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Pure
silica (SiO2) has a "glass melting point" of over
2,300°C (4,172°F). While pure silica can be made into
glass for special applications, other substances are added to
common glass to simplify processing. One is sodium carbonate
(Na2CO3), which lowers the melting point to about 1,500°C
(2,732°F) in soda-lime glass; "soda" refers to
the original source of sodium carbonate in the soda ash obtained
from certain plants. However, the soda makes the glass water
soluble, which is usually undesirable, so lime (calcium oxide
(CaO), generally obtained from limestone), some magnesium oxide
(MgO) and aluminum oxide are added to provide for a better chemical
durability. The resulting glass contains about 70 to 74 percent
silica by weight and is called a soda-lime glass. Soda-lime
glasses account for about 90 percent of manufactured glass.
As well as soda and lime, most common glass has other ingredients
added to change its properties. Lead glass, such as lead crystal
or flint glass, is more 'brilliant' because the increased refractive
index causes noticeably more "sparkles", while boron
may be added to change the thermal and electrical properties,
as in Pyrex. Adding barium also increases the refractive index.
Thorium oxide gives glass a high refractive index and low dispersion,
and was formerly used in producing high-quality lenses, but
due to its radioactivity has been replaced by lanthanum oxide
in modern glasses. Large amounts of iron are used in glass that
absorbs infrared energy, such as heat absorbing filters for
movie projectors, while cerium(IV) oxide can be used for glass
that absorbs UV wavelengths. Besides the chemicals mentioned,
in some furnaces recycled glass ("cullet") is added,
originating from the same factory or other sources. Cullet leads
to savings not only in the raw materials, but also in the energy
consumption of the glass furnace. However, impurities in the
cullet may lead to product and equipment failure. Fining agents
such as sodium sulfate, sodium chloride, or antimony oxide are
added to reduce to bubble content in the glass. A further raw
material used in the production of soda-lime and fiber glass
is calumite, which is a glassy granular by-product of the iron
making industry, containing mainly silica, calcium oxide, alumina,
magnesium oxide (and traces of iron oxide). For obtaining the
desired glass composition, the correct raw material mixture
(batch) must be determined by glass batch calculation.Properties
such as density and melting point vary greatly depending on
the material added to the silica: density can range from light
display glass with 2.37 g/cm³ to high lead-content flint
glass with 7.2 g/cm³, while melting points can range from
500 to 1650°C. These ranges can be exceeded, but usually
at the cost of stability or practicality.
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