It was made popular by the cheap production costs, greater coverage.

[1] Although barium sulfate is almost completely inert, zinc sulfide degrades upon exposure to UV light, leading to darkening of the pigment.

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.

The cobalt salts help to stabilize zinc sulfide so it will not have as severe a reaction to UV exposure.