Glass

Name: Glass

Description: Glass is a noncrystalline material that can maintain indefinitely, if left undisturbed, its overall form and amorphous microstructure at a temperature below its glass transition temperature. The standard definition of a glass (or vitreous solid) requires the solid phase to be formed by rapid melt quenching. Glass is therefore formed via a supercooled liquid and cooled sufficiently rapidly from its molten state through its glass transition temperature, Tg, that the supercooled disordered atomic configuration at Tg, is frozen into the solid state. Generally, the structure of a glass exists in a metastable state with respect to its crystalline form, although in certain circumstances, for example in atactic polymers, there is no crystalline analogue of the amorphous phase. By definition as an amorphous solid, the atomic structure of a glass lacks any long range translational periodicity. However, by virtue of the local chemical bonding constraints glasses do possess a high degree of short-range order with respect to local atomic polyhedra. Amorphous solids may also be formed by methods other than melt quenching, such as vapour deposition or the sol-gel method.

The common form of glass used for household objects such as light bulbs and windows contain about 70–72 % by weight of silicon dioxide (SiO2). The most common form of glass is soda-lime glass, which contains nearly 30 % sodium and calcium oxides or carbonates. Pyrex is borosilicate glass containing about 10 % boric oxide. Lead crystal is a form of lead glass that contains a minimum of 24 % lead oxide. Silica glass may be produced by using sand as a raw material (or "quartz sand") that contains almost 100 % crystalline silica in the form of quartz. Although it is almost pure quartz, it may still contain a small amount (less than 1 %) of iron oxides that would color the glass, so this sand is usually depleted before production to reduce the iron oxide amount to less than 0.05 %. Large natural single crystals of quartz are pure silicon dioxide, and upon crushing are used for high quality specialty glasses. Synthetic amorphous silica, an almost 100 % pure form of quartz, is the raw material for the most expensive specialty glasses. The most common method for glass production is using molten tin, where the molten glass floats on top of the tin, thus giving it the name "float glass". Glass may be formed into smooth and impervious surfaces. Under tension, glass is brittle and will break into sharp shards. Under compression, pure glass can withstand a great amount of force. The properties of glass can be modified or changed with the addition of other compounds or heat treatment.