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A glass is a type of material that is purely amorphous, or one having no long range order. The opposite of an amorphous material is crystalline. Typically the term glass refers to the solid silicon dioxide, however in principle, given a fast enough cooling rate, any liquid can be made into an amorphous solid. The cooling process reduces the mobility of the molecules or atoms in the material, eventually causing them to be fixed in position. If the cooling rate is faster than the rate at which molecules can organize into a more thermodynamically favorable crystalline state, then an amorphous solid will be formed. As the arrangement of the atoms or molecules in a glass is extremely random, consisting of both ionic and covalent bonds, glasses are often likened to highly viscous liquids rather than solids [2].

Due to entropy, many polymers, organic molecules and even metals can be made amorphous solids by cooling even at slow rates. In contrast, if molecules are small or have sufficient time to organize into a structure with two- or three-dimensional order, then a crystalline solid will be formed. For example, water typically forms a crystalline solid when frozen rather than a glass due to its small molecular size and ability to quickly rearrange. The addition of another liquid, such as ethanol will break the long range order of the water (or ice crystals) and instead a glass will be formed upon freezing.

Well formed glasses are usually optically transparent, however colored glass can be obtained by the addition of homogeneously distributed charged ions. These charged ions cause them to be selective in their absorption of visible lightwaves. The frequencies of the spectrum which are not absorbed are either reflected back or transmitted for our physical observation. For example, iron oxide may be added in very small amounts to clear glass to make it green.

Silicon dioxide, or silica, is a common glass material. Despite the material being fully bonded, no long range crystalline order is present [1]

--Cassidy 18:47, 12 September 2009 (UTC)



[2] Soft condensed matter By Richard Anthony Lewis Jones