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"Apart from replacing the roof and spire, the main task was decontamination. Everything was covered in lead oxide powder. But that meant we could restore and clean – which explains why the cathedral looks so beautiful today."

Spices such as turmeric, cinnamon and paprika have been known to be mixed with lead chromate or lead oxide, compounds that mirror the spices’ colors, said Karen Everstine, technical director for FoodchainID, a company that tracks food supply chains.

Plumbonacrite is a byproduct of lead oxide, allowing the researchers to say with more certainty that Leonardo likely used the powder in his paint recipe.

The discovery, Gonzalez said, confirmed for the first time what art historians had previously only hypothesized: that Leonardo most likely used lead oxide powder to thicken and help dry his paint as he began working on the portrait that now stares out from behind protective glass in the Louvre Museum in Paris.

Leonardo is thought to have dissolved lead oxide powder, which has an orange color, in linseed or walnut oil by heating the mixture to make a thicker, faster-drying paste.