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In Christian iconography, it was gold that signified the aura of sanctity, but, as Baroque masters experimented with the effects of extreme light and dark, the hunt for a deep, light-fast yellow became urgent.

Worse, when Dr Stenger and his colleagues dug out photographs taken of them when they were new, the researchers were dismayed to find that the photographs were not light-fast either, and that they too had faded over the years.