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Jai Ganesh

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Glass

Glass

Gist

The standard definition of a glass (or vitreous solid) is a non-crystalline solid formed by rapid melt quenching. However, the term "glass" is often defined in a broader sense, to describe any non-crystalline (amorphous) solid that exhibits a glass transition when heated towards the liquid state.

Summary

Glass is an inorganic solid material that is usually transparent or translucent as well as hard, brittle, and impervious to the natural elements. Glass has been made into practical and decorative objects since ancient times, and it is still very important in applications as disparate as building construction, housewares, and telecommunications. It is made by cooling molten ingredients such as silica sand with sufficient rapidity to prevent the formation of visible crystals.

Glass is treated in detail in a number of articles. Stained glass and the aesthetic aspects of glass design are described in stained glass and glassware. The composition, properties, and industrial production of glass are covered in industrial glass. The physical and atomic characteristics of glass are treated in amorphous solid.

The varieties of glass differ widely in chemical composition and in physical qualities. Most varieties, however, have certain qualities in common. They pass through a viscous stage in cooling from a state of fluidity; they develop effects of colour when the glass mixtures are fused with certain metallic oxides; they are, when cold, poor conductors both of electricity and of heat; most types are easily fractured by a blow or shock and show a conchoidal fracture; and they are but slightly affected by ordinary solvents but are readily attacked by hydrofluoric acid.

Details

Glass is an amorphous (non-crystalline) solid. Because it is often transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window panes, tableware, and optics. Some common objects made of glass are named after the material, e.g., a "glass" for drinking, "glasses" for vision correction, and a "magnifying glass".

Glass is most often formed by rapid cooling (quenching) of the molten form. Some glasses such as volcanic glass are naturally occurring, and obsidian has been used to make arrowheads and knives since the Stone Age. Archaeological evidence suggests glassmaking dates back to at least 3600 BC in Mesopotamia, Egypt, or Syria. The earliest known glass objects were beads, perhaps created accidentally during metalworking or the production of faience, which is a form of pottery using lead glazes.

Due to its ease of formability into any shape, glass has been traditionally used for vessels, such as bowls, vases, bottles, jars and drinking glasses. Soda–lime glass, containing around 70% silica, accounts for around 90% of modern manufactured glass. Glass can be coloured by adding metal salts or painted and printed with vitreous enamels, leading to its use in stained glass windows and other glass art objects.

The refractive, reflective and transmission properties of glass make glass suitable for manufacturing optical lenses, prisms, and optoelectronics materials. Extruded glass fibres have applications as optical fibres in communications networks, thermal insulating material when matted as glass wool to trap air, or in glass-fibre reinforced plastic (fibreglass).

Microscopic structure

The standard definition of a glass (or vitreous solid) is a non-crystalline solid formed by rapid melt quenching. However, the term "glass" is often defined in a broader sense, to describe any non-crystalline (amorphous) solid that exhibits a glass transition when heated towards the liquid state.

Glass is an amorphous solid. Although the atomic-scale structure of glass shares characteristics of the structure of a supercooled liquid, glass exhibits all the mechanical properties of a solid. As in other amorphous solids, the atomic structure of a glass lacks the long-range periodicity observed in crystalline solids. Due to chemical bonding constraints, glasses do possess a high degree of short-range order with respect to local atomic polyhedra. The notion that glass flows to an appreciable extent over extended periods well below the glass transition temperature is not supported by empirical research or theoretical analysis (see viscosity in solids). Though atomic motion at glass surfaces can be observed, and viscosity on the order of {10}^{17}–{10}^{18} Pa·s can be measured in glass, such a high value reinforces the fact that glass would not change shape appreciably over even large periods of time.

Formation from a supercooled liquid

For melt quenching, if the cooling is sufficiently rapid (relative to the characteristic crystallization time) then crystallization is prevented and instead, the disordered atomic configuration of the supercooled liquid is frozen into the solid state at Tg. The tendency for a material to form a glass while quenched is called glass-forming ability. This ability can be predicted by the rigidity theory. Generally, a glass exists in a structurally 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.

Glass is sometimes considered to be a liquid due to its lack of a first-order phase transition where certain thermodynamic variables such as volume, entropy and enthalpy are discontinuous through the glass transition range. The glass transition may be described as analogous to a second-order phase transition where the intensive thermodynamic variables such as the thermal expansivity and heat capacity are discontinuous. However, the equilibrium theory of phase transformations does not hold for glass, and hence the glass transition cannot be classed as one of the classical equilibrium phase transformations in solids.

Occurrence in nature

Glass can form naturally from volcanic magma. Obsidian is a common volcanic glass with high silica (SiO2) content formed when felsic lava extruded from a volcano cools rapidly. Impactite is a form of glass formed by the impact of a meteorite, where Moldavite (found in central and eastern Europe), and Libyan desert glass (found in areas in the eastern Sahara, the deserts of eastern Libya and western Egypt) are notable examples. Vitrification of quartz can also occur when lightning strikes sand, forming hollow, branching rootlike structures called fulgurites. Trinitite is a glassy residue formed from the desert floor sand at the Trinity nuclear bomb test site. Edeowie glass, found in South Australia, is proposed to originate from Pleistocene grassland fires, lightning strikes, or hypervelocity impact by one or several asteroids or comets.

Additional Information:

Introduction

Glass is a useful and unique material. It usually lets light shine through, but it blocks liquids and air. Glass is easily breakable, but it can also be made very strong. And glass can be formed into all kinds of shapes.

Glass feels hard like a solid, but it is built more like a liquid. The tiny particles that make up true solids are arranged in a specific order. The particles in glass are arranged randomly, as in a liquid.

How Glass Is Made

The main ingredient for glass is pure silica, or sand. It takes very high temperatures to make glass from sand. By adding certain chemicals to the silica, the process needs much less heat. Chemicals can also make the glass stronger or add colors to it.

The silica and chemicals are called the batch. To begin, glassmakers add to the batch some glass that has already been made. This scrap glass helps the silica to melt.

Once the batch is melted, glassmakers remove any bubbles or streaks. Then the melted glass can be molded into shapes or rolled into sheets and allowed to harden.

Modern machines can quickly and easily create huge numbers of glass items. But artists still create unique items through a method called glassblowing. Glassblowers blow air through a tube into melted glass to create different shapes.

Uses

People use glass in countless ways. Glass windows allow light to pass through but keep out cold and rain. Electric lightbulbs are made of glass. Glass containers store all kinds of things, including food, drinks, and chemicals. Glass mirrors reflect images. Lenses made of high-quality glass are used in microscopes and telescopes. Fiber-optic cables are bundles of tiny glass threads that carry information in the form of light. These cables are used to link telephones and computers over long distances.

History

The oldest known glass is more than 4,000 years old. Ancient Egyptians made glass beads and jars in about 2500 bce. Glassblowing probably began in Syria more than 2,000 years ago.

Glassmaking became important in Europe after the 1100s. Many European churches were decorated with stained-glass windows. In stained-glass windows, strips of lead hold pieces of colored glass together.

People in the American colonies started making glass in 1608. In 1825 a company in Pittsburgh, Pennsylvania, invented a glass-pressing machine. The machine helped to make glass in huge amounts.

In the 1900s glass became easier to make, less expensive, and stronger. Glass windows and containers became everyday features of most homes.

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