Editing Invisible ink (section)

==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.<ref name="Dooley">{{cite journal|last1=Dooley|first1=John F.|title=Review of ''Prisoners, Lovers, and Spies: The Story of Invisible Ink from Herodotus to al-Qaeda'' by Kristie Macrakis|journal=Cryptologia|date=25 July 2015|volume=40|issue=1|pages=107–112|doi=10.1080/01611194.2015.1028684|s2cid=825438}}</ref> This was part of his list of the 20 different methods of secret communications<ref>{{Cite book|title=Codes, Ciphers and Spies: Tales of Military Intelligence in World War I|last=Dooley|first=John|publisher=Copernicus Books|year=2016|isbn=9783319294148 |url=https://books.google.com/books?id=lc3tCwAAQBAJ&pg=PA210 |location=New York|pages=210}}</ref> 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.<ref name=":0">{{Cite book|title=Codes: The Guide to Secrecy From Ancient to Modern Times|last=Mollin|first=Richard|publisher=CRC Press|year=2005|isbn=9781584884705|location=Boca Raton, FL|pages=9 |url=https://books.google.com/books?id=xm_13wqVjKAC }}</ref> 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.<ref name=":0" />

[[Philo of Byzantium]] may be the first writer known to describe an invisible ink using a reagent around 217{{endash}}218 BC, with [[oak galls]] and [[vitriol]].<ref name="Macrakis">{{cite book|last1=Macrakis|first1=Kristie|title=Prisoners, lovers, & spies: the story of invisible ink from Herodotus to al-Qaeda|date=2014|publisher=Yale University Press|location=New Haven|isbn=978-0300179255|pages=11–13|url=https://books.google.com/books?id=Yiv4AgAAQBAJ&pg=PA11|access-date=12 January 2017|url-status=live|archive-url=https://web.archive.org/web/20171221020035/https://books.google.com/books?id=Yiv4AgAAQBAJ&pg=PA11|archive-date=21 December 2017}}</ref> These ingredients were used to make [[oak gall ink]].<ref name="Wheeler">{{cite book|last1=Wheeler|first1=Jo|first2= Katy|last2= Temple|title=Renaissance secrets, recipes & formulas|date=2009|publisher=Victoria and Albert Museum|location=London|isbn=9781851775774 |url=https://books.google.com/books?id=Dfw3AQAAIAAJ |page=98 }}</ref> People soon discovered that they could write invisibly with one of the ingredients and then cause the writing to appear by adding the other.<ref name="Reisert" /> [[Pliny the Elder]] and the Roman poet [[Ovid]] gave advice on the use of plant juices and milk to write secret messages.<ref>{{cite book|title=Intelligence Digest Supplement|date=1957|publisher=University of Michigan|page=xxvi}}</ref>

Lemons were also used as organic inks by [[Arabs]] around 600{{nbsp}}AD, and during the 16th century in Europe.<ref name="Reisert">{{cite journal|last1=Reisert|first1=Sarah|title=Under Cover of Ink|journal=Distillations|date=2015|volume=1|issue=4|pages=46–47|url=https://www.sciencehistory.org/distillations/magazine/under-cover-of-ink|access-date=23 March 2018|archive-date=24 March 2018|archive-url=https://web.archive.org/web/20180324102051/https://www.sciencehistory.org/distillations/magazine/under-cover-of-ink|url-status=dead}}</ref>

[[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, {{lang|la|[[Magia Naturalis]]}} (1558, 1589).<ref name="Lukes">{{cite journal|last1=Lukes|first1=Igor|title=Book Reviews Prisoners, Lovers, and Spies: The Story of Invisible Ink from Herodotus to al-Qaeda. By Kristie Macrakis. New Haven, CT: Yale University Press, 2014. Pp. xiv+377. $27.50.|journal=The Journal of Modern History|date=September 2016|volume=88|issue=3|pages=638–640|doi=10.1086/687421}}</ref><ref name="Macrakis" />{{rp|24}} 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.<ref name="Reisert" /><ref name="Macrakis" /> In World War II, neutral or acidic solutions of [[phenolphthalein]], a chemical compound extracted from pills for [[constipation]], were used as invisible ink.<ref name=":1">{{Cite book|title=Chemistry in the Marketplace|last1=Selinger|first1=Ben|last2=Barrow|first2=Russell|publisher=CSIRO Publishing|year=2017|isbn=9781486303328 |url=https://books.google.com/books?id=soYmDwAAQBAJ&pg=PA297 |location=Clayton, Victoria|pages=297}}</ref> It is colorless but turns pink when exposed to alkali such as [[ammonia]] and [[Sodium bicarbonate|bicarbonate soda]].<ref name=":1" />