The history of glass is an old, mysterious story. Natural glass has existed since the beginning of time. Obsidian is a natural black volcanic glass that was used by primitive people for tools, knives, arrowheads, and decoration. It is uncertain when and where man discovered combining enough heat with the right materials could make glass. Archeologists have dated the oldest man-made glass to about 7000 B.C. Glass’s natural properties are what create its sparkling brightness and often-translucent form.
Glass’s history has many uncertainties. The New Book of Knowledge states that according to the Roman natural historian Pliny (A.D. 23-79) glass was discovered along the coast of Syria. According to this account, some seamen stumbled upon glass when cooking there dinner. Since they could not find big enough rocks to place their cooking pots on, they used blocks of niter, a form of soda. After their fires died down, they found that the niter had fused the sand underneath into a glassy substance (“Glass”).
After this discovery the craft was born. Early glass articles consisted mainly of small beads and charms. Glass was considered as a gem, valued equally as a precious stone. Not until the discovery of the blowpipe was glass changed from a luxury product into something that lower class people could afford and enjoy (“Glass”).
Glass is one of a few products that can be made from inexpensive raw materials. According to The New Book of Knowledge glass has a base of pure silica sand, which is composed of the mineral quartz, a compound of elements silicon and oxygen (“Glass”).
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In order to allow the sand (former) to melt more readily at a lower temperature of about 2600 degrees Fahrenheit, fluxing agents need to be added, like Soda ash, potash, and lithium carbonate. Fluxes however, make the glass chemically unstable, making it liable to dissolve in water or form unwanted crystals. Therefore stabilizers are also needed to make the glass uniform and keep its special structure intact. These include limestone, litharge, magnesia, barium carbonate, strontium carbonate, and zinc oxide (The Glassy State In-Brief Par 6-8).
Other substances can be added to adapt glass for special purposes. Because of the iron present in all sand, glass naturally has a greenish cast. In order to decolorize the glass selenium (a non-metallic element somewhat like sulfur) needs to be added. This gives the glass a red tint that balances out the green, leaving it clear. Almost any other color can be obtained by adding different chemical oxides. For instance, cobalt is added for blue, manganese for purple, gold for ruby, uranium for yellow, and chromium or iron for green (The New Book of Knowledge “Glass”).
In order to obtain the beauty and usefulness of glass right mixture of properties is important.
According to The World Book Encyclopedia, “Glass is made in somewhat the same way that a cook makes hard candy.” The glassmaker mixes together a large amount of sand and small amounts of lime, soda, or other materials to give the glass special qualities. This mixture is then heated in a furnace until it becomes a syrupy mass, which then forms glass when it cools (“Glass”).
Different recipes are used to form different types of glass; most of these recipes have not been changed for hundreds of years. Soda-Lime Glass is used for many industrial and art objects like plate and window glass, containers, and electric-light bulbs. The composition for Soda-Lime Glass is about 72% silica (sand); 15% sodium oxide (soda); 9% calcium oxide (lime); and the other 4% consists of minor ingredients (“Glass”).
Soda-Lead Glass, commonly known as lead glass, is made by substituting lead oxide for calcium oxide. Lead glass is easy to melt and much softer than Soda-Lime glass (“Glass”).
... than ceramics USES OF GLASS USES OF CERAMIC TYPES OF GLASS a) FUSED SILICA GLASS b) SODA-LIME GLASS c) BOROSILICATE GLASS d) LEAD CRYSTAL GLASS COMPOSITE MATERIALS * Composite ... called monomers. * Polymerisation is a process of combining monomers to form a long chain of molecules. polymerisation polymer monomer * Polymers can ...
However, it is much more expensive to make and has many valuable optical properties. Therefore, Soda-Lead glass is used primarily for the finest tableware and art objects. In addition, lead oxide gives many electrical properties to the glass. Another type of glass is Borosilicate Glass. This is a glass that is resistant to heat-shock. This contains about 80% silica, 4% alkali, 2% alumina, and about 13%boric acid (“Glass”).
This glass is great for chemical and electrical purposes since it is about three times as heat-shock resistant as Soda-Lime Glass. Without this type of glass it would not be possible to have glass cookware or glass pipelines. A glass that is even more resistant to cracking is 96% Silica Glass. This glass can be heated until red-hot and then plunged into ice-cold water without cracking. The World Book Encyclopedia states that this glass is formed from a special borosilicate composition treated chemically to leave a porous skeleton. This skeleton then shrinks up to 35% when fired at high temperatures. The pores close and the final non-porous glass is obtained (“Glass”).
This glass is used mostly for chemical ware.
There are four main ways to form and shape glass: blowing, pressing, drawing, and casting. The art of glass blowing without the use of molds is about 2,000 years old (“GLASS”).
