Invisible ink
[3] 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.
[6] Pliny the Elder and the Roman poet Ovid gave advice on the use of plant juices and milk to write secret messages.
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.
[6][4] In World War II, neutral or acidic solutions of phenolphthalein, a chemical compound extracted from pills for constipation, were used as invisible ink.
There are specialty security maker pens formulated for writing on non-porous surfaces such as glass, plastic, metal, etc.
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: From practical experience "6" and "9" were usually incompatible.
The agency was known to supply special inks to its field agents, rather than have them depend upon improvisation from obtainable everyday chemicals.
[11] Any invisible ink can be made visible by someone who is sufficiently determined, but the limitation is generally time available and the fact that one cannot apply hours of effort to every single piece of paper.
Also, key words in the visible letter, such as "heat" or any other odd code name, in an out of place context may alert a censor to the presence of invisible ink.
For example, invisible writing with soap water may be made visible either by heat, reaction with phenolphthalein, viewing under ultraviolet light, or by placing the page inside an iodine fume cupboard.
The writing is rendered visible by heating the paper, either on a radiator, by ironing it, using a hair dryer, or by placing it in an oven.
[19] This exemption made the recipes for invisible ink the oldest classified documents held by the National Archives until their declassification in 2011.
In 2012, Sotheby's London discovered Basquiat's signature painted in invisible ink on the 1982 work, Orange Sports Figure.
[21] In 2012, the Hayward Gallery exhibition, Invisible: Art about the Unseen, 1957-2012, included the 1989 work, Magic Ink, by Gianni Motti.
The illustrations, drawn on the walls and floor of the Music Gallery Performance Space, were only visible under UV light.
Disappearing inks typically rely on the chemical reaction between thymolphthalein and a basic substance such as sodium hydroxide.
