Some of the older pigments are lead-based and difficult to acquire because of EU health-and-safety regulations.

WSJ: Forging Ahead With a Goya Classic

Glaze is mostly translucent, but da Vinci would also slip in small amounts of pigments, such as manganese and lead oxides.

ECONOMIST: The da Vinci method

The albino condition, caused by a lack of colouring pigments in the skin, can lead to the animal being susceptible to sunburn, but the zoo said its wallaby enclosure had lots of shade and it should lead a "normal life".

BBC: Rare albino wallaby born at Linton Zoo