What Turns Litmus Paper Blue

Substance used for writing which is invisible and tin after exist 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 tin afterward be made visible by some ways, such as heat or ultraviolet light. Invisible ink is one form of steganography.

History [edit]

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 blazon of ink to exist used.^[1] This was function of his listing of the xx different methods of secret communications^[2] in a book called On the Defense of Fortifications. 1 of the techniques that involved steganography involved puncturing a tiny pigsty above or below letters in a certificate to spell out a secret bulletin.^[three] This did not include an invisible ink but the Germans improved on the method during World State of war I and Earth War Ii. They used invisible ink and microdots instead of pinpricks.^[3]

Philo of Byzantium may be the first writer known to draw an invisible ink using a reagent around 217–218 BC, with oak galls and vitriol.^[four] These ingredients were used to make oak gall ink.^[5] People soon discovered that they could write invisibly with ane of the ingredients and then cause the writing to appear by adding the other.^[6] Pliny the Elder and the Roman poet Ovid gave advice on the use of plant juices and milk to write hole-and-corner messages.^[7] Lemons were besides used as organic inks by Arabs around 600AD, and during the 16th century in Europe.^{[half dozen]}

Giovanni Battista della Porta is credited with the first recipe for a sympathetic ink, derived from alum and vinegar, too as the beginning book on secret writing and invisible inks, Magia Naturalis(1558, 1589).^[8] ^[iv] ^: 24 Since so, a wide diverseness of invisible inks have been used for all sorts of secretive purposes. A formula like 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, forth 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.^[ix] It is colorless but turns pink when exposed to alkali such every bit ammonia and bicarbonate soda.^[9]

General application and use [edit]

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. Once dry, the written surface looks as if it were bare, with a similar texture and reflectivity as the surrounding surface.

The ink can exist afterward fabricated visible by different methods according to the type of invisible ink used. The ink may be revealed by heat or past application of an appropriate chemical, or it may exist made visible past viewing under ultraviolet light. Inks which are developed past a chemical reaction may depend on an acid-base reaction (like litmus paper), reactions like to the blueprint procedure, or any of hundreds of others. Programmer fluids may be practical using a spray bottle, but some developers are in the form of vapor, e.g. ammonia fumes used to develop phenolphthalein ink.

There are also toy invisible ink pens which accept 2 tips—ane tip for invisible ink writing, and another tip for developing the ink. Invisible ink is sometimes used to impress parts of pictures or text in books for children to play with, always including a "decoder pen" which is used to show the invisible parts of texts or pictures, thus revealing answers to questions printed in regular ink or completing missing parts of pictures.

Security marker pens or UV markers with fluorescent ink that glows when illuminated with a UV light is oftentimes used to invisibly mark valuable household items in case of burglary. There are specialty security maker pens formulated for writing on non-porous surfaces such as glass, plastic, metal, etc. The mark tin can be read by using a blacklight or other UV light source. Security mark pens can be obtained commercially and are widely used equally a crime countermeasure.

Some commercially available invisible inks glow very brightly, in a variety of colors, under ultraviolet lite. This makes them suitable for use in readmission such equally hand stamping.

There are some invisible ink types that can only be invisible when applied to certain types of surfaces, but are still visible on others.

Some vendors now offer invisible ink for use in computer inkjet printers. Such inks are ordinarily visible under ultraviolet light. Typical uses include printing information on business organisation forms for use by the form processor, without cluttering up the visible contents of the form. For example, some Us Post mail sorting stations use UV-visible ink to print bar codes on mailed envelopes giving routing information for utilise by postal service handling equipment further down the line before delivery.

Very rarely, invisible ink has been used in art. It is usually adult, though non always. At that place are artists who use the upshot in conjunction with invisible and other reactive inks and paints to create a multifariousness of effects when used in conjunction with UV lights.

An E2E voting system chosen Scantegrity Two uses invisible ink to enable the voter to obtain a confirmation code only for the voted option.^[10]

Properties of an "ideal" invisible ink [edit]

What an "ideal" invisible ink is depends on its intended utilize. For example, property mark should ideally be done with ink easily read nether ultraviolet light, whereas in espionage such an ink would be considered besides easily detectable since a large number of letters may exist screened relatively quickly using UV light.

