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Metallic additives in the glass mix can produce a variety of colours. Here cobalt has been added to produce a bluish coloured decorative glass
One of the most obvious characteristics of ordinary glass is that it is transparent to visible light (not all glassy materials are). The transparency is due to an absence of atomic transition states in the range of visible light, and to the fact that such glass is homogeneous on all length scales greater than about a wavelength of visible light (inhomogeneities cause light to be scattered, breaking up any coherent image transmission). The molecular structure of ordinary glass does not allow light at a wavelength of lower than 400 nanometers (also known as ultraviolet light or UV) to pass. This limitation is unacceptable in some applications, and special formulations of glass have been developed that do allow UV light to be transmitted.
Glass can be made so pure that hundreds of kilometers of glass are transparent at infrared wavelengths in fibre optic cables.
Sodium is generally used to lower the otherwise impossibly high temperatures needed to work glass. Additional soda or potash is sometimes added to further lower the melting point.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 very high refractive index, and is used in producing high-quality lenses. Large amounts of iron are used in glass that absorbs infrared energy, such as heat absorbing filters for movie projectors, while cerium can be used for glass that absorbs UV wavelengths (biologically damaging ionizing radiation).
Metals and metal oxides are added to glass during its manufacture to change its color. Manganese can be added in small amounts to remove the green tint lent by iron, or in higher concentrations to give glass an amethyst color. Like manganese, selenium can be used in small concentrations to decolorize glass, or in higher concentrations to impart a reddish color. Small concentrations of cobalt (0.025 to 0.1%) yield blue glass. Tin oxide with antimony and arsenic oxides produce an opaque white glass, first used in Venice to produce an imitation porcelain. 2 to 3% of copper oxide produces a turquoise color. Pure metallic copper produces a very dark red, opaque glass, which is sometimes used as a substitute for gold ruby glass. Nickel, depending on the concentration, produces blue, or violet, or even black glass. Adding titanium produces yellowish-brown glass. Metallic gold, in very small concentrations (around 0.001%), produces a rich ruby-colored glass, while lower concentrations produces a less intense red, often marketed as "cranberry". Uranium (0.1 to 2%) can be added to give glass a fluorescent yellow or green color. Uranium glass is typically not radioactive enough to be dangerous, but if ground into a powder, such as by polishing with sandpaper, and inhaled, it can be carcinogenic. Silver compounds (notably silver nitrate) can produce a range of colors from orange-red to yellow. The way the glass is heated and cooled can significantly affect the colors produced by these compounds. The chemistry involved is complex and not well understood. New colored glasses are frequently discovered.
Glass is sometimes created naturally from volcanic magma. This glass is called obsidian. Obsidian was long used to make extremely sharp knives using primitive tools. In many countries, including the United States, obsidian collection is prohibited by law in some places.
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