Invisible ink is a substance used for writing, which is either invisible on application, or soon thereafter, and which later on can be made visible by some means. The use of invisible ink is a form of steganography, and has been used in espionage. Other uses may include anticounterfeiting, property marking, hand stamping for readmission, children's games, and marking for the purpose of identification in manufacturing. Steganography is the art and science of writing hidden messages in such a way that no one apart from the intended recipient knows of the existence of the message; this is in contrast to cryptography, where the existence of the message itself is not disguised, but the content is obscured. ... Spy and Secret agent redirect here. ...

Application and use

Invisible ink is applied to a writing surface with a fountain pen, toothpick or even a finger dipped in the liquid. Once dry, the surface should appear blank and of similar texture as surrounding material. A fountain pen is a writing instrument, more specifically a pen, that contains a reservoir of water-based ink that is fed to a nib through a feed via a combination of gravity and capillary action. ... Wood toothpicks A Toothpick A toothpick is a piece of plastic or other substance such as wood (in this case it may be known as an interdental woodstick) used to remove detritus from the teeth, usually after a meal. ...

A cover message should be written over the invisible message, since a blank sheet of paper might arouse suspicion that an invisible message is present. This is best done with a ballpoint pen, since fountain pen writing may 'run' when it crosses a line of invisible ink, thus giving a clue that invisible ink is present. Similarly, invisible ink should not be used on ruled paper, since it may alter or streak the colour of the lines.

The ink is later made visible by different methods according to the type of invisible ink used. This may be by heat, by application of a chemical appropriate to the ink used, or without development by viewing under ultraviolet light. The invisible inks which depend on a chemical reaction generally depend on an acid-base reaction (like litmus paper) similar to the blueprint process. Developer fluids may be applied using a spray bottle, but some developers are in the form of vapours, e.g. ammonia fumes for developing phenolphthalein ink. Note: Ultraviolet is also the name of a 1998 UK television miniseries about vampires. ... Vapours of hydrogen chloride in a beaker and ammonia in a test tube meet to form a cloud of a new substance, ammonium chloride A chemical reaction is a process that results in the interconversion of chemical substances. ... Acids and bases: Acid-base reaction pH Self-ionization of water Buffer solutions Systematic naming Acid-base extraction Acidity function Proton affinity Acids: Strong acids Weak acids Superacids Lewis acids Mineral acids Organic acids Bases: Strong bases Weak bases Superbases Lewis bases Organic bases edit is a chemical reaction that... The term litmus test can be literal or metaphorical. ... Modern blueprint of the French galleon La Belle. ... Ammonia is a compound with the formula NH3. ... Phenolphthalein is a sensitive chemical with the formula C20H14O4 (often written as HIn in shorthand notation). ...

One can obtain toy invisible ink pens which have two tips - one tip for invisible ink writing, and another tip for developing the ink. Also, invisible ink is sometimes used to print parts of pictures or text in books for children to play with, particularly while they are travelling. A "decoder pen" is included with these books and children may rub this pen over invisible parts of texts or pictures, thus revealing answers to questions printed in regular ink or completing missing parts of pictures. A chained book in the Bodleian Library at Oxford University A book is a set or collection of written, printed, illustrated, or blank sheets, made of paper, parchment, or other material, usually fastened together to hinge at one side, and within protective covers. ... The decoder pen, yes-no pen, yes-know pen or magic pen book is a combination of decoder pen or marker specially designed to reveal invisible ink-encoded pictures or writing, in the form of answers to questions or hidden parts of pictures, with specially created childrens books with...

Other ink pens can be obtained commercially that fluoresce when illuminated with a UV light. These inks are simply applied and then identified using a black light or other UV light source. These inks are invisible to the naked eye and are only revealed when illuminated. They are widely used for property marking as a crime countermeasure.

There is a commercially available red invisible ink which is only invisible when applied to certain types of surfaces, but visible on others.

Some vendors now offer invisible ink for use in computer inkjet printers. [1] Such inks are usually visible under ultraviolet light. Typical uses include printing information on business forms for use by the form processor, without cluttering up the visible contents of the form. For example, some United States Postal Service mail sorting stations use UV-visible ink to print bar codes on mailed envelopes giving routing information for use by mail handling equipment further down the line before delivery. Inkjet printers are a type of computer printer that operates by propelling tiny droplets of liquid ink onto paper. ... The United States Postal Service (USPS) is an independent establishment of the executive branch of the United States government (see 39 U.S.C. Â§ 201) responsible for providing postal service in the U.S. Within the United States, it is colloquially referred to simply as the post office. ...

