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Making invisible ink to write and reveal secret messages is a great science project to try, even if you think you don't have the right chemicals. Why? Because just about any chemical can be used as invisible ink if you know how to use it.
What 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 write your message with the ink using a cotton swab, dampened finger, fountain pen, or toothpick. Let the message dry. You might also 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 use. Most invisible inks are made visible by heating the paper. Ironing the paper and 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.
Ways to Make Invisible Ink
Anyone can write an invisible message, assuming you have paper, because bodily fluids can be used as invisible ink. If you don't feel like collecting urine, here are some alternatives:
Heat-Activated Invisible Inks
You can reveal the message by ironing the paper, setting it on a radiator, placing it in an oven (set to lower than 450 degrees F), or holding it up to a hot light bulb.
To write the message you can use:
- Any acidic fruit juice (e.g., lemon, apple, or orange juice)
- Onion juice
- Baking soda (sodium bicarbonate)
- White wine
- Diluted cola
- Diluted honey
- Soapy water
- Sucrose (table sugar) solution
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 in doubt, paint or spray a suspected message with a base (such as sodium carbonate solution) or an acid (such as lemon juice). Some of these inks will reveal their message when heated (e.g., vinegar).
Examples of such inks include:
- 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 diluted 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 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)
- Bodily 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.