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What is Invisible Ink?

By D. Poupon
Updated Feb 20, 2024
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Invisible ink is a fluid used to write hidden messages that do not appear unless exposed by a revealing process. Invisible inks can be classified into three main categories: those that are revealed by heat, those revealed by chemical reactions and those that are visible under ultraviolet light. Some common household invisible inks are diluted fruit juices, vinegar and laundry detergent, all which can be applied by a paintbrush, special invisible ink pen or even a toothpick. Historically used in times of war by governments and insurgents alike, invisible ink's peacetime applications include children's invisible ink books and property markers. The chemical processes of invisible inks are well known, so a variety of detection methods exist.

The simplest invisible ink experiment consists of dipping a brush in lemon juice and writing on a piece of blank white paper. When the "ink" dries, it will be invisible to the naked eye, but if the paper is held up to a moderate heat source such as a light bulb, a radiator or an iron, the lemon juice writing will brown before the rest of the paper, exposing the message. Many other mildly acidic liquids also will work, such as apple juice, white wine, cola and even urine.

Spies are more likely to use invisible inks that are developed by a chemical process, because they are more secure and harder to detect. During the Cold war, German secret police used cerium oxalate to write hidden messages and used a solution of manganese sulfate, hydrogen peroxide and other chemicals to reveal the texts. The United States government has kept its formula used during World War I classified. Similar chemical recipes are used for children's entertainment, where hidden texts are printed in an invisible ink book, and an invisible ink pen is used to give clues and to reveal the book's ending. Some well-known chemical ink and developer combinations are vinegar and red cabbage water as well as cornstarch and iodine, both diluted in water.

Invisible inks that can be detected when exposed to ultraviolet or black lights have many modern applications. Using a special marker, invisible ink is applied to money or valuable goods, so that in the case of theft, the article can be identified by pawn shops or police and can be returned to the rightful owner. Many nightclubs use invisible inks to stamp patrons' hands for readmission, and some tattoos use ink that is visible only in black light. Copyrights sometimes are printed on documents with ultraviolet-sensitive ink, preventing photocopies from being mistaken for originals. Desperate prisoners with limited resources might write hidden messages to the outside world using bodily fluids, which contain phosphors that are detectable by ultraviolet lights.

A few reliable detection methods have made invisible inks mostly a part of history. Depending on the ink used, telltale odors and pen scratches might give away the use of invisible inks. When invisible writing that is sensitive to ultraviolet light or heat is suspected, it can be revealed easily. Chemically reactive inks are more difficult to detect, but iodine vapors indicate whether paper fibers have been disturbed by a pen.

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Discussion Comments

By stoneMason — On Apr 23, 2014

Milk can also be used as invisible ink. I tried it the other day. I used a cotton swab dipped in milk and wrote some things on paper. Then I microwaved it for a few minutes and the writing darkened. I wasn't sure if the microwave would be safe for this but it worked. I could have ironed it as well but I felt too lazy to pull out the iron. Surprisingly, the milk turned fairly dark when exposed to heat, more than vinegar! I would have expected vinegar to be darker.

I'm planning to buy some invisible ink (yes, they actually sell it!) which can be seen only under UV light. That should be more fascinating.

By ysmina — On Apr 23, 2014

I don't see how many of these methods would be effective considering that things like lemon juice, vinegar and iodine are all liquids. So even if the writing is not visible to the naked eye, it will be visible that the paper was wet at one point. Paper becomes uneven and creases when it's wet. So anyone who pays attention to detail will know that something was done to the paper. It probably wouldn't take long to find out that something was written on it.

Such methods can be children's entertainment at most.

By SteamLouis — On Apr 22, 2014

As far as I know, the invisible ink recipes used in World War I have been declassified. I remember this because I read an article about it. One of the invisible ink recipes turned out to be lemon juice and there was a big joke about it. Considering that this information was kept classified for a very long time, I would have expected the invisible ink recipe to be more complicated than lemon juice. I guess not.

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