Very rarely, invisible ink has been used in art. It is usually developed, though not always. There are artists who use the effect in conjunction with invisible and other reactive inks and paints to create a variety of effects when used in conjunction with UV lights.

Invisible ink types

Inks developed by heat

Some of these are organic substances that oxidize when heated, which usually turns them brown. For this type of 'heat fixed' ink, any acidic fluid will work. As a rule of thumb, the most secure way to use any particular ink is by diluting it - usually with water - near to the point when it begins to get difficult to develop.

• Milk
• Lemon, apple or orange juice
• Onion juice
• Sugar solution
• Diluted honey
• Diluted cola drink
• Vinegar or wine
• Soap water

The writing is made visible by heating the paper, either on a radiator, by ironing it, or by placing it in an oven. A 100W light bulb is less likely to damage the paper. HVAC may also stand for High-voltage alternating current HVAC is an initialism that stands for heating, ventilation and air-conditioning. This is sometimes referred to as climate control. ... Radiators and convectors are types of heat exchangers designed to transfer thermal energy from one medium to another for the purpose of cooling and heating. ... An iron Ironing or smoothing is the work of using a heated tool to remove wrinkles from washed clothes. ... Oven depicted in a painting by Millet An oven is an enclosed compartment for heating, baking or drying. ... The light bulb is one of the most significant inventions in the history of the human race, illuminating the darkness of the evening and bringing light indoors at all times in order focus on the task at hand. ...

Inks developed by chemical reaction

In most cases, one substance changes color when mixed with an acid or base.

• Phenolphthalein, developed by ammonia fumes or sodium carbonate.
  • Phenolphthalein is a pH indicator that turns pink in the presence of a base such as ammonia.
• Vinegar, revealed by red cabbage water.
  • Vinegar contains acetic acid that affects the pH indicator in red cabbage water.
  • Vinegar may also be developed by being dried and put over a candle or in the oven, at a temperature below 232 degrees Celsius or 451 degrees Fahrenheit, the ignition temperature of paper.
• Ammonia, developed by red cabbage water.
• Copper sulfate, developed by sodium iodide.
• Copper sulfate, developed by sodium carbonate.
• Copper sulfate, developed by ammonium hydroxide.
• Lead(II) nitrate, developed by sodium iodide.
• Iron sulfate, developed by sodium carbonate.
• Iron sulfate, developed by potassium ferricyanate.
• Cobalt chloride, developed by potassium ferricyanide.
• Copper sulfate, developed by potassium ferricyanide.
• Iron sulfate, developed by sodium sulfide.
• Starch, developed by iodine solution (ink turns dark blue, paper turns light blue).
• Lemon juice, developed by iodine solution (ink turns white, paper turns light blue).
• Sodium chloride (table salt), developed by silver nitrate.

