Unlocking the Secrets of Invisible Ink

Have You Ever Wondered …

How does invisible ink work?

What common household substances can be used to make invisible ink?

What things can you do to reveal a message written in invisible ink?

Steganography is the practice of concealing a file, message, image, or video within another. The use of invisible inks is one of the earliest known examples of steganography. Invisible ink today is mostly considered child’s play, but in the not too distant past, its use and the recipes were considered classified government information.

Using the suggested inks and reagents provided here, write a secret message to a friend. Then get creative and see how many kinds of invisible ink you can find.

Types of Invisible Inks

There are two categories into which invisible inks fall ~ organic fluids and sympathetic inks. You can find many heat-activated invisible inks right inside your kitchen. Another type of invisible ink is chemically activated. Read on to learn more about each.

Organic or Heat-Activated Invisible Inks

Organic fluids consist of the natural methods your likely already familiar: lemon juice, vinegar, milk, or onion juice, to name a few. These organic invisible inks can be revealed through heat, such as with fire, irons, or light bulbs.

The organic fluids alter the fibers of the paper so that the secret writing has a lower burn temperature and turns brown faster than the surrounding paper when exposed to heat. To activate or develop the ink, simply iron the paper, set it on a radiator, place it in an oven (set lower than 450° F), or hold it up to a hot light bulb.

• any acidic fruit juice (e.g., lemon, apple, or orange juice)
• onion juice
• sodium bicarbonate NaHCO₃ solution (baking soda)
• vinegar
• white wine
• diluted cola
• milk
• soapy water
• sucrose solution (table sugar)
• bodily fluids

Sympathetic Inks

Sympathetic inks contain one or more chemicals and require the application of a specific “reagent” to be activated, such as another chemical or a mixture of chemicals. Most of these inks work using pH indicators, requiring the recipient to paint or spray a suspected message with a base (like sodium carbonate Na₂CO₃ or washing soda solution) or an acid (like lemon juice). Some of these inks will reveal their message when heated.

• lemon juice, activated by iodine solution
• starch (e.g., corn starch or potato starch), activated by iodine solution
• vinegar or dilute acetic acid CH₃COOH, activated by red cabbage water
• ammonia NH₃, activated by red cabbage water
• sodium bicarbonate NaHCO3 (baking soda), activated by grape juice
• sodium chloride NaCl (table salt), activated by silver nitrate
• phenolphthalein (pH indicator), activated by ammonia fumes or sodium carbonate Na₂CO₃ (or another base)
• lead nitrate, activated by sodium iodide
• iron sulfate, activated by sodium carbonate, sodium sulfide, or potassium ferricyanide

CAUTION: Some of the chemicals suggested here can be hazardous if misused. Always use caution when working with chemicals. Read the information on the chemical label before you start, and always wear protective safety equipment such as goggles, gloves, and aprons. Adult supervision required.

Ultraviolet Light Activated Invisible Inks

Most of the inks that become visible when you shine an ultraviolet or black light on them will also become visible if you heat the paper. Here are are few ‘glow-in-the-dark’ ideas to try:

• dilute laundry detergent (the bluing agent glows)
• tonic water (quinine glows)
• vitamin B-12 dissolved in vinegar

The History of Invisible Ink

The history of invisible ink is incredibly fascinating and swings wildly between high-tech methods and the humblest of approaches. Invisible ink was a key method for spy communications throughout history. Prisoners, Lovers, and Spies is an historical account of invisible ink and the secret communications revealed through thrilling stories about scoundrels, heroes, and their ingenious methods for concealing messages.

The Catholic Mary, Queen of Scots, kept under luxurious house arrest for eighteen years by her Protestant cousin Elizabeth I, advised correspondents to write to her employing two commonly used substances: alum (hydrated potassium aluminum sulfate) or nutgall (the tannic acid secreted in swellings generated by parasitic wasps colonizing oak trees). Letters written in alum required the recipient to soak the paper in water, while nutgall needed a solution of ferrous sulphate as a reagent.

During World War II, chemist Linus Pauling worked on an unusual wartime project, formulating new kinds of invisible ink that would resist all known reagents. Pauling and his colleagues experimented with invisible inks made from pneumococcus bacteria (an inert strain so as not to spread pneumonia). The ink-ified microbe would react to an antibody, and then become visible once dipped in a dye solution. However, the ink never passed the experimental stage.

Visit The Art of Manliness for a more detailed look at how invisible inks have been used in espionage and naval intelligence.