The ink can be later made visible by different methods according to the type of invisible ink used. Once dry, the written surface looks as if it were blank, with a similar texture and reflectivity as the surrounding surface. 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. It is colorless but turns pink when exposed to alkali such as ammonia and bicarbonate soda. In World War II, neutral or acidic solutions of phenolphthalein, a chemical compound extracted from pills for constipation, were used as invisible ink. 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 4 other Germans, who all died for their efforts either by suicide or execution, along with John Hahn, an English baker) during World War I. : 24 Since then, a wide variety of invisible inks have been used for all sorts of secretive purposes. 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).
Lemons were also used as organic inks by Arabs around 600 AD, and during the 16th century in Europe. Pliny the Elder and the Roman poet Ovid gave advice on the use of plant juices and milk to write secret messages. People soon discovered that they could write invisibly with one of the ingredients and then cause the writing to appear by adding the other. These ingredients were used to make oak gall ink. 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. They used invisible ink and microdots instead of pinpricks. This did not include an invisible ink but the Germans improved on the method during World War I and World War II. 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 was part of his list of the 20 different methods of secret communications in a book called On the Defense of Fortifications.
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He mentions it in discussing how to survive under siege but does not indicate the type of ink to be used. One of the earliest writers to mention an invisible ink is Aeneas Tacticus, in the 4th century BC.
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