Phenolphthalein is a sensitive chemical with the formula C20H14O4 (often written as HIn in shorthand notation). ... Ammonia is a compound with the formula NH3. ... Sodium carbonate (also known as washing soda or soda ash), Na2CO3, is a sodium salt of carbonic acid. ... The correct title of this article is . ... Vinegar is sometimes infused with spices or herbs—as here, with oregano. ... Acetic acid, also known as ethanoic acid, is an organic chemical compound best recognized for giving vinegar its sour taste and pungent smell. ... A blank sheet of paper Paper is a commodity of thin material produced by the amalgamation of fibers, typically vegetable fibers composed of cellulose, which are subsequently held together by hydrogen bonding. ... Ammonia is a compound with the formula NH3. ... Copper (II) sulfate (CuSO4) is the most common copper salt, made by the action of sulfuric acid on the base copper oxide. ... Sodium iodide (NaI) is used in polymerase chain reactions (PCR) Categories: Chemistry stubs ... Copper (II) sulfate (CuSO4) is the most common copper salt, made by the action of sulfuric acid on the base copper oxide. ... Sodium carbonate (also known as washing soda or soda ash), Na2CO3, is a sodium salt of carbonic acid. ... Copper (II) sulfate (CuSO4) is the most common copper salt, made by the action of sulfuric acid on the base copper oxide. ... Main article: ammonia Ammonium hydroxide, although it doesnt exist as an isolatable chemical compound, is a name sometimes given to an aqueous solution of ammonia. ... Lead(II) nitrate is a chemical compound, the inorganic salt of nitric acid and lead. ... Sodium iodide (NaI) is used in polymerase chain reactions (PCR) Categories: Chemistry stubs ... Iron(II) sulfate, also known as ferrous sulfate and as copperas (FeSO4) is an example of an ionic compound. ... Sodium carbonate (also known as washing soda or soda ash), Na2CO3, is a sodium salt of carbonic acid. ... Iron(II) sulfate, also known as ferrous sulfate and as copperas (FeSO4) is an example of an ionic compound. ... Cobalt(II) chloride (CoCl2) is a chemical compound composed of cobalt and chlorine. ... Potassium ferricyanide (K3[Fe(CN)6]) also known as red prussiate, Prussian red or potassium hexacyanoferrate(III), is a coordination compound that is stable at room temperature and pressure and forms ruby red crystals and powder. ... Copper (II) sulfate (CuSO4) is the most common copper salt, made by the action of sulfuric acid on the base copper oxide. ... Potassium ferricyanide (K3[Fe(CN)6]) also known as red prussiate, Prussian red or potassium hexacyanoferrate(III), is a coordination compound that is stable at room temperature and pressure and forms ruby red crystals and powder. ... Iron(II) sulfate, also known as ferrous sulfate and as copperas (FeSO4) is an example of an ionic compound. ... Sodium sulfide, or Na2S, is a water soluble chemical compound. ... Starch (CAS# 9005-25-8) is a complex carbohydrate which is soluble in water; it is used by plants as a way to store excess glucose. ... General Name, Symbol, Number iodine, I, 53 Chemical series halogens Group, Period, Block 17, 5, p Appearance violet-dark gray, lustrous Standard atomic weight 126. ... Binomial name Citrus × limon (L.) Burm. ... General Name, Symbol, Number iodine, I, 53 Chemical series halogens Group, Period, Block 17, 5, p Appearance violet-dark gray, lustrous Standard atomic weight 126. ... Jordanian and Israeli salt evaporation ponds at the south end of the Dead Sea Sodium chloride, also known as common salt, table salt, or halite, is a chemical compound with the formula NaCl. ... R-phrases , S-phrases , , , , Flash point non-flammable Supplementary data page Structure and properties n, εr, etc. ...

Inks visible under ultraviolet light

Some inks glow faintly (fluoresce) when under an ultraviolet lamp. This is a property of many substances. There are commercially available inks that glow very brightly when illuminated using a black light or UV light. Invisible inks with fluorescent properties can be obtained in a variety of colors and even have formulations for non-porous surfaces so they can be used on glass, plastics, etc.. Fluorescence induced by exposure to ultraviolet light in vials containing various sized Cadmium selenide (CdSe) quantum dots. ... UV redirects here. ...

• laundry detergents that contain optical brighteners.

Other inks work in a near opposite way by absorbing ultraviolet light. When they are used on fluorescent paper, the written-on areas fluoresce less than the surrounding paper area when under an ultraviolet lamp. This is especially a property of inks with a yellow tint. This article or section does not adequately cite its references or sources. ... Optical brighteners, optical brightening agents, fluorescent brightening agents or fluorescent whitening agents are dyes that absorb light in the ultraviolet and violet region of the electromagnetic spectrum, and re-emit light in the blue region. ... Fluorescence induced by exposure to ultraviolet light in vials containing various sized Cadmium selenide (CdSe) quantum dots. ...

Security marker pens with fluorescent ink may also be used to invisibly mark valuable household items in case of burglary. The owner of a recovered, stolen item which has been marked in this way can be traced simply by using an ultraviolet lamp. Items can also be marked for a variety of property marking purposes and identification. They can also be used in readmissions such as hand stamping. For other uses, see Security (disambiguation). ... A marker pen, or marker, is a term used to refer to various kinds of pen which have their own ink-source and usually a tip made of some porous material. ...

Some UV-revealed writing will be readable on a photocopy, due to the relatively strong ultraviolet component in light from the photocopier scanning head.

Inks which disturb the surface of paper

This includes virtually all invisible inks, but pure distilled water can also be used in this way. Application of any fluid will disturb the paper surface fibers or sizing.

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

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

Disappearing inks

Inks that are visible for a period of time without the intention of being made visible again are called disappearing inks. Disappearing inks typically rely on the chemical reaction between thymophthalein and a basic substance such as alcohol or ethanol. Thymophthalein, which is normally colorless, turns blue in solution with the base. When the base evaporates, the chemical reaction stops and the color disappears.[2] Through modifications to the chemical solution, the rate of evaporation can be controlled. Pens are now also available that can be erased simply by swiping a special pen over the original text. Disappearing inks have been been used in gag squirtguns, for time-limited secret messages[3], and for fraudulent purposes.[4][5]

Interception of secret messages

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. Successful use of invisible ink depends on not arousing suspicion.

