Inorganic, phosphorescent Zinc sulfide powders. Recommended for use with water-based binders (except fresco). Since excessive grinding weakens their phosphorescent qualities, pigments should be mixed gently with a binder, or ground lightly. Humidity and ultraviolet rays can darken these pigments. If the colours are exposed to direct light, keep the humidity of the environment under 50%. Applied in favourable conditions, the special properties of these pigments can last for years.