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Posted By: davidcycleback

The same longwave blacklight you use on baseball cards is useful in identifying modern repro baseball caps. This is as much modern thread, tags and cloth fluoresces brightly under blacklight. Post WWII Manufacturers add the same type of brightly fluorescing chemicals to thread as they do card stock.

In my experience with modern dark cloth caps, most of the cap will not fluoresce, but one or more small areas will usually give it away. This can include a brightly fluorescing tag or emblem or inside stitching.

This also illustrates that modern alterations to old jerseys can often be identified with blacklight. On a vintage 1930 jersey, a modern patch can fluoresce. Even with a vintage 1930 patch recently sewn onto a vintage 1930 jersey, the new thread can give it away.

http://www.cycleback.com/blacklight.html