People have been sending secret messages since ancient times. Countless methods have been devised to protect information—and just as many to uncover it. Nowadays, digital methods are in fashion, and ciphers and chemical inks are more like toys, but there was a time when lives depended on them.
Methods for concealing information fall into two main categories. One group includes cryptographic techniques, which convert the message into an unreadable, encoded form for unauthorized parties. Cryptography has evolved from ancient secret alphabets to today’s digital systems. At one time, it was the privilege of elites—used by popes, kings, and generals—and curious commoners often lost their heads over it. Today, cryptography permeates our everyday lives, though we are generally unaware of the key exchanges, signature schemes, and encryption protocols we use when paying by card, filing taxes, browsing the internet, making calls, buying train tickets, or unlocking our cars.
The other group of methods, steganography, conceals the very existence of the message. While cryptography protects the content, steganography hides that there is anything to conceal. Herodotus even mentions such a method: a message was written on the shaved scalp of a courier, who was sent on his way after his hair grew back. Whether this is a legend or people just had more time on their hands back then is debatable. What is certain is that invisible inks have been known and used for over 2,000 years. Messages were typically written between the lines of unrelated text. Once dry, the writing disappears, and the paper shows nothing suspicious—until the recipient reveals the hidden message. In some methods, the writing disappears again after development; in others, it remains visible. Invisible ink was a favorite tool of spies during both World Wars—and even throughout the Cold War. Today, it’s still used for marking, stamp ink, and children’s toys.
Spycraft Tools
Invisible ink is first mentioned by the Greek military writer Aeneas Tacticus in his treatise on siegecraft in the 4th century BCE, though he doesn’t give a recipe. The Byzantine engineer and physicist Philo Mechanicus wrote about using oak galls and iron sulfate in the late 3rd century BCE. This is the recipe for classical gall ink—not invisible ink. The iron ions give gall ink its color. The gallic acid in galls forms a blue-black iron-tannate complex with iron sulfate. When dry, insoluble iron-tannate precipitates and oxidizes, darkening further. People soon discovered that writing with iron sulfate alone created invisible writing, which could later be developed with gall extract.
Spies often had to concoct their own invisible inks in the field. They were thoroughly trained in how to make chemical inks from accessible materials, and they sometimes experimented with the most unlikely substances—from onion juice to milk, and even semen.
During the American Revolutionary War, George Washington ordered the development of a more reliable invisible ink. It was created by James Jay (brother of Founding Father John Jay). Instead of heat-reactive organic inks, he used a two-solution system: one for writing, the other as a developer. This “white ink” was used by Benjamin Franklin, Silas Deane, and Washington’s New York spies Abraham Woodhull (alias Samuel Culper Sr.) and Robert Townsend (alias Samuel Culper Jr. or Agent 723).
According to a WWII-era British SOE (Special Operations Executive) training manual, a good chemical ink must:
- be water-soluble
- not leave greasy stains
- have no suspicious smell
- leave no sparkling crystals on paper
- be invisible under UV light
- not damage or color the paper
- resist iodine or common developers
- have as few known developers as possible
- be undevelopable by heat
- consist of easily obtainable, everyday materials that don’t arouse suspicion
Unfortunately, many of these criteria conflict. In truth, all invisible inks can be detected by someone who knows how. Therefore, spies were taught to use the weakest possible solution to reduce the risk of exposure. The carrier material also had to look innocent. Warring nations tried to restrict potential carriers. During WWII, the U.S. banned floral gift cards, crossword puzzles, newspaper clippings, sewing patterns, and children’s drawings by mail. Suspicious letters were re-copied by censors onto clean paper.
Traditional Methods
In WWII, the Germans used colorless lead nitrate ink, which could be developed with sodium sulfide. The reaction produces black lead sulfide, and the writing remains permanently visible. Lead nitrate is dangerous and toxic.
Copper sulfate is friendlier, though still toxic like many metal salts. It’s used in vineyards as a fungicide (Bordeaux mixture) and as a preservative in food (E519). A few grams suffice for ink, which can be bought at pharmacies. Dissolve as much of the sky-blue crystals as possible in a thimble of water. Write with a sharpened skewer or quill. Avoid steel nibs—they can corrode. Initially, the ink appears pale green due to copper aquacomplexes. When dry, only white, anhydrous copper sulfate remains. To develop it, warm 10% ammonium hydroxide (ammonia water) slightly. Hold the paper over the vapor—don’t inhale! The writing turns deep blue from copper tetraammine complex formation.
