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Tempered glass

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Glass

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Literature provides many definitions of glass or glassy state. That state is most often defined based on internal structure, namely, structure without long range order. The way basic elements are arranged in the spatial network of glass is similar to the arrangement of molecules in liquid or even gas.

These molecules are not able to move or the possibility is extremely low because of very high viscosity. From the thermodynamics point of view, glass is a non-durable material – energetic state of amorphous network is higher than its crystalline equivalent. For that reason, each type of glass tends to crystallisation, but it does not happen even after a long time for its viscosity. In normal conditions viscosity is the same as in case of crystalline solid matters.  Many types of glass just never crystallises, even in the Universe time-scale.

The statement that glass is a liquid or supercooled liquid is imprecise, as, for its rheological properties, it is a solid matter. Supercooled liquid means only that the substance when cooled below its crystallisation point still remains in liquid state with all the consequences, i.e. it is still liquid and its molecules can move around – in other words – its viscosity remains low. In case of glass, both regular one with siliceous bonds and glassified metals, it is, by no means, a solid state substance. There are known many other crystalline materials that “float” much more intensely than glass and their solid state have never been denied. Sometimes, in order to justify the floating nature of glass the fact of uneven thickness of glass, e.g. in old churches’ windows is raised. But the explanation is much more simple: glass produced with old-fashioned method just had no equal thickness on the whole surface. Stained glass manufacturers usually cut fragments of glass so that the thicker part – more stable – was placed in bottom parts.

Traditional glasses – i.e. such glasses that naturally transform from alloy to glassy state have short range ordered structure – it is one of the reasons why the alloy’s viscosity increases significantly with the temperature drop. The phenomenon causes, in the consequence, that the material solidifies “freezing“ the liquid structure inside. In other words, internal resistance is so high that it prevents its crystallisation.  For alloy’s crystallisation the conditions allowing the crystallisation from the thermodynamic point of view are necessary to exist and additionally, the viscosity of the alloy shall be low enough to enable molecules to move. The lower temperature the lower crystallisation ability (ability to grow crystals). In case of metals, in order to obtain the glossy state, a very rapid cooling is necessary (cooling “speed” of about  106 K/s) that will prevent crystalline structure creation.

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