Invisible ink
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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.
Invisible ink is applied to a writing surface with a fountain pen, toothpick or even a finger dipped in the liquid. Once dry, the paper should appear blank or the ink is not an invisible ink. The ink is later developed (made visible) by different methods according to the type of invisible ink used. This can be by heat, by viewing under ultraviolet light, or by applying a chemical appropriate for the ink used.
Invisible inks which depend on a chemical reaction, generally depend on an acid-base reaction (like litmus paper) similar to the blueprint process. 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.
A cover message should be written over the invisible message, since a blank sheet of paper might arouse suspicion. 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.
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.
Very rarely, invisible ink has been used in art. It is usually developed, though when it is not, it makes a mockery of the concept of "visual art".
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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.
- Milk
- Lemon, apple or orange juice
- Onion juice
- Sugar solution
- Diluted honey
- Diluted cola drink
- Vinegar or wine
- Saliva, urine, semen or blood serum.
- Soap water
The writing is made visible by heating the paper, either on a radiator, by ironing it, or by placing it in an oven. A 100W 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.
- Phenolphthalein, developed by ammonia fumes or sodium carbonate.
- Phenolphthalein is a pH indicator that turns pink in the presence of a base such as ammonia.
- Vinegar, revealed by red cabbage water.
- Vinegar contains acetic acid that affects the pH indicator in red cabbage water.
- Ammonia, developed by red cabbage water.
- Copper sulfate, developed by sodium iodide.
- Copper sulfate, developed by sodium carbonate.
- Copper sulfate, developed by ammonium hydroxide.
- Lead nitrate, developed by sodium iodide.
- Iron sulfate, developed by sodium carbonate.
- Iron sulfate, developed by potassium ferricyanate.
- Cobalt chloride, developed by potassium ferricyanide.
- Copper sulfate, developed by potassium ferricyanide.
- Iron sulfate, developed by sodium sulfide.
- Starch, developed by iodine solution (ink turns dark blue, paper turns light blue).
- Lemon Juice, developed by iodine solution (ink turns white, paper turns light blue).
- Sodium Chloride (table salt), developed by silver nitrate.
Inks visible under ultraviolet light
Some inks glow faintly (fluoresce) when under an ultraviolet lamp. This is a property of many substances, including many of the invisible inks above.
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.
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.
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 his 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.
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 after 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 insecure. World War II SOE agents were trained not to risk their lives through reliance on insecure inks. The SOE training manual taught the following criteria for identifying an "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. As an indication of security, most of the inks mentioned here were already known by the end of World War I.
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.
Further reading
SOE Syllabus: Lessons in Ungentlemanly Warfare, World War II (Surrey: Public Record Office, 2001) Most of the material in this article is found in the chapter "Invisible Inks".de:Geheimtinte