Although barium sulfate is almost completely inert, zinc sulfide degrades upon exposure to UV light, leading to darkening of the pigment. The severity of this UV reaction is dependent on a combination of two factors; how much zinc sulfide makes up the pigments formulation, and its total accumulated UV exposure. Depending on these factors the pigment itself can vary in shade over time, ranging from pure white all the way to grey or even black. To suppress this effect, a dopant may be used, such as a small amount of cobalt salts, which would be added to the formulation. This process creates cobalt-doped zinc sulfide. The cobalt salts help to stabilize zinc sulfide so it will not have as severe a reaction to UV exposure.
In terms of refractive index and opacity, lithopone surpasses zinc oxide and lead oxide. Lithopone's high refractive index allows it to efficiently scatter and reflect light, thereby increasing the opacity of various media. Whether you need to enhance the opacity of paints, inks or plastics, lithopones deliver outstanding results, ensuring your final product is completely opaque.