Invisible ink
substance used for writing which is invisible and can later be made visible
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.
The public source identifies “Invisible ink” as substance used for writing which is invisible and can later be made visible. This brief keeps that definition visible, then builds a research path around Invisible, substance and used.
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This entry incorporates text from “Invisible ink” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.