Zinc
White

Zinc oxide had been known to alchemists as philosopher’s wool (the white smoke condensed on the upper walls of furnaces in which brass was being smelted) since at least the Renaissance, but it was not made commercially as a paint pigment until the mid-nineteenth century. The French chemist Edme-Jean Leclaire developed the so-called French or indirect method for producing oil-grade zinc white over the period 1835-1844, with large-scale industrial production following in the mid-1840s. In 1834, Winsor & Newton in London had already marketed zinc oxide for watercolour under the trade name “Chinese white.” The exotic naming was a marketing decision associated with the qualities of Chinese porcelain, though the pigment’s oil-grade production and the watercolour launch were two distinct developments.

The pigment caught on quickly for two practical reasons. Unlike lead white, it was non-toxic and did not blacken in the sulphur-laden urban air of nineteenth-century European cities, where coal smoke routinely converted lead carbonate to black lead sulphide on exterior surfaces. And unlike most earth pigments, it ground to a cool, almost blueish white that lent itself to mixtures requiring a clean cool tone (pale skies, snow scenes, glazed flesh highlights).

In oil paint, the story is more complicated. Zinc white was adopted more broadly as an oil pigment from the second half of the nineteenth century onwards, appearing in paintings by Cezanne and the early Impressionists. As early as the 1890s, concerns were being raised: the English chemist A.H. Church noted in 1890 that zinc white in oil “often shows a tendency to crack and scale” when used freely. Conservators continued to document a recurring problem as the twentieth century progressed: zinc-white oil paint films, after several decades, developed brittle horizontal cracks and tended to delaminate from the canvas in sheets. The chemistry was eventually understood. Zinc ions form metal soaps with the linseed oil binder over time, the resulting zinc carboxylates crystallise into rigid microcrystals, and the paint film loses its elastic strength. The phenomenon is now well-documented in the conservation literature, and modern best practice cautions against using pure zinc white in oil, particularly as a thick body-colour or impasto.

For watercolour the problem does not arise (there is no oil for the zinc to form soaps with), and zinc white remains the standard cool watercolour white, sold as both Chinese white and “zinc white” by every major manufacturer. Modern oil paint manufacturers offer zinc white either alone (with conservation warnings) or as a “mixing white” blended with titanium dioxide for stability; pure zinc-white oil paint is now mainly used by painters who specifically want its cooler, more transparent character and accept the long-term cracking risk.