Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: Matthew on 11/07/2008 10:07:24
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Matthew asked the Naked Scientists:
What is the ingredients to make glass?
What do you think?
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Pure silica (SiO2) has a "glass melting point"— at a viscosity of 10 Pa·s (100 P)— of over 2300 °C (4200 °F). While pure silica can be made into glass for special applications (see fused quartz), other substances are added to common glass to simplify processing. One is sodium carbonate (Na2CO3), which lowers the melting point to about 1500 °C (2700 °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 aluminium 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.[9] 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index and low dispersion, and was formerly used in producing high-quality lenses, but due to its radioactivity has been replaced by lanthanum oxide in modern glasses.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 (biologically damaging ionizing radiation).
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 the bubble content in the glass.[9]
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).[11]
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If you plan to smash the glass over someone's head then a better set of ingredients is sugar, glucose syrup and water.
On the other hand, for far infra red transparency, you can use arsenic sulphide glass.
Please don't get these mixed up.