Sodium carbonate or bicarbonate (baking soda) also works as developers, producing greenish-blue copper carbonate and hydroxide (like the patina on statues).
Similarly, light green ferrous sulfate (iron(II) sulfate) can be used. When developed, it produces brownish-red writing.
Sodium bicarbonate (baking soda) in water can also serve as invisible ink, developed with grape juice. Writing with starch solution can be revealed with iodine tincture (blue color). A dilute salt solution can be revealed with silver nitrate (dark brown). SOE agents also used lead nitrate revealed with sodium iodide.
Heat-Reactive Inks
Cobalt(II) chloride solution is a classic heat-developable ink. At room temperature, it’s pink, and invisible on pink paper. It forms a hydrate with water from the solution. When heated (e.g., with an iron), it loses this water and turns blue—revealing the message.
Similar substances are found in the kitchen: apple juice, onion juice, diluted Coca-Cola or honey, sugar solution, white or red wine, and milk all work. Lemon juice is the best. The writing is completely invisible until heated (e.g., ironed), which forms yellow-brown carbon compounds and reveals the text.
UV-Fluorescent Inks
UV-visible markings are used for security on currency and documents. HP even sells UV ink for its printers. Optical brighteners in detergents also work—they absorb 340–370 nm UV light and emit blue-violet (420–470 nm) fluorescence, which makes laundry appear whiter. You could try making UV ink from detergent, but the additives make it tricky. Better to use a standalone optical brightener (e.g., Dr. Beckmann Super Whitener), filtered and diluted in water.
You may also find a great option in your spice rack: turmeric. Its pigment, curcumin (E100), consists of two compounds with rich conjugated double bonds. Upon UV light exposure, electrons absorb energy and fluoresce in orange light when returning to the ground state.
To extract the dye, soak a teaspoon of turmeric in 2–3 teaspoons of 96% ethanol overnight, then filter. The solution makes an effective secret ink. It’s hard to wash off—your fingers may glow after cleaning wiht soap too. To use it in a fountain pen, dilute 3–5 parts of the solution with 95 parts distilled water. Avoid floating particles—they clog pens. Store the ink in a dark place in a brown bottle. Even so, it won’t last more than a few weeks. More durable fluorescent inks, like those used on banknotes, are inorganic and much more expensive.
Vitamin B-12 in vinegar or tonic water also makes UV-visible inks.
Reversible Inks
Phenolphthalein is a common acid-base indicator. Also used as a laxative (though possibly carcinogenic), it’s available in tablet form. At pH<0, it’s orange. Between pH 0–8.2, it’s colorless. From pH 8.2–13, it turns pink, then colorless again above pH 13. This transition is gradual.
Writing with concentrated phenolphthalein produces invisible text. Exposing the paper to ammonium hydroxide vapor reveals red text. After a while, the vapor dissipates and the writing disappears again—this cycle is repeatable.
Other similar dyes, like thymolphthalein, also work. Thymolphthalein is blue above pH 10 and colorless at neutral pH. When the paper dries, atmospheric CO₂ neutralizes the alkali, and the writing vanishes. It can be revived with ammonia vapor, sodium hydroxide, washing soda, or bleach. Over time, these too evaporate or break down, and the text disappears again.
Red cabbage juice is another home indicator. If you write with vinegar, the text is invisible. Brushing over with pale blue cabbage juice turns it red. However, this is not reversible.
The Real Spy Ink
Finally, a recipe courtesy of the CIA: a German WWI spy ink, declassified in 2011. In 1999, when other war records were released, the CIA requested this one remain classified for another 12 years due to “ongoing national security relevance.” The secret? Dissolved aspirin. The developer, however, is more complex—requiring uncommon chemicals like quinine hydrochloride.
https://www.cia.gov/library/readingroom/docs/CIA-RDP11X00001R000100010006-4.pdf
Sources:
David E. Newton: Encyclopedia of Cryptology. 1997. Santa Barbara, CA: Instructional Horizons, Inc.