I've got a pretty specific example. In a book I have, Atlantic by Simon Winchester, there is an interesting note on the scuttling of the German fleet at Scapa Flow:
Many of the hulls remain - and high-quality German steel salvaged from some of the remaining wrecks is occasionally brought to the surface today, much prized for certain very delicate scientific experiments, since it was fashioned, forged, and cut long before the spread of radioactive contamination, which has affected most metals that have been made since Hiroshima. - Atlantic, page 256
What is this saying? That the spread of nuclear materials have contaminated historical artifacts?
Not as such. Oxygen is used in blast furnaces while producing steel. The nuclear detonations of the previous century released nuclear contamination into the global atmosphere. The oxygen used in blast furnaces comes from the atmosphere, one way or another. As such it contains that nuclear contamination, albeit a tiny amount. So any steel produced now will be ever so slightly contaminated as well.
Some extremely sensitive equipment (aerospace instruments, geigercounters) is more accurate without any nuclear contaminants, so metal produced before 1940 is still used for them. Scuttled ships are particularly good, since they're submerged and as such not directly exposed to atmospheric contaminants.