Zinc Sulfide Phosphor: Zinc Oxide and Sulfur Fired Twice with Halide Fluxes and a Trace of Copper (Rothschild, US 2,546,239)

Glow-in-the-dark zinc sulfide needs very pure zinc sulfide, normally precipitated from carefully cleaned solutions. Rothschild made it straight from zinc oxide and sulfur with salt as a flux, cleaned it by washing and floating, then fired it again at 1200 C with more fluxes, zinc dust and a tenth of a gram of copper chloride: the copper is what makes it glow. Result: zinc sulfide phosphor with a yellow-green afterglow. Source: Rothschild, US 2,546,239 (1951), Example and claim 6. Picture drawn by Youblob.