Invisible inks are inherently "insecure" against a determined and well-equipped inspector, which must be balanced against the logistical difficulty in conveying 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 post. Apart from in dictatorships where large numbers of personnel are employed to spy on fellow nationals, screening of posted mail service is only viable in item situations, such as letters to and from a detail suspect or facility.

The British SOE training manual used in the Second World War identified the post-obit properties of an "ideal" invisible ink:

Mixes with water.
Non-volatile, i.e. no pronounced smell.
Not depositing crystals on paper, i.e. not easily seen in glancing light.
Invisible nether ultraviolet light.
Does not decompose or discolor the paper eastward.chiliad. silverish nitrate.
Nonreactive with iodine, or with whatsoever of the other usual developers.
Potential developers for the ink should be as few equally possible.
Should non develop nether estrus.
Easily obtainable and has at least one plausible innocent utilize by the holder.
Not a compound of several chemicals, as this would violate No. vii.

From practical experience "half-dozen" and "9" were commonly incompatible. SOE agents were trained not to run a risk their lives through reliance on insecure inks, about of which were from Globe War I. In general, SOE used invisible inks equally a back-up method of communication when other, more secure communication techniques were unavailable. The bureau was known to supply special inks to its field agents, rather than have them depend upon improvisation from obtainable everyday chemicals. When agents were forced to improvise, they were brash to dilute their invisible ink every bit much as possible to reduce chances of detection.^[11]

Screening letters for secret letters [edit]

Any invisible ink can exist made visible by someone who is sufficiently adamant, but the limitation is generally time available and the fact that one cannot apply hours of effort to every single piece of paper. Thus successful use of invisible ink depends on not arousing suspicion that invisible ink may be present.

Telltale signs of invisible ink, such as pen scratches from a sharp pen, roughness, or changed reflectivity of the paper (either more dull or more shiny, normally from using undiluted ink), can be obvious to a careful observer who simply makes use of strong light, a magnifying drinking glass and his or her nose. Also, fundamental words in the visible letter, such as "oestrus" or any other odd lawmaking name, in an out of place context may alert a censor to the presence of invisible ink. Invisible ink is not effective with glossy or very smooth paper types, since the sizing of these papers prevents ink from being absorbed deep into the newspaper and it is easily visible, especially if the paper is examined nether glancing lite. At that place are, yet, commercially available inks for non-porous surfaces that are only visible under ultraviolet light and are otherwise virtually invisible on such surfaces.

Using either ultraviolet light or an iodine smoke cupboard, messages can be quickly screened for invisible ink and also read without outset permanently developing the invisible ink. Thus, if a censor uses this method to intercept messages, the alphabetic character may and so be sent to the intended recipient, who will be unaware that the secret message has already been intercepted by a tertiary party.

A "screening station" theoretically could involve visual and olfactory inspection, an test under ultraviolet lite and so the heating of all objects in an oven before finally trying exposure to iodine fumes to produce optimal security in optimal time.

Invisible ink types [edit]

For practical reasons, the inks are listed here according to their method of development. It must exist understood nonetheless that some inks - specially those of organic origin or those consisting of a mixture of several chemicals - may exist made visible past several methods. For example, invisible writing with soap h2o may be made visible either past heat, reaction with phenolphthalein, viewing nether ultraviolet lite, or by placing the page within an iodine smoke closet.

Inks adult by heat [edit]

Some of these are organic substances that oxidize when heated, which usually turns them brownish. For this blazon of "heat fixed" ink, whatever acidic fluid volition piece of work. The almost secure style to utilise any of the following substances for invisible ink is past dilution, usually with water, close to the point when they become hard to develop.

Cola drink
Honey solution, saccharide (carbohydrate turns into caramel by dehydration)
Lemon, apple, orangish or onion juice (organic acids and the paper forms ester under heat)
Milk (lactose dehydrates)
Bodily fluids such as claret serum.
Soap solution (carboxylic partially oxidizes)
Wine, or vinegar
Cobalt chloride, which turns bluish when heated and becomes invisible over again after a while (if not overly heated)

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. A 100-watt calorie-free bulb is less likely to harm the paper.

Inks developed by chemical reaction [edit]

In near cases, these substance changes colour when mixed with an acid or base.

