Electrical insulators and dielectric materials are ceramics with intrinsically low electrical conductivity. Internal porosity lowers bulk electrical conductivity. Insulators for DC or low frequency AC devices must have a high resistivity, good thermal resistivity, and low dilation coefficient in order to have sufficient resistance to thermal shock. The porosity must be very low. The surface vitrification must be perfect to avoid water absorption and improve mechanical resistance. The most commonly used materials are porcelains, mainly ternary compounds (Al2O3-SiO2-MgO). Such insulators are used especially as supports for electric oven resistors or heating elements.

Ceramics used for high frequency insulator applications require a low dielectric constant and a small loss tangent. The careful choice of raw materials saves the surface from having to be vitrified. A vitrified surface would increase the value of the dissipation factor.

For very high frequencies (VHF), high-purity dense alumina, fired at temperatures above 1600°C, is generally used. For powerful tubes, large alumina insulators must be assembled on metallic electrodes. This application is used for television or satellite transmitter tubes, for microwave generators used for heating, and for powerful lasers.