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 Types Of Glass
Nearly all commercial glasses fall into one of six basic categories or
types. These categories are based on chemical composition. Within each
type, except for fused silica, there are numerous distinct
compositions.
1. Soda-lime glass is the most common (90% of glass made), and least
expensive form of glass. It usually contains 60-75% silica, 12-18% soda,
5-12% lime. Resistance to high temperatures and sudden changes of
temperature are not good and resistance to corrosive chemicals is only fair.
2. Lead glass has a high percentage of lead oxide (at least 20% of the
batch). It is relatively soft, and its refractive index gives a
brilliance that may be exploited by cutting. It is somewhat more expensive
than soda-lime glass and is favored for electrical applications because of
its excellent electrical insulating properties. Thermometer tubing and
art glass are also made from lead-alkali glass, commonly called lead
glass. This glass will not withstand high temperatures or sudden changes
in temperature.
3. Borosilicate glass is any silicate glass having at least 5% of boric
oxide in its composition. It has high resistance to temperature change
and chemical corrosion. Not quite as convenient to fabricate as either
lime or lead glass, and not as low in cost as lime, borosilicate's cost
is moderate when measured against its usefulness. Pipelines, light
bulbs, photochromic glasses, sealed-beam headlights, laboratory ware, and
bake ware are examples of borosilicate products.
4. Aluminosilicate glass has aluminum oxide in its composition. It is
similar to borosilicate glass but it has greater chemical durability and
can withstand higher operating temperatures. Compared to borosilicate,
aluminosilicates are more difficult to fabricate. When coated with an
electrically conductive film, aluminosilicate glass is used as resistors
for electronic circuitry
5. Ninety-six percent silica glass (see: GLASS AND THE SPACE ORBITER )
is a borosilicate glass, melted and formed by conventional means, then
processed to remove almost all the non-silicate elements from the
piece. By reheating to 1200°C the resulting pores are consolidated. This
glass is resistant to heat shock up to 900°C.
6. Fused silica glass (see: GLASS AND THE SPACE ORBITER ) is pure
silicon dioxide in the non-crystalline state. It is very difficult to
fabricate, so it is the most expensive of all glasses. It can sustain
operating temperatures up to 1200°C for short periods.
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