Wolfgang Beltracchi is an artist who got his start as a forger of classic masters like Picasso, Van Gogh, and others. But he and those in his business found a foil in Peggy Guggenheim, Dr. Elena Basner, and a cadre of scientists that found a technique that searches for traces of the isotopes cesium-137 and strontium-90.

The reason is that these isotopes are only created by fission of Uranium 235, and between 1945 and 1963, 522 open-air atomic bomb blasts scattered these isotopes into the atmosphere, which then got into the soil, made its way into flax plants, which were used to create linseed oil, which was used as a binding agent in paint.

So if a painting shows traces of these isotopes in the paint, there is no way that it was created before 1945.

This technique has foiled hundreds of art forgers in the years since and has proven to be one of the most difficult challenges for future Wolfgang Baltracchis.