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Sand's Versatile Gift
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# : |
14567 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
3 / 1988 |
2,000 Words |
| Author
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Stanton Wormley
Stanton Wormley is a free-lance writer who resides in
Bridgton, Maine. |
The first glass was naturally occurring obsidian, a brownish-black, brittle, translucent material formed when silicon dioxide, or silica, is fused by the heat of a volcanic eruption. Obsidian was used to make tools such as knives, scrapers, and spear points, as well as decorative and ceremonial items by early humans.
Striving to imitate this utilitarian material, man first began to make glass by melting sand (which is primarily composed of silicon dioxide) together with soda ash (sodium carbonate and limestone (calcium carbonate). This "soda-lime" glass is still a commercially important glass, being used today for containers, window glass, light bulbs, low-cost glassware and so forth. It is inexpensive and easy to melt and shape, and accounts for almost 90 percent of the total annual world production of glass. It is, however, susceptible to thermal shock and high temperature and has only moderate resistance to chemical attack.
All silicon-based glass formulations fall into one of six categories. Soda-lime glass and lead glass have generally similar properties. They are the most easily worked and least durable glasses. Borosilicate glass, made by adding boric oxide to the glass melt, is more resistant to acid and thermal attack than soda-lime glass and generally has more dimensional stability. Aluminosilicate glass (which contains aluminum oxide) has properties similar to borosilicate glass, but can tolerate even greater temperatures. Ninety-six percent silica glass is extremely resistant to thermal shock and high temperature, being inferior in these properties only to fused silica glass, which is composed completely of silicon dioxide. The high silica content glasses are also the most dimensionally stable.
The theoretical strength of glass is considerably greater than that of steel. In fact, though, glass surfaces have many minute flaws, resulting in weakened strength. Hence, some glasses are thermally or chemically strengthened and such glasses are used for industrial piping, automobile and aircraft windows, protective glass, ophthalmic lenses, and home glassware. Strengthened glass is especially valuable in laboratories, where it is used to make equipment subjected to high stress, such as centrifuge tubes.
Chemical and Thermal Properties
The chemical inertness of glass is one of its most useful properties. The ability of glass to resist chemical attack, while not being totally impervious to it, is responsible for its wide use
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