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, usually from using undiluted ink) can be obvious to a careful observer who simply makes use of strong light, a magnifying glass and their nose. Also, key words in the visible letter, such as 'red cabbage' or 'heat', in an odd context may alert a censor to the use of invisible ink. Invisible ink should not be used with glossy or very smooth paper types, since the sizing of these papers prevents ink from being absorbed deep into the paper and it is easily visible, especially when the paper is examined under glancing light. There are, however, commercially available inks for non-porous surfaces that are only visible under ultraviolet light and are otherwise virtually invisible on these kinds of surfaces. This article or section does not cite any references or sources. ...

Using either ultraviolet light or an iodine fume cupboard, messages can be quickly screened for invisible ink and also read without first permanently developing the invisible ink. Thus, if a censor uses this method to intercept messages, he may then let the letter be sent to the intended recipient who will be unaware that the secret message has already been intercepted by a third party.

A "screening station" could theoretically involve visual and olfactory inspection, an examination under ultraviolet light and then the heating of all objects in an oven before finally trying exposure to iodine fumes. In theory, some invisible inks may even show up using a camera sensitive to infrared light. Image of two girls in mid-infrared (thermal) light (false-color) Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of radio waves. ...

Properties of an "ideal" invisible ink

Most invisible inks are insecure. World War II SOE agents were trained not to risk their lives through reliance on insecure inks, most of which inks were of World War I vintage. The SOE training manual identified the following properties of the "ideal" invisible ink: The Special Operations Executive (SOE), sometimes referred to as the Baker Street Irregulars after Sherlock Holmess fictional group of spies, was a World War II organization initiated by Winston Churchill and Hugh Dalton in July 1940 as a mechanism for conducting warfare by means other than direct military engagement. ...

1. Very water soluble, i.e. non greasy.
2. Non-volatile, i.e. no pronounced smell.
3. Not depositing crystals on paper, i.e. not easily seen in glancing light.
4. Invisible under ultraviolet light.
5. Does not decompose or discolour the paper e.g. not silver nitrate.
6. Unreactive with iodine, or with any of the other usual developers.
7. Potential developers for the ink should be as few as possible.
8. Should not develop under heat.
9. Easily obtainable and has at least one plausible innocent use by the holder.
10. Not a compound of several chemicals, as this would contradict "7".

In practice, "6" and "9" are usually incompatible. The SOE was known to supply special inks to their field agents, rather than depend on improvisation from obtainable everyday chemicals. The Special Operations Executive (SOE), sometimes referred to as the Baker Street Irregulars after Sherlock Holmess fictional group of spies, was a World War II organization initiated by Winston Churchill and Hugh Dalton in July 1940 as a mechanism for conducting warfare by means other than direct military engagement. ...

Invisible inks are not inherently "secure", but this has to be balanced against the fact that it is technically difficult to carry out mass screening of posted letters. It is easier to perform large-scale undetected screening of millions of electronic communications than it is to manually inspect even a small fraction of conventional posted letters. Apart from in dictatorships with large numbers of personnel employed to spy on their fellow nationals, inspection of posted mail can only be used in particular situations, for example focusing on the letters of a particular suspect or the screening of letters entering and leaving a particular facility. A radome at RAF Menwith Hill, a site with satellite downlink capabilities that some believe to be used by ECHELON. ECHELON is a name used to describe a highly secretive world-wide signals intelligence and analysis network said to be run by the UKUSA Community (composed of intelligence agencies of...

As an indication of security, most of the inks mentioned here were already known by the end of World War I. The U.S. Central Intelligence Agency requested (successfully) in 1999 that World War I era invisible ink technology remain exempt from mandatory declassification, based on their disputed claim that invisible ink was still relevant to national security [6]. For other uses, see United States (disambiguation) and US (disambiguation). ... The Central Intelligence Agency (CIA) is an intelligence agency of the United States government. ... 1999 (MCMXCIX) was a common year starting on Friday, and was designated the International Year of Older Persons by the United Nations. ... Declassification is the process of documents that formerly were classified becoming available to the public. ... Security measures taken to protect the Houses of Parliament in London, England. ...

References

• SOE Syllabus: Lessons in Ungentlemanly Warfare, World War II (Surrey: Public Record Office, 2001).