This news is about a month old, but it's new to me! Russian curator Elena Basner thinks she might have developed a foolproof way of determining whether a work of art was made before or after 1945. She tests the paint for radioactive isotopes. From the Times Online
The first nuclear bomb was successfully tested in July 1945 in New Mexico. On August 6, 1945, an atomic bomb was dropped on Hiroshima and three days later a second, more powerful bomb was dropped on Nagasaki. About 550 further explosions were carried out by the United States, Britain, the Soviet Union and France before most countries signed the Limited Test Ban Treaty in 1963. China tested its first Bomb the next year.
Dr Basner’s team argue that this activity released isotopes into the environment that do not occur naturally. Tiny traces of these isotopes, caesium-137 and strontium-90, permeated soil and plant life and ended up in all postwar paintings through the natural oils used as binding agents for paints.
Any work of art purporting to be more than 63 years old that registers trace amounts of the two isotopes can therefore be definitively declared a fake, Dr Basner said.
The article goes on to point out that it would be possible for a forger to circumvent this method of detection by using paints and canvasses from the relevant period. I can understand how a forger could obtain an old canvas, but where would they get old paint?