How to Make Invisible Ink With Black Light
Black lights don't actually make invisible ink, but they can reveal certain types of ink that can't be seen in normal daylight or under incandescent lighting. These special fluorescent inks absorb ultraviolet wavelengths from black light bulbs and re-emit them as visible light. Due to this characteristic, this type of ink is usually invisible in normal light but glows bright enough to read under black lights, making it good for secret messages.
Things You'll Need
- Laundry detergent with optical brighteners
- Dark colored paper
Find a detergent that contains optical brighteners. Not all detergents list ingredients, but generally detergents that are labeled as bio-degradable do not contain optical brighteners, while most others do. For detergents that do list ingredients, look for benzoxazolyl, diaminostilbene disulfonate, naphthotriazolylstilbene, benzimidazoyl or naphthylimide. The detergent may also state simply that it contains brightening agents.
Mix the detergent with enough water so that it is nearly as thin as water. The thicker the mixture the brighter it will be under black light. If it is too thick, however, it will be visible under normal light.
Put the detergent mixture in a jar or 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.
Dip a paintbrush in the detergent mixture 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.
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.
- Experiment with the ratio of water and detergent to find the combination that glows as brightly as possible without showing in regular light.
- Don't saturate the paper too much or the outline of the letters will show where the paper was wet and then dried.
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