Sympathetic ink

Invisible ink, also known as security 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. Invisible ink is one form of steganography.

One of the earliest writers to mention an invisible ink is Aeneas Tacticus, in 4th century BCE. He mentions it in discussing how to survive under siege, but does not indicate the type of ink to be used.Philo of Byzantium may be the first writer known to describe an invisible ink using a reagent around 280-220 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 CE, 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 John 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 during World War I.

Invisible ink can be applied to a writing surface with a specialty purpose stylus, stamp, fountain pen, toothpick, calligraphy pen, 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.

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