Calcium sulfide (CaS) is an inorganic compound with the chemical formula CaS. It appears as a white to light gray crystalline solid with a cubic rock-salt crystal structure. It is sparingly soluble in water but reacts with it, and it is soluble in acids with decomposition. Industrially, it is often produced by the reduction of calcium sulfate with carbon or by the reaction of hydrogen sulfide with calcium oxide or calcium carbonate. The primary application of calcium sulfide is as a precursor material in the production of luminescent materials, phosphors, and certain types of ceramics. It acts as a host lattice for activators like bismuth or rare-earth elements to create electroluminescent and photoluminescent devices. Historically, it was used in luminous paints and as a depilatory agent in leather processing. It also finds niche use as a flotation agent in ore concentration and as a chemical intermediate in some laboratory syntheses. Calcium sulfide must be handled with care as it reacts with moisture and acids to release hydrogen sulfide (H₂S), a highly toxic and flammable gas. It is typically stored under dry, inert conditions. Its use in mainstream industrial applications has declined in favor of safer or more efficient alternatives, but it remains a material of interest in specific advanced material research contexts.