Invisible ink, also known as security ink or sympathetic ink, is a substance used for writing, which is invisible either on application or soon thereafter, and can later be made visible by some means, such as heat or ultraviolet light. Invisible ink is one form of steganography.
History One of the earliest writers to mention an invisible ink is Aeneas Tacticus, in the 4th century BC. He mentions it in discussing how to survive under siege but does not indicate the type of ink to be used. This was part of his list of the 20 different methods of secret communications in a book called On the Defense of Fortifications. One of the techniques that involved steganography involved puncturing a tiny hole above or below letters in a document to spell out a secret message. This did not include an invisible ink but the Germans improved on the method during World War I and World War II. They used invisible ink and microdots instead of pinpricks. Philo of Byzantium may be the first writer known to describe an invisible ink using a reagent around 217–218 BC, with oak galls and vitriol. These ingredients were used to make oak gall ink. People soon discovered that they could write invisibly with one of the ingredients and then cause the writing to appear by adding the other. Pliny the Elder and the Roman poet Ovid gave advice on the use of plant juices and milk to write secret messages. Lemons were also used as organic inks by Arabs around 600 AD, and during the 16th century in Europe. Giovanni Battista della Porta is credited with the first recipe for a sympathetic ink, derived from alum and vinegar, as well as the first book on secret writing and invisible inks, Magia Naturalis (1558, 1589). Since then, a wide variety of invisible inks have been used for all sorts of secretive purposes. A formula similar to oak gall ink was created by James Jay and used by George Washington and the Culper Spy Ring during the American Revolution and lemon juice was used by the 'Lemon Juice Spies' (Carl Muller and four other Germans, who all died for their efforts either by suicide or execution, along with John Hahn, an English baker) during World War I. In World War II, neutral or acidic solutions of phenolphthalein, a chemical compound extracted from pills for constipation, were used as invisible ink. It is colorless but turns pink when exposed to alkali such as ammonia and bicarbonate soda.
General application and use Invisible ink can be applied to a writing surface with a specialty purpose stylus, stamp, fountain pen, toothpick, calligraphy pen, Cotton swab, or even a finger dipped in the liquid. Once dry, the written surface looks as if it were blank, with a similar texture and reflectivity as the surrounding surface. The ink can be later made visible by different methods according to the type of invisible ink used. The ink may be revealed by heat or by application of an appropriate chemical, or it may be made visible by viewing under ultraviolet light. Inks which are developed by 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 vapor, e.g. ammonia fumes used to develop phenolphthalein ink. There are also toy invisible ink pens which have two tips—one tip for invisible ink writing, and another tip for developing the ink. Invisible ink is sometimes used to print parts of pictures or text in books for children to play with, always including a "decoder pen" which is used to show the invisible parts of texts or pictures, thus revealing answers to questions printed in regular ink or completing missing parts of pictures.
Security marker pens or UV markers with fluorescent ink that glows when illuminated with a UV light is often used to invisibly mark valuable household items in case of burglary. There are specialty security maker pens formulated for writing on non-porous surfaces such as glass, plastic, metal, etc. The mark can be read by using a blacklight or other UV light source. Security marker pens can be obtained commercially and are widely used as a crime countermeasure. Some commercially available invisible inks glow very brightly, in a variety of colors, under ultraviolet light. This makes them suitable for use in readmission such as hand stamping. There are some invisible ink types that can only be invisible when applied to certain types of surfaces, but are still visible on others. 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. An E2E voting system called Scantegrity II uses invisible ink to enable the voter to obtain a confirmation code only for the voted selection.
Properties of an "ideal" invisible ink What an "ideal" invisible ink is depends on its intended use. For example, property marking should ideally be done with ink easily read under ultraviolet light, whereas in espionage such an ink would be considered too easily detectable since a large number of letters may be screened relatively quickly using UV light. Invisible inks are inherently "insecure" against a determined and well-equipped inspector, which must be balanced against the logistical difficulty in carrying out mass-screening of posted mail. It is easier to perform large-scale undetected screening of millions of electronic communications, than to mass-screen even a small fraction of conventional mail. Apart from in dictatorships where large numbers of personnel are employed to spy on fellow nationals, screening of posted mail is only feasible in particular situations, such as letters to and from a particular suspect or facility. The British SOE training manual used in the Second World War identified the following properties of an "ideal" invisible ink:
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