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glass
A material formed by the rapid cooling of certain molten liquids so that they fail to crystallize but retain an amorphous structure. Glasses are in fact supercooled liquids which, however, have such high viscosity that they behave like solids for all practical purposes. Some glasses may spontaneously crystallize or devitrify. Few materials form glasses, and almost all that are found naturally or used commercially are based on silica and the silicates.
Natural glass is formed by rapid cooling of magma, producing chiefly obsidian, or rarely by complete thermal metamorphism (see also tektite). The earliest known manufactured glass was made in Mesopotamia in the 3rd millennium BC. Glass was shaped by molding or core-dipping, until the invention of glassblowing by Syrian craftsmen in the 1st century BC. Essentially still used, the process involved gathering molten glass on the end of a pipe, blowing to form a bubble, and shaping the vessel by further blowing, swinging, or rolling it on a surface. They also blew glass inside a shaped mold; this is now the chief process used in mechanized automatic glassblowing.
Modern glass products are very diverse, including windows, bottles and other vessels, optical devices, building materials, fiberglass products, etc. Most are made of soda-lime glass. Although silica itself can form a glass, it is too viscous and its melting point is too high for most purposes. Adding soda lowers the melting point, but the resultant sodium silicate is water-soluble (see water glass), so lime is added as a stabilizer, together with other metal oxides as needed for decolorizing, etc. The usual proportions are 70% SiO2, 15% Na2O, 10% CaO.
Crown glass, used in optical systems for its low dispersion, is of this type, with barium oxide (BaO) often replacing the lime. Flint glass, or crystal, is a brilliant clear glass with high optical dispersion, used in high-quality glassware and to make lenses and prisms. It was originally made from crushed flints to give pure, colorless silica; later, sand was used, with increasing amounts of lead (II) oxide. For borosiliate glass, used where high thermal stresses must be withstood, see Pyrex.
The manufacture of the various kinds of glass begins by mixing the raw materials – sand, limestone, sodium nitrate or carbonate, etc. – and melting them in large crucibles in a furnace. The molten glass, having been refined (free from bubbles) by standing, is formed to the shape required and then annealed. Some safety glass is not annealed, but rapidly cooled to induce superficial compressive stresses which yield greater strength. Plate glass is made by passing a continuous sheet of soft glass between rollers, grinding and polishing it on both sides, and cutting it up so as to eliminate flaws. A newer method (the float glass process) involves pouring the molten glass into molten metal, such as tin, and to allow it to cool slowly; the surface touching the metal is perfectly flat and needs no polishing. Special glass products include foam glass, made by sintering a mixture of glass and an agent that gives off a gas on heating, used for insulation; photosensitive glass, which darkens reversibly in bright light; and fiberglass.
Related category
• INORGANIC CHEMISTRY
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