Phenolphthalein, commonly used as a pH indicator, turns pink in the presence of a base such as ammonia fumes or sodium carbonate.
Vinegar, is revealed by ruby-red cabbage water. Vinegar contains acerb acrid that affects the pH indicator in cherry cabbage h2o. Vinegar may also be adult past heat, as higher up.
Ammonia, developed by red cabbage water.
Copper sulfate, developed by sodium iodide, sodium carbonate, ammonium hydroxide or potassium ferricyanide.
Lead(II) nitrate, adult past sodium iodide.
Iron(Ii) sulfate, developed by sodium carbonate or potassium ferricyanate.
Cobalt(2) chloride, developed by potassium ferricyanide.
Fe(III) sulfate, adult past sodium sulfide.
Starch, developed by iodine solution which turns starch dark blueish and the paper light blue.
Lemon juice, developed by iodine solution (ink turns white, paper turns light blueish).
Sodium chloride (common table salt), adult by silver nitrate.
Cerium oxalate developed by manganese sulfate and hydrogen peroxide^[12] ^[13]

Inks visible under ultraviolet light [edit]

A US$20 bill showing a visible stripe nether an ultraviolet low-cal

Some inks glow faintly (fluoresce) when under an ultraviolet lamp. This is a holding of many substances, particularly organic substances and body fluids.

Other inks piece of work in a nigh opposite way by absorbing ultraviolet light but without fluorescing. When these are used on fluorescent newspaper, the inked areas fluoresce less than the surrounding paper area when under an ultraviolet lamp. This is peculiarly a belongings of inks with a yellow tint.

Some UV-visible inks may be detected on a photocopy, due to the relatively stiff ultraviolet component in light from the photocopier scanning head.

Examples of inks revealed by ultraviolet low-cal are:

Laundry detergents containing optical brighteners
Soap
Body fluids, serum, saliva
Sunscreen
Lemon juice

Inks which alter the surface of paper [edit]

This includes virtually all invisible inks, but pure distilled water tin can also exist used in this way. Awarding of whatsoever fluid will alter the newspaper surface fibers or sizing.

Fumes created from heating iodine crystals volition develop the writing, which will appear brown because the iodine sticks preferentially to the contradistinct areas of the newspaper. Exposing the paper to stiff sunlight will return the writing to its invisible state, equally will using a bleach solution.

Slightly dampening paper with a sponge or by steam and then drying it before writing a message will forestall writing from being developed by this method, only overdoing dampening volition result in telltale paper cockling.

Disappearing inks [edit]

Inks that are visible for a catamenia of time without the intention of being made visible once again are called disappearing inks. Disappearing inks typically rely on the chemical reaction between thymolphthalein and a bones substance such as sodium hydroxide. Thymolphthalein, which is usually colorless, turns blue in solution with the base. As the base reacts with carbon dioxide (always nowadays in the air), the pH drops below ten.v and the colour disappears. Pens are now also available that tin be erased just by swiping a special pen over the original text. Disappearing inks have been used in gag squirtguns, for express-time secret messages, for security reasons on non-reusable passes, for fraudulent purposes, and for dress-making and other crafts where measurement marks are required to disappear.^[14] ^[15] ^[16]

Modernistic usage [edit]

As an indication of security, most invisible inks were already known past the end of World War I. Nonetheless, in 1999, the U.S. Fundamental Intelligence Agency successfully requested that Earth War I invisible ink remain exempt from mandatory declassification, based on the premise that invisible ink was notwithstanding relevant to national security.^[17] The World War I documents remained classified until 2011.^[eighteen] ^[19]

Former MI-half dozen agent Richard Tomlinson stated that Pentel Rolling Writer rollerball pens were extensively used by MI-half-dozen agents to produce secret writing in the form of invisible messages while on missions.^[xx]

In 2002, a gang was indicted for spreading a riot between federal penitentiaries using coded telephone messages, and messages in invisible ink.^[21]

References [edit]

