How can Cold War bomb tests expose a painting that claims to be older than 1950?

Cold War bomb tests left a dated signature in the air. Above-ground nuclear tests caused a sudden increase of carbon-14 in the atmosphere, almost doubling its relative concentration, and every plant that grew afterwards took it up. A canvas, a wooden frame or the oil in a paint binder made from such plants therefore carries a mark showing it came from after the 1950s.

Ordinary radiocarbon dating cannot do this job alone. Carbon-14 has a half-life of about 5,730 years: when an organism dies, its carbon-14 decays, so the amount left shows how long ago it died. That clock is far too slow to separate a painting of 1920 from one of 1990. The bomb pulse works differently. Carbon-14 in the atmosphere almost doubled in less than ten years, reaching its maximum in 1963-1965. Since the 1963 test ban, the relative concentration has been decreasing at 4% per year.

The first modern artwork exposed this way involved a painting from the Peggy Guggenheim Collection, supposed to be a Fernand Léger of 1913-14. The canvas had a high radiocarbon concentration and dated to 1959 or 1962, at least four years after Léger's death.

A forger can fight back. In a study of a painting by the forger Robert Trotter, the canvas was inconclusive because, like many forgers, he used old canvases, but the oil binder in his paint came from seeds harvested after 1950, so the picture could not date from the 1860s.