[alephnil]

The presence of Cesium-137 and Strontium-90 has also been used to reveal art forgeries. Before atomic bombs were tested, these isotopes did not exist in nature. Art produced before 1945 will not contain these isotopes, while art produced after that time usually will.

[mulmen]

How does this work? The old art still exists but how does it avoid exposure? Why is exposure only relevant at the time of creation? Will old paint used in modern times display the isotope?

[pulisse]

Traces of the isotopes in question are detectable in the pigments used in recently produced art. Surface contamination isn't what's being tested.

[azernik]

The issue is usually in contamination of raw materials.

Similar to carbon-14 dating - the isotopic ratio of carbon in the atmosphere is maintained by cosmic radiation hitting the carbon-12 and carbon-13 atoms, but as soon as the carbon is incorporated into solid matter it ceases to be part of the process that renews the carbon-14 levels.