Notes

^ Dooley, John F. (25 July 2015). "Review of Prisoners, Lovers, and Spies: The Story of Invisible Ink from Herodotus to al-Qaeda by Kristie Macrakis". Cryptologia. forty (1): 107–112. doi:10.1080/01611194.2015.1028684. S2CID 825438.
^ Dooley, John (2016). Codes, Ciphers and Spies: Tales of Military machine Intelligence in Earth State of war I. New York: Copernicus Books. p. 210. ISBN9783319294148.
^ ^a ^b Mollin, Richard (2005). Codes: The Guide to Secrecy From Ancient to Modern Times. Boca Raton, FL: CRC Press. p. ix. ISBN9781584884705.
^ ^a ^b ^c Macrakis, Kristie (2014). Prisoners, lovers, & spies : the story of invisible ink from Herodotus to al-Qaeda. New Haven: Yale University Printing. pp. eleven–13. ISBN978-0300179255. Archived from the original on 21 December 2017. Retrieved 12 January 2017.
^ Wheeler, Jo; Temple, Katy (2009). Renaissance secrets, recipes & formulas. London: Victoria and Albert Museum. p. 98. ISBN9781851775774.
^ ^a ^b ^c Reisert, Sarah (2015). "Under Cover of Ink". Distillations. i (four): 46–47. Retrieved 23 March 2018.
^ Intelligence Digest Supplement. Academy of Michigan. 1957. p. xxvi.
^ Lukes, Igor (September 2016). "Volume Reviews Prisoners, Lovers, and Spies: The Story of Invisible Ink from Herodotus to al-Qaeda. By Kristie Macrakis. New Haven, CT: Yale Academy Press, 2014. Pp. 14+377. $27.l". The Journal of Mod History. 88 (3): 638–640. doi:10.1086/687421.
^ ^a ^b Selinger, Ben; Barrow, Russell (2017). Chemistry in the Marketplace. Clayton, Victoria: CSIRO Publishing. p. 297. ISBN9781486303328.
^ Chaum, David; Richard Carback; Jeremy Clark; Aleksander Essex; Stefan Popoveniuc; Ronald 50. Rivest; Peter Y. A. Ryan; Emily Shen; Alan T. Sherman (2008). "Scantegrity II: Cease-to-End Verifiability for Optical Scan Election Systems using Invisible Ink Confirmation Codes". Proceedings of USENIX/Authentic EVT. Archived from the original on 2008-12-03.
^ Rigden, Denis (2001). SOE Syllabus: Lessons in Ungentlemanly Warfare, World State of war 2. ISBN9781903365182.
^ "Cold War Invisible Ink Secrets Unlocked". ScienceDaily. Archived from the original on 2014-03-20. Retrieved 2014-06-10 .
^ MacRakis, Kristie; Bell, Elizabeth Thousand.; Perry, Dale 50.; Sweeder, Ryan D. (2012). "Invisible Ink Revealed: Concept, Context, and Chemical Principles of "Cold War" Writing". Journal of Chemical Education. 89 (4): 529–532. Bibcode:2012JChEd..89..529M. doi:10.1021/ed2003252.
^ "Bad Sign". CFO Magazine. April 1, 2002.
^ Exhibit A: Schemes and Dreams Epitomize Archived 2009-01-15 at the Wayback Machine Television receiver.com
^ Dritz Quilting Fine Point Disappearing Ink Marking Pen Archived 2016-12-27 at the Wayback Machine
^ The formula for invisible ink will remain classified Archived 2015-12-22 at the Wayback Machine St. Petersburg Times, June 23, 1999
^ Miller, Nib (2001-06-xiii). "The Very Visible Battle Over Invisible Ink". Los Angeles Times. Archived from the original on 2010-07-06. Retrieved 2010-05-02 .
^ Ure, Laurie (2011-04-19). "Spy agency reveals invisible ink formula". CNN. Archived from the original on 2014-01-10. Retrieved 2011-04-19 .
^ Tomlinson, Richard: The Big Breach: From Height Hole-and-corner to Maximum Security, pg 44. Mainstream Publishing 2001 ISBN 1-903813-01-8
^ "ARYAN Prison house GANG LINKS WITH MAFIA Drugs, money & the Gambinos". Daily News. New York. 2002-11-03. ^{[ permanent dead link ]}

Further reading

Kristie Macrakis (2015). Prisoners, Lovers, and Spies: The Story of Invisible Ink from Herodotus to al-Qaeda. Yale University Press. ISBN978-0300212600.
John A. Nagy (2009). Invisible Ink: Spycraft of the American Revolution. Westholme Publishing. ISBN978-1594161414.