Invisible ink

Invisible ink

Invisible ink is a substance used for writing, which is either invisible on application, or soon thereafter, and which later on can be made visible by some means. The use of invisible ink is a form of steganography, and has been used in espionage. Other uses may include anticounterfeiting, property marking, hand stamping for readmission, children's games, and marking for the purpose of identification in manufacturing.

Application and use

Invisible ink is applied to a writing surface with a specialty purpose stylus, stamp, fountain pen, toothpick or even a finger dipped in the liquid. Once dry, the surface should appear blank and of similar texture as surrounding material.

A cover message should be written over the invisible message, since a blank sheet of paper might arouse suspicion that an invisible message is present. This is best done with a ballpoint pen, since fountain pen writing may 'run' when it crosses a line of invisible ink, thus giving a clue that invisible ink is present. Similarly, invisible ink should not be used on ruled paper, since it may alter or streak the colour of the lines.

The ink is later made visible by different methods according to the type of invisible ink used. This may be by heat, by application of a chemical appropriate to the ink used, or without development by viewing under ultraviolet light. The invisible inks which depend on a chemical reaction may depend on an acid-base reaction (like litmus paper), reactions similar to the blueprint process, or any of hundreds of others. Developer fluids may be applied using a spray bottle, but some developers are in the form of vapours, e.g. ammonia fumes for developing phenolphthalein ink.

One can obtain toy invisible ink pens which have two tips - one tip for invisible ink writing, and another tip for developing the ink. Also, invisible ink is sometimes used to print parts of pictures or text in books for children to play with, particularly while they are travelling. A "decoder pen" is included with these books and children may rub this pen over invisible parts of texts or pictures, thus revealing answers to questions printed in regular ink or completing missing parts of pictures.

Other ink pens can be obtained commercially that fluoresce when illuminated with a UV light. These inks are simply applied and then identified using a black light or other UV light source. These inks are invisible to the naked eye and are only revealed when illuminated. They are widely used for property marking as a crime countermeasure.

There is a commercially available red invisible ink which is only invisible when applied to certain types of surfaces, but visible on others.

Former MI-6 agent Richard Tomlinson alleges that Pentel Rolling Writer rollerball pens were extensively used by SIS agents to produce secret writing (invisible messages) while on missions.

Some vendors now offer invisible ink for use in computer inkjet printers. Such inks are usually visible under ultraviolet light. Typical uses include printing information on business forms for use by the form processor, without cluttering up the visible contents of the form. For example, some United States Postal Service mail sorting stations use UV-visible ink to print bar codes on mailed envelopes giving routing information for use by mail handling equipment further down the line before delivery.

Very rarely, invisible ink has been used in art. It is usually developed, though not always. There are artists who use the effect in conjunction with invisible and other reactive inks and paints to create a variety of effects when used in conjunction with UV lights.

An E2E voting system called Scantegrity II uses invisible ink to enable the voter to obtain a confirmation code only for the voted selection.

Invisible ink types

Inks developed by heat

Some of these are organic substances that oxidize when heated, which usually turns them brown. For this type of 'heat fixed' ink, any acidic fluid will work. As a rule of thumb, the most secure way to use any particular ink is by diluting it - usually with water - near to the point when it begins to get difficult to develop.

The writing is made visible by heating the paper, either on a radiator, by ironing it, or by placing it in an oven. A 100 W light bulb is less likely to damage the paper.

Inks developed by chemical reaction

In most cases, one substance changes color when mixed with an acid or base.

Inks visible under ultraviolet light

Some inks glow faintly (fluoresce) when under an ultraviolet lamp. This is a property of many substances. There are commercially available inks that glow very brightly when illuminated using a black light or UV light. Invisible inks with fluorescent properties can be obtained in a variety of colors and even have formulations for non-porous surfaces so they can be used on glass, plastics, etc..

Other inks work in a near opposite way by absorbing ultraviolet light. When they are used on fluorescent paper, the written-on areas fluoresce less than the surrounding paper area when under an ultraviolet lamp. This is especially a property of inks with a yellow tint.

Security marker pens with fluorescent ink may also be used to invisibly mark valuable household items in case of burglary. The owner of a recovered, stolen item which has been marked in this way can be traced simply by using an ultraviolet lamp. Items can also be marked for a variety of property marking purposes and identification. They can also be used in readmissions such as hand stamping.

Some UV-revealed writing will be readable on a photocopy, due to the relatively strong ultraviolet component in light from the photocopier scanning head.

