How to Make Invisible Ink With Black Light
Black lights don't really "make" invisible ink, but they can reveal certain types of invisible ink. Fluorescent inks absorb ultraviolet wavelengths from black light bulbs and re-emit them as visible light. Some of these are invisible under normal light but glow bright enough to read under black lights. These make good invisible inks for secret messages.
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Instructions
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Mix an optical brightening detergent with water. Water it down until it has the consistency of water. The thicker it is, the brighter it will be under black light. If it is too thick, however, it will be visible under normal light. Follow the link below in Resources for a guide to which detergents have optical brighteners.
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Put the detergent in a jar or other waterproof, transparent container. Turn off the lights and turn on a black light. Hold the jar of ink up to the light. It should glow brightly.
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Dip a paintbrush in the ink and write on a piece of paper. Bleached notebook paper will glow under black light, so the message will have less contrast. Write on dark or black paper to get the best contrast.
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Wait until the ink dries. Turn off the lights and turn on the black light. Shine it close to the invisible ink to see the message. You may have to shine it closer than before to see the writing.
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Make some other inks. You can mix soap with water to get invisible ink, or write in pure kerosene. You could even use your own saliva. You can also crush up B-12 pills in distilled vinegar and mix them until the pills dissolve.
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Tips & Warnings
Be careful with kerosene because it is highly flammable.