A hollow pipe about four to five feet long is dipped into molten glass. Some of this molten substance sticks to the pear-shaped end of the blowpipe. Then the workman lifts the blowpipe and forms a glass bubble, or hollow bulb by gently blowing into the pipe. This glass bulb can then be squeezed, stretched out, twirled, reheated, and cut until the wanted shape is acquired. After obtaining the shape, the red-hot glass is then broken from the pipe and set aside to cool. If glass were allowed to cool quickly it would crack, so the article is placed inside a lehr were it cools slowly and evenly (The New Book of Knowledge “Glass”).
This cooling process is known as annealing. Pressing is another technique used to shape glass. This is done by dropping a hot glob of glass into a mold, and then pressing it with a plunger until it spreads and fills the inside of the mold (“Glass”).
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In order for an article to be pressed it is necessary that it is a shape that the plunger can be withdrawn. Baking dishes, glass blocks, lenses, and many other articles are made this way. This process can be done by hand or by machinery, with single or multiple molds.
A process known as drawing makes all sheet glass, plate glass, tubing, and fiberglass. A wide sheet of glass is drawn from a melting tank of molten glass, cooled, and cut to size (“Glass”).
Casting is another way to shape glass. Casting involves filling any mold with hot glass and letting it cool. When the glass cools the mold is removed and the desired shape is obtained. This method is occasionally used for large architectural or art pieces. The 200-inch-wide telescope disk for the Mount Palomar Observatory was made this way since there was no other way to make such a huge piece of glass (“Glass”).
Before a glass article can be decorated some finishing operations are needed. All excess glass must be removed from blown glassware. This is usually done while the glass is still hot and soft enough to cut. If the excess glass was not removed before cooling then it can be cut off by a process called scoring. This involves cutting the glass with a diamond or steel wheel, and then snapping of the excess glass with sudden pressure (“Glass”).
By polishing with fine abrasives or by flames in a fire-polishing machine, the cut edges can then be smoothed to match the piece (“Glass”).
Once all the excess glass is removed the article can be decorated.
There are five major techniques used for decorating. These include, etching, sandblasting, cutting, copper-wheel engraving, and fired decorations. Etching is done by first painting the desired pattern or design onto the article with an acid-resistant chemical, the glass is then dipped into hydrofluoric acid, which dissolves and eats away at the glass that is exposed (“Glass”).
The insides of electric light bulbs are frosted in this way, as well as many intricate designs on art pieces. Sandblasting is used to obtain a translucent surface that is usually rougher than the one obtained by etching. Often a rubber stencil is placed over the article, and then compressed air blows coarse, rough-grained sand at the class to form a design (“Glass”).
Ovenware, plate glass, and widow glass are often decorated or made translucent by sandblasting. Another decorating technique is cutting. This is a process of actually wearing away at the glass by holding it against sandstone or carborundum wheels (“Glass”).
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The desired pattern is first drawn onto the glass article so that the workman has something to follow. After the cuts are made the glass is then polished with fine abrasives to restore its original luster.
To create delicate, detailed, flowing designs, a process called copper wheel engraving is used. This process involves cutting the glass with dozens of abrasive-fed copper wheels, of all different sizes (“Glass”).
Most glass masterpieces have been decorated in this way, since such great detail can by obtained. According to The World Book Encyclopedia fire decorations are done by applying colored enamels and lusters to glass by hand painting, using decalcomanias, or by silk screening. When these enamels are heated to the right temperature they fuse to the glass and become part of the article (“Glass”).
Many colored glass products are decorated in this way.
Since it requires a large amount of highly skilled labor, the cost of man-made glass is high. The growing demand for lower-priced industrial glass items caused manufacturers to look for cheaper methods of glass making. At about 1900, glass manufacturing began to be mechanized (The New Book of Knowledge “Glass”).
Today machine-made glass manufacturing is one of the larger mass-production industries. Many kinds of glass items are now made with the use of automatic machines. The raw materials are fed into one end of a huge rectangular furnace where they are melted into molten glass. The glass then flows out in a steady stream and is cut to exact size automatically by shears. The globs of molten glass are then dropped into a waiting mold and are carried through a series of operations that form and shape the glass (“Glass”).
Once the article is finished it is then mechanically put into an annealing lehr to be cooled.
There are countless uses for glass. Glass is used for windows, containers, lenses, bottles, structural material, and lighting. With out glass lighting, our homes would be dark dreary places, and automobiles would be unable to operate after sunset. Television, or computer monitor screens would not exist, and many people would be unable to see without their eyeglasses.
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It is hard to believe that this beautiful, sparkling, transparent material can be formed from simply heating dull, ugly sand. Ever since man discovered glass, and its many uses, it has been an essential part of life. Man continues to find new uses for glass, and new ways of improving it. Even with all the technological advances, glass still has a major role in today’s world.