It’s easy to make invisible ink and reveal secret messages. Really, any liquid that dries invisible works, if you know how to use it. Here are several ways to make invisible ink and reveal the messages.
How Invisible Ink Works
Some types of invisible ink work by very slightly disturbing the fibers in paper. Heat reveals the damage and shows the message. Other types of ink are invisible until a chemical reaction occurs. Another type of invisible ink is hidden under ordinary light, but appears under an ultraviolet or black light.
Usually, you write an invisible ink message using a cotton swab, fountain pen, toothpick, or even an ink-dipped finger. Make the invisible message less conspicuous by writing a cover message. Use a pencil, crayon, or ballpoint pen for the cover message, so it won’t run into the secret message when you reveal the invisible ink.
Invisible Ink Revealed by Water
Probably the simplest way to write a secret message that you can easily read involves water and no ink.
- Thoroughly wet a sheet of paper. This is the page for your invisible message.
- Smooth the water paper over a completely flat surface, like a mirror or a glass table top.
- Place a second sheet of paper (dry) over the wet paper. Write a message using a pencil.
- Remove the top sheet of paper. At this point, you can see the depressions made by the pencil on the wet paper. Set your secret message out in the sun to dry. Peel the wet paper from the surface. Be careful because the paper is fragile. If you’re impatient, dry it with a blow dryer. When dry, the message is invisible.
- Reveal the invisible message by soaking the paper in water.
Pressing on the wet paper disturbs the cellulose fibers in the paper just enough that the impression remains when the paper dries. Soaking the paper in water swells the fibers and reveals the message.
Invisible Ink Revealed by Heat
Heat reveals many invisible inks. Writing on paper disturbs its fibers and slightly damages the paper. Heat darkens the message more than it darkens the intact paper. Reveal messages using heat by placing the paper in an over (cooler than 450 °F), ironing the message, or holding it near a hot light bulb.
Write the message using invisible ink that is activated by heat. Typically, these “inks” are weak acids or bases. Anything that contains sugar works because the sugar caramelizes and darkens with heat, revealing the message.
- Lemon juice
- Apple juice
- Onion juice
- Baking soda mixed with a little water
- Vinegar
- White wine
- Diluted cola
- Diluted honey
- Milk
- Soapy water
- Solution of sugar in water
- Blood plasma
- Urine
Invisible Ink Revealed by a Chemical Reaction
Heat-activated invisible inks are not particularly good at concealing secret messages because they are so easily revealed. A better invisible ink only shows itself when a chemical reaction causes a color change. Some of these inks involve acid-base reactions, while other reactions form a colored precipitate.
Here are chemically-activated inks and ways of revealing them:
- Starch (e.g., corn starch or potato starch), developed by iodine solution
- Lemon juice, developed by iodine solution (or by heat)
- Phenolphthalein (a pH indicator that is colorless at neutral pH), developed by ammonia fumes or sodium carbonate (or another base)
- Thymolphthalein (another pH indicator), developed by ammonia fumes or sodium carbonate (or another base)
- Vinegar or diluted acetic acid, developed by red cabbage juice in water (or by heat)
- Ammonia, developed by red cabbage water
- Sodium bicarbonate (baking soda), developed by colored grape juice
- Sodium chloride (table salt) in water, 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(II) sulfate, developed by sodium carbonate or potassium ferricyanide
- Iron(III) sulfate, developed using sodium sulfide
- Cobalt(II) chloride, developed by potassium ferricyanide
- Cerium oxalate, developed by a mixture of manganese sulfate and hydrogen peroxide
Invisible Ink Revealed by Black Light
Some inks are invisible under ordinary light, but glow under ultraviolet or black light. Other inks absorb ultraviolet light and appear as dark messages on fluorescent paper. Many inks that respond to ultraviolet light reveal their messages when photocopied.
Here are some inks that reveal messages under black light:
- Lemon juice
- Laundry detergent
- Soap
- Saliva
- Blood serum
- Sunscreen
- Tonic water that contains quinine
- Vitamin B12 dissolved in water
Most of these inks are also revealed by heat.
Disappearing Ink
While not ordinarily considered an invisible ink, disappearing ink is colorless when dry and develops a color in a chemical reaction. Disappearing ink consists of the pH indicator thymolphthalein or phenolphthalein mixed with sodium hydroxide. Sodium hydroxide is a base that colors the indicator. But, as the ink reacts with carbon dioxide in air, its pH lowers and the indicator becomes colorless. Swiping across the message with a base in water reveals the message. Examples of common bases include baking soda or washing soda in water.
References
- Dooley, John F. (2015). “Review of Prisoners, Lovers, and Spies: The Story of Invisible Ink from Herodotus to al-Qaeda by Kristie Macrakis”. Cryptologia. 40 (1): 107–112. doi:10.1080/01611194.2015.1028684
- Dooley, John (2016). Codes, Ciphers and Spies: Tales of Military Intelligence in World War I. New York: Copernicus Books. ISBN 9783319294148.
- MacRakis, Kristie; Bell, Elizabeth K.; Perry, Dale L.; Sweeder, Ryan D. (2012). “Invisible Ink Revealed: Concept, Context, and Chemical Principles of “Cold War” Writing”. Journal of Chemical Education. 89 (4): 529–532. doi:10.1021/ed2003252
- Mollin, Richard (2005). Codes: The Guide to Secrecy From Ancient to Modern Times. Boca Raton, FL: CRC Press. ISBN 9781584884705.
- Wheeler, Jo; Temple, Katy (2009). Renaissance Secrets, Recipes & Formulas. London: Victoria and Albert Museum. ISBN 9781851775774.