[drcy]

If it fluoresces very bright white or very bright white with a bluish tinge like a piece of today's computer paper, it's modern material. Use a piece of today's computer paper or recent baseball card for comparison. If it doesn't, that doesn't prove it Pre-War, but much to most modern white cloth and paper fluoresce very brightly. Modern (post WWII) white cloth and paper commonly has fluorescing chemicals added during manufacturing, that's why it fluoresces so brightly under blackight. That's why our white clothes and even white shoelaces glow in the dark under Halloween party blacklights. The chemicals are intentionally added to white materials to make them appear brighter in sunlight, as sunlight contains ultraviolet rays.

A blacklight is one of the quickest ways to identify many modern reprints and counterfeits of antique cards, posters, postcards, photos and other paper collectibles.