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?
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.