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Why is glass generally an electrical insulator at ordinary temperatures?

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Glass is generally an effective electrical insulator at ordinary temperatures because its high viscosity severely impedes the movement of charged metallic ions that would otherwise carry an electric current.1

While many glasses contain mobile ions—particularly univalent alkali ions—that are capable of conducting electricity, their ability to move through the glassy structure is restricted by the material's stiff, amorphous nature. Because viscosity is a function of temperature, the electrical conductivity of glass increases significantly as it is heated; for example, soda-lime-silica glass can be melted electrically once it reaches approximately 1,000 °C (1,800 °F).1

In contrast to typical insulating glasses, electronic conduction can occur in specific families, such as chalcogenides and oxide glasses containing transition-metal ions, where electrons can move between energy levels or hop between ions of differing valence. However, for most common commercial glasses, the structural resistance to ion mobility remains the primary factor ensuring their insulating properties at room temperature.21