Inks which disturb the surface of paper

This includes virtually all invisible inks, but pure distilled water can also be used in this way. Application of any fluid will disturb the paper surface fibers or sizing.

Fumes created from heating iodine crystals will develop the writing, which will appear brown because the iodine sticks preferentially to the disturbed areas of the paper. Exposing the paper to strong sunlight will return the writing to its invisible state, as will using a bleach solution.

Slightly dampening paper with a sponge or by steam and then drying it before writing a message, will prevent writing from being developed by this method. But overdoing dampening will result in telltale paper cockling.

Disappearing inks

Inks that are visible for a period of time without the intention of being made visible again are called disappearing inks. Disappearing inks typically rely on the chemical reaction between thymolphthalein and a basic substance such as sodium hydroxide. Thymolphthalein, which is normally colorless, turns blue in solution with the base. As the base reacts with carbon dioxide (always present in the air), the pH drops below 10,5 and the color disappears. Pens are now also available that can be erased simply by swiping a special pen over the original text. Disappearing inks have been used in gag squirtguns, for time-limited secret messages, for security reasons on non-reusable passes, and for fraudulent purposes.

Interception of secret messages

Any invisible ink can be made visible by someone who is sufficiently determined, but the limitation is generally time available and the fact that one cannot apply hours of effort to every single piece of paper. Successful use of invisible ink depends on not arousing suspicion.

Telltale signs of invisible ink, such as pen scratches from a sharp pen, roughness or changed reflectivity of the paper (either more dull or more shiny, usually from using undiluted ink) can be obvious to a careful observer who simply makes use of strong light, a magnifying glass and their nose. Also, key words in the visible letter, such as 'red cabbage' or 'heat', in an odd context may alert a censor to the use of invisible ink. Invisible ink should not be used with glossy or very smooth paper types, since the sizing of these papers prevents ink from being absorbed deep into the paper and it is easily visible, especially when the paper is examined under glancing light. There are, however, commercially available inks for non-porous surfaces that are only visible under ultraviolet light and are otherwise virtually invisible on these kinds of surfaces.

Using either ultraviolet light or an iodine fume cupboard, messages can be quickly screened for invisible ink and also read without first permanently developing the invisible ink. Thus, if a censor uses this method to intercept messages, he may then let the letter be sent to the intended recipient who will be unaware that the secret message has already been intercepted by a third party.

A "screening station" could theoretically involve visual and olfactory inspection, an examination under ultraviolet light and then the heating of all objects in an oven before finally trying exposure to iodine fumes. In theory, some invisible inks may even show up using a camera sensitive to infrared light.

Properties of an "ideal" invisible ink

Most invisible inks are unsecure. World War II SOE agents were trained not to risk their lives through reliance on insecure inks, most of which inks were of World War I vintage. The SOE training manual identified the following properties of the "ideal" invisible ink:

  1. Very water soluble, i.e. non greasy.
  2. Non-volatile, i.e. no pronounced smell.
  3. Not depositing crystals on paper, i.e. not easily seen in glancing light.
  4. Invisible under ultraviolet light.
  5. Does not decompose or discolour the paper e.g. not silver nitrate.
  6. Unreactive with iodine, or with any of the other usual developers.
  7. Potential developers for the ink should be as few as possible.
  8. Should not develop under heat.
  9. Easily obtainable and has at least one plausible innocent use by the holder.
  10. Not a compound of several chemicals, as this would contradict "7".

In practice, "6" and "9" are usually incompatible. The SOE was known to supply special inks to their field agents, rather than depend on improvisation from obtainable everyday chemicals.

Invisible inks are not inherently "secure", but this has to be balanced against the fact that it is technically difficult to carry out mass screening of posted letters. It is easier to perform large-scale undetected screening of millions of electronic communications than it is to manually inspect even a small fraction of conventional posted letters. Apart from in dictatorships with large numbers of personnel employed to spy on their fellow nationals, inspection of posted mail can only be used in particular situations, for example focusing on the letters of a particular suspect or the screening of letters entering and leaving a particular facility.

CIA resistance to declassification

As an indication of security, most of the inks mentioned above were already known by the end of World War I. The U.S. Central Intelligence Agency requested (successfully) in 1999 that World War I era invisible ink technology remain exempt from mandatory declassification, based on their disputed claim that invisible ink was still relevant to national security.


  • SOE Syllabus: Lessons in Ungentlemanly Warfare, World War II (Surrey: Public Record Office, 2001).

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