Glasses are amorphous so they have no long-range order and no symmetry. There is, however, a great deal of short-range order. If crystallization is prevented during cooling, an amorphous glass will form with short-range order inherited from the liquid. The critical cooling rate to prevent crystallization varies greatly from one material to another. Silicate glasses cannot crystallize unless the cooling rates are extremely slow. On the other hand, extremely rapid cooling is required to prevent crystallization of metals.

Structure of Silicate Glasses

The basic structural units of silicate glasses are tetrahedra with Si4+ ions in the center bonded covalently to O2− ions at each corner. In pure silica all corner oxygen ions are shared by two tetrahedra (Figure 13.1). The result is a covalently bonded glass with a very high viscosity at elevated temperatures.

The compositions of typical commercial glasses are quite complex. Soda-lime glasses may contain 72% SiO2, 14% Na2O, 11% CaO, and 3% MgO. The Na+, Ca2+, and Mg2+ ions are bonded ionically to some of the corner O2− ions (Figure 13.2). With these alkali and alkaline earth oxides, not all of the oxygen ions are covalently bonded to two tetrahedra. This lowers the viscosity at high temperatures.