The application of chemical thermodynamics to the design of long‐life, high‐temperature devices is advocated and illustrated by its use in selecting the material components for a high‐temperature lead vapor Raman cell. It is shown that the reduction of alumina and of a calcia–alumina sealing frit by tantalum lead to equilibrium aluminum and calcium partial pressures orders of magnitude greater than those over pure alumina at the desired cell operating temperature.