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Glass is an amorphous or non-crystalline solid that is brittle in nature and the structure is obtained by melt-quenching process. The glass structure is also obtained by using sol-gel and vapor deposition technique. Glass is weak in tension because of its non-crystalline molecular structure. When load is applied beyond the strength limit, glass breaks without any prior warning, unlike steel and aluminium where plastic deformation occurs. Therefore, the atomic structure of the glass is different from the structure of the crystalline materials.  


The main constituent of glass is silica sand. Almost, all glass contains at least 50% silica. The structure of the glass is formed by bonding silicon and oxygen ions. Network modifiers (or intermediates) and network formers such as sodium (Figure 1(c)), may interrupt the continuity or contribute to the network structure of glass. There are two main differences between crystalline and amorphous solids; crystalline solids are formed by repeating geometric arrangement of atoms whereas; amorphous solids have random atomic arrangement. The second difference is their phase transformation behavior (Figure 2) when they are heated. For example, the transformation phases of crystalline and amorphous 

solids can be understand by examining silica (SiO2) which can exist in either state. When silica is in crystal form and is heated at a temperature Tm (freezing or melting point), it becomes liquid. It is observed that at melting point the specific volume of crystalline solids changes abruptly which causes sharp changes in physical properties. Whereas the amorphous structure of silica softens gradually (start softening at temperature Tg) when those are heated because there is a wide temperature range between the solid and liquid state. The temperature Tg is the glass transition temperature of the solids. 


https://en.wikipedia.org/wiki/Glass