recent times is fiber optics, or optical wave guides. These ultrapure, very fine glass fibers are a most crucial part of modern communications technology, wherein glass fibers link telephones, televisions, and computers. A single strand of glass optical fiber that has a protective plastic coating looks much like a human hair. The glass fiber has an inner core of ultrapure fused silica, which is coated with another silica glass that acts as a light-refractive barrier. Lasers are used to convert sound waves and electrical impulses to pulses of light that are sent, static-free, through the inner glass core. Glass fibers can transmit many times more information than can be carried by charges moving in a copper wire. In fact, one pound of glass optical wave guides can transmit as much information as can be transmitted via 200 tons of copper wire. Today millions of miles of optic fibers are crisscrossing not only the United States, but also the entire planet.
Windows need to be cleaned. In 2000 a new glass that largely cleans itself when it comes into contact with rain was introduced. This low-maintenance glass was developed by Pilkington Glass Works, the company that invented the float process. It is made by depositing a microscopically thin coating of titanium dioxide (TiO2) on hot sheet glass during its manufacture in the float process. As dirt collects on the window, the Sun's ultraviolet rays promote a catalytic reaction at the glass surface that breaks down and loosens surface dirt.
With so many millions of windows in buildings and vehicles everywhere, we tend to take sheets of flat glass for granted. Throughout most of human history, however, there were no sheets of flat, transparent glass. Even as recently as the eighteenth century, glass windows were quite uncommon.
In a very limited way glass windows did start to appear in the Roman world during the third century, but they were generally small glass fragments set in bronze or wooden frames. In that era most windows were not glass, but were thin sheets of translucent horn or marble, or perhaps panes of mica (isinglass). Around 600 C.E., during the Byzantine period, glass windows (usually made of small pieces of colored glass) began to appear in the large churches, but glass windows in houses and other secular buildings remained quite rare until the end of the eighteenth century.
The principal method for making flat glass during the 1700s called for blowing a hot glass bubble, securing an iron rod to the bubble's other side, and then cutting the bubble free from the blowing pipe. The tulip-shaped hot glass was then rotated rapidly around the iron rod axis until the centrifugal force forced the glass tulip to open up and form a disk. The rod was then removed from the glass (leaving a spot in the middle of the glass disk that looked rather like a bull's eye). This method was the source of the old "bull's-eye" windows that can still sometimes be found in English pubs. The windows were limited in size and poor in optical quality (besides having a bull's-eye at their centers).
The chief method for making flat glass during the 1800s was the cylinder method. The first step was to blow a large glass bubble (compressed air was often used); it would then be swung back and forth until the bubble became elongated and acquired a cylindrical shape; finally the cylinder was split lengthwise, reheated, and allowed to flatten on an iron table. The resulting pane of glass was not really flat, and it had a lot of optical distortion, but the method was used widely to make sheet glass. For example, it was used to produce the 300,000 panes of glass that were used to build London's Crystal Palace, the huge greenhouse constructed for the London World's Fair Exhibition of 1851.
By the twentieth century these methods were replaced by an innovative technique invented by a Belgian named Foucault, who had learned how to draw up continuous sheets from a tank filled with molten glass. Even this glass was of nonuniform thickness and had some roughness at its surface, therefore, for high quality flat glass, it had to be ground and polished.
Then, in 1959, the Pilkington Glass Works in Britain introduced the "float glass" process. In the float process, molten glass is allowed to flow continuously onto a mirrorlike surface of molten tin at 1,000oC (1,832oF). At this temperature the glass spreads out and becomes a layer that is about 6 millimeter (1/4 inch) thick. If the layer is stretched as it cools, a thickness of 2 millimeter (0.08 inch) can be achieved. The glass is allowed to advance on the hot liquid tin until, at 600oC (1,112oF), it becomes solid enough to be lifted off the molten tin surface. It is then annealed (heated to relieve any strain) before being cut into desired sheet lengths. The float glass method rapidly replaced the Foucault drawing process, and today it is the standard method for making flat glass. A large modern float glass plant can produce 5,000 tons of glass sheet per week, and it can be operated 24 hours a day, 365 days a year, for several years before serious repairs are apt to be needed. Float glass has uniform thickness and bright fire-polished surfaces that need no grinding or polishing.
The drawing of glass fibers had long been of interest, but glass fibers found little use until the twentieth century. Articles such as wedding gowns made from glass fiber cloth were largely curiosities, made for show rather than use. In the 1930s glass researchers learned to feed molten glass into platinum bushings having hundreds of tiny holes. Fine glass filaments of 10 to 50 microns were rapidly drawn downward and assembled as bundles or strands of glass fiber. Today a major use of glass cloth or filaments is to strengthen the plastics used to make fiberglass-reinforced composites. These composites are widely used in making boats, from canoes to yachts, and bodies for cars, such as the Corvette.
An even larger poundage market is that of glass wool insulation. In a process much like that used to make cotton candy, fine glass fibers are spun, sprayed with an organic bonding agent, and then heat-cured and cut into mats of various sizes, to be used for insulating buildings and appliances.
Surely the most significant glass fiber development in recent times is fiber optics, or optical wave guides. These ultrapure, very fine glass fibers are a most crucial part of modern communications technology, wherein glass fibers link telephones, televisions, and computers. A single strand of glass optical fiber that has a protective plastic coating looks much like a human hair. The glass fiber has an inner core of ultrapure fused silica, which is coated with another silica glass that acts as a light-refractive barrier. Lasers are used to convert sound waves and electrical impulses to pulses of light that are sent, static-free, through the inner glass core. Glass fibers can transmit many times more information than can be carried by charges moving in a copper wire. In fact, one pound of glass optical wave guides can transmit as much information as can be transmitted via 200 tons of copper wire. Today millions of miles of optic fibers are crisscrossing not only the United States, but also the entire planet.
Windows need to be cleaned. In 2000 a new glass that largely cleans itself when it comes into contact with rain was introduced. This low-maintenance glass was developed by Pilkington Glass Works, the company that invented the float process. It is made by depositing a microscopically thin coating of titanium dioxide (TiO2) on hot sheet glass during its manufacture in the float process. As dirt collects on the window, the Sun's ultraviolet rays promote a catalytic reaction at the glass surface that breaks down and loosens surface dirt.