Zinc white vs titanium white

A white hides what is under it by scattering light, and it scatters more the more its refractive index differs from the binder around it. Zinc oxide sits at about 2.0, close to lead white; titanium dioxide at 2.5 to 2.7. The difference is the whole story of their covering power: a layer of titanium white hides a dark ground that zinc white would let through.

The same property is zinc’s use. Because it covers little, it lightens a colour without burying it, and it can be glazed over a passage to cool or soften it. Titanium, used the same way, turns a mixture chalky very quickly.^{[5, 7, 9]}

Painters noticed early that zinc white dries to a hard, brittle film that tends to crack. Conservation research has since tied the problem to specific failures: at the Hirshhorn Museum, zinc oxide underlayers were linked to cracking and delamination in Abstract Expressionist paintings, and studies of zinc soaps forming at the boundaries between layers explain how a paint layer can let go of the one beneath.

The practical conclusion drawn by paint makers and conservators is not to ban zinc, but to avoid it as the main white in grounds and lower layers, and to keep it to small proportions elsewhere, which is what most titanium–zinc blends do.^{[2, 10, 11, 12]}

Winsor & Newton sold zinc white as a watercolour, "Chinese white", from 1834, and it was a regular oil colour by about 1850. Titanium white went into production in Norway and the United States in 1916–19 and reached artists’ paint around 1920. A titanium white in a painting therefore means the passage is no older than that, which is why these whites matter to conservators as much as to painters.^{[1, 8, 12]}