Things You'll Need
Optical brightening detergent
Black light ink is any liquid that reabsorbs the ultraviolet rays from a black light and re-emits them as visible light. The obvious result is that, when exposed to a black light, black light inks will glow. There are a variety of ways to make black light ink, some of which render the written message invisible until exposed to the black light. Other black light inks just make the message glow under the black light.
Dip a small paintbrush into a container of kerosene, wipe off excess, and write on a piece of paper with the kerosene.
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Allow the kerosene to dry completely in a well-ventilated area.
Tightly close the kerosene container once you're done with it.
Mix optical brightening detergent--available in the laundry aisle of most supermarkets--with water.
Keep adding water until the mixture is thin enough to be painted onto paper with a small paintbrush.
Add a little of the mixture to the paper and check to make sure it dries clear. If it doesn't dry clear, you need to thin out the brightener with more water. If you're not worried about making the message invisible but simply want it to glow, you don't need to worry about this step.
Paint your message onto the paper and let dry.
Expose the paper to a black light in order to see your invisible message.
Mix commercially available glow powders (see Resources) into a water or vegetable-glycerin base.
Match the color of glow powder you choose to use to the paper you're writing on. This takes some care, and you may need to experiment a little, but the result should be a pigment that blends seamlessly into the color of the paper. If you're not worried about making the message invisible but just want it to glow under a black light, choose whatever color pigment you like.
Dip a small paint brush in the glow powder mixture and paint your message onto the paper or other surface.
Let the ink mixture dry, then expose it to a black light to see the painted message glow in the dark.
Spit on a clean dish.
Dip your paintbrush into the dish and use it to write a short message.
Wait until the saliva dries.
Expose the written-on area to a black light. The message should glow.
Make sure the kerosene stored away from heat sources. Take care not to ignite the kerosene writing when exposing the written-on paper to the black light.