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Invisible Inks

Write & Reveal Secret Messages

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This smiley face was made with invisible ink. The face became visible when the paper was heated.

This smiley face was made with invisible ink. The face became visible when the paper was heated.

Anne Helmenstine

Sometimes people tell me they can't do any science projects because they don't have any chemicals. There are some activities that don't require any chemicals you don't already have. A great example is invisible ink.

Invisible ink is any substance that you can use to write a message that is invisible until the ink is revealed. You use the ink by writing your message with it using a cotton swab, dampened finger, fountain pen, or toothpick. Let the message dry. You may want to write a normal message on the paper so that it doesn't appear to be blank and meaningless. If you write a cover message, use a ballpoint pen, pencil, or crayon, since fountain pen ink could run into your invisible ink. Avoid using lined paper to write your invisible message, for the same reason.

How you reveal the message depends on the ink you used. Most invisible inks are made visible by heating the paper. Ironing the paper or holding it over a 100-watt bulb are easy ways to reveal these types of messages. Some messages are developed by spraying or wiping the paper with a second chemical. Other messages are revealed by shining an ultraviolet light on the paper.

Make Invisible Ink

Anyone can write an invisible message, assuming you have paper, because body fluids can be used as invisible ink. If you don't feel like collecting urine, here are some alternatives:

Heat-Activated Invisible Inks
Iron the paper, set it on a radiator, place it in an oven (set lower than 450° F), hold it up to a hot light bulb.

  • any acidic fruit juice (e.g., lemon, apple, or orange juice)
  • onion juice
  • baking soda (sodium bicarbonate)
  • vinegar
  • white wine
  • dilute cola
  • diluted honey
  • milk
  • soapy water
  • sucrose (table sugar) solution
  • urine
Inks Developed by Chemical Reactions
These inks are sneakier, because you have to know how to reveal them. Most of them work using pH indicators, so when it doubt, paint or spray a suspected message with a base (like sodium carbonate solution) or an acid (like lemon juice). Some of these inks will reveal their message when heated (e.g., vinegar).
  • phenolphthalein (pH indicator), developed by ammonia fumes or sodium carbonate (or another base)
  • thymolphthalein, developed by ammonia fumes or sodium carbonate (or another base)
  • vinegar or dilute acetic acid, developed by red cabbage water
  • ammonia, developed by red cabbage water
  • sodium bicarbonate (baking soda), developed by grape juice
  • sodium chloride (table salt), developed by silver nitrate
  • copper sulfate, developed by sodium iodide, sodium carbonate, potassium ferricyanide, or ammonium hydroxide
  • lead(II) nitrate, developed by sodium iodide
  • iron sulfate, developed by sodium carbonate, sodium sulfide, or potassium ferricyanide
  • cobalt chloride, developed by potassium ferricyanide
  • starch (e.g., corn starch or potato starch), developed by iodine solution
  • lemon juice, developed by iodine solution
Inks Developed by Ultraviolet Light (Black Light)
Most of the inks that become visible when you shine a black light on them also would become visible if you heated the paper. Glow-in-the-dark stuff is still cool. Here are some chemicals to try:
  • dilute laundry detergent (the bluing agent glows)
  • body fluids
  • tonic water (quinine glows)
  • vitamin B-12 dissolved in vinegar
Any chemical that weakens the structure of paper can be used as an invisible ink, so you might find it fun to discover other inks around your home or lab.

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