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A POISONOUS yellow pigment used by painters for hundreds of years is helping
art buffs to date works of art more accurately than before.

“Naples yellow” is a pigment made of lead antimony oxide that artists have
used since the 1600s. Oil paints containing it are toxic, but over the years,
manufacturers have tweaked the physical structure of the pigment to make it
safer, cheaper and more stable.

Joris Dik and his colleagues at the University of Amsterdam looked at the
structure of Naples yellow in over 100 European paintings of known age. They
bombarded the paint samples with electrons and monitored the X-rays emitted.
This allowed them to create a “map” of where and when artists used different
varieties of the pigment, they told a meeting of the American Crystallographic
Association in Los Angeles last month.

Marina Aarts from Christie’s auction house in Amsterdam says dating paintings
using pigments can be crucial. “One dealer had bought a painting in a flea
market,” says Aarts. “I thought it was 19th century but we tested the pigments
and it turned out to be 16th century.” That made the painting six times as
valuable.

Matthys De Keyzer, an art expert from the Netherlands Institute for Cultural
Heritage, cautions that pigments can’t confirm where a painting originates.
“It’s tricky,” he says. “There was a lot of trading of pigments.”