Electroluminescent becomes extremely deterioration‐resistant after firing or refiring in pure sulfur. Additional air bake is permissible. The emission does not deteriorate toward zero, but toward a finite equilibrium which, in best cases, may be more than 50% of the original intensity. These “hypermaintenance” phosphors permit high brightness (by excitation with high frequency) to be maintained over long time. The deterioration is considered to be due to sulfur vacancies diffusing from the particle surfaces into the interior.