Why must ceramic lamp envelopes resist both high heat and corrosion?

Ceramic lamp envelopes must resist high heat and corrosion because they are used in electric discharge lamps, where enclosed gases are energized by an applied voltage to produce light. This process pushes materials to their thermochemical limits, as the plasma contained within the inner envelope can reach temperatures as high as 1,200 °C (2,200 °F).1

To function effectively, these envelopes must be made of a refractory material that can withstand these extreme temperatures while remaining optically transparent to transmit the light generated by the discharge. For instance, in high-pressure sodium-vapour lamps, alumina is used because it provides the necessary heat resistance and chemical stability to contain the hot, corrosive sodium discharge.1