How do scientists catch a perfect art forgery?

Scientists catch skilled forgers by looking for things the forger could not control: materials that did not exist in the artist's lifetime, chemical traces from the wrong place or time, and physical clues such as cracks and dust that behave differently when faked. A forgery can fool the eye, but it is very hard to fake the chemistry of a paint film that has aged for centuries.

The classic case is Han van Meegeren, a Dutch painter who forged Vermeers in the 1930s and 1940s. He was careful: he bought genuine seventeenth-century canvases, ground his own pigments from materials like lapis lazuli, and hardened his paint with Bakelite, an early plastic, by baking the canvas, so it would pass the solvent tests of the day. In 1937 the leading Vermeer expert called his Supper at Emmaus 'the masterpiece of Johannes Vermeer'. Hermann Göring traded 137 looted paintings for another of his fakes.

After the war he was charged with treason for selling a Dutch treasure to the Nazis, a crime punishable by death. His defence was to confess and, to prove it, paint a new 'Vermeer' in front of witnesses. Chemists then found the Bakelite in his paint, a substance invented only in the twentieth century. Even so, some experts kept arguing, until in 1967 scientists measured radioactive lead in his white paint and found it nothing like the lead of genuine seventeenth-century Dutch paintings.