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 Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...

light. Invisible ink is one form of

steganography Steganography ( ) is the practice of representing information within another message or physical object, in such a manner that the presence of the information is not evident to human inspection. In computing/electronic contexts, a computer file, ...

History

One of the earliest writers to mention an invisible ink is

Aeneas Tacticus Aeneas Tacticus ( grc-gre, Αἰνείας ὁ Τακτικός; fl. 4th century BC) was one of the earliest Greek writers on the art of war and is credited as the first author to provide a complete guide to securing military communications. Poly ...

, 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

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 World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...

and

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...

. They used invisible ink and microdots instead of pinpricks.

Philo of Byzantium Philo of Byzantium ( el, , ''Phílōn ho Byzántios'', ca. 280 BC – ca. 220 BC), also known as Philo Mechanicus, was a Greek engineer, physicist and writer on mechanics, who lived during the latter half of the 3rd century BC. Although he was f ...

may be the first writer known to describe an invisible ink using a reagent around 217218 BC, with

oak galls Oak apple or oak gall is the common name for a large, round, vaguely apple-like gall commonly found on many species of oak. Oak apples range in size from in diameter and are caused by chemicals injected by the larva of certain kinds of gall ...

and

vitriol Vitriol is the general chemical name encompassing a class of chemical compound comprising sulfates of certain metalsoriginally, iron or copper. Those mineral substances were distinguished by their color, such as green vitriol for hydrated iron(II ...

. 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 Gaius Plinius Secundus (AD 23/2479), called Pliny the Elder (), was a Roman author, naturalist and natural philosopher, and naval and army commander of the early Roman Empire, and a friend of the emperor Vespasian. He wrote the encyclopedic '' ...

and the Roman poet

Ovid Pūblius Ovidius Nāsō (; 20 March 43 BC – 17/18 AD), known in English as Ovid ( ), was a Roman poet who lived during the reign of Augustus. He was a contemporary of the older Virgil and Horace, with whom he is often ranked as one of the th ...

gave advice on the use of plant juices and milk to write secret messages. Lemons were also used as organic inks by

Arabs The Arabs (singular: Arab; singular ar, عَرَبِيٌّ, DIN 31635: , , plural ar, عَرَب, DIN 31635, DIN 31635: , Arabic pronunciation: ), also known as the Arab people, are an ethnic group mainly inhabiting the Arab world in Wester ...

around 600AD, and during the 16th century in Europe.

Giovanni Battista della Porta Giambattista della Porta (; 1535 – 4 February 1615), also known as Giovanni Battista Della Porta, was an Italian scholar, polymath and playwright who lived in Naples at the time of the Renaissance, Scientific Revolution and Reformation. Giamb ...

is credited with the first recipe for a sympathetic ink, derived from

alum An alum () is a type of chemical compound, usually a hydrated double salt, double sulfate salt (chemistry), salt of aluminium with the general chemical formula, formula , where is a valence (chemistry), monovalent cation such as potassium or a ...

and

vinegar Vinegar is an aqueous solution of acetic acid and trace compounds that may include flavorings. Vinegar typically contains 5–8% acetic acid by volume. Usually, the acetic acid is produced by a double fermentation, converting simple sugars to et ...

, as well as the first book on secret writing and invisible inks, ''

Magia Naturalis ' (in English, ''Natural Magic'') is a work of popular science by Giambattista della Porta first published in Naples in 1558. Its popularity ensured it was republished in five Latin editions within ten years, with translations into Italian (1560 ...

''(1558, 1589). 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 Sir James Jay (October 16, 1732 – 1815) was an Thirteen Colonies, American physician and politician. He was brother of John Jay, one of the Founding Fathers of the United States. While initially a supporter of American independence, he later cha ...

and used by

George Washington George Washington (February 22, 1732, 1799) was an American military officer, statesman, and Founding Father who served as the first president of the United States from 1789 to 1797. Appointed by the Continental Congress as commander of th ...

and the

Culper Spy Ring The Culper Ring was a network of spy, spies active during the American Revolutionary War, organized by Major Benjamin Tallmadge and General George Washington in 1778 during the British New York and New Jersey campaign, occupation of New York Cit ...

during the

American Revolution The American Revolution was an ideological and political revolution that occurred in British America between 1765 and 1791. The Americans in the Thirteen Colonies formed independent states that defeated the British in the American Revolut ...

and lemon juice was used by the 'Lemon Juice Spies' (Carl Muller and 4 other Germans, who all died for their efforts either by suicide or execution, along with John Hahn, an English baker) during World War I. In World War II, neutral or acidic solutions of

phenolphthalein Phenolphthalein ( ) is a chemical compound with the chemical formula, formula carbon, C20hydrogen, H14oxygen, O4 and is often written as "HIn", "HPh", "phph" or simply "Ph" in shorthand notation. Phenolphthalein is often used as an indicator in ...

, a chemical compound extracted from pills for

constipation Constipation is a bowel dysfunction that makes bowel movements infrequent or hard to pass. The stool is often hard and dry. Other symptoms may include abdominal pain, bloating, and feeling as if one has not completely passed the bowel movement ...

, were used as invisible ink. It is colorless but turns pink when exposed to alkali such as

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...

and bicarbonate soda.

General application and use

Invisible ink can be applied to a writing surface with a specialty purpose

stylus A stylus (plural styli or styluses) is a writing utensil or a small tool for some other form of marking or shaping, for example, in pottery. It can also be a computer accessory that is used to assist in navigating or providing more precision w ...

stamp Stamp or Stamps or Stamping may refer to: Official documents and related impressions * Postage stamp, used to indicate prepayment of fees for public mail * Ration stamp, indicating the right to rationed goods * Revenue stamp, used on documents to ...

fountain pen A fountain pen is a writing instrument which uses a metal nib to apply a water-based ink to paper. It is distinguished from earlier dip pens by using an internal reservoir to hold ink, eliminating the need to repeatedly dip the pen in an inkw ...

toothpick A toothpick is a small thin stick of wood, plastic, bamboo, metal, bone or other substance with at least one and sometimes two pointed ends to insert between teeth to remove detritus, usually after a meal. Toothpicks are also used for festive ...

calligraphy pen A dip pen or nib pen or pen nib usually consists of a metal nib with capillary channels like those of fountain pen nibs, mounted in a handle or holder, often made of wood. Other materials can be used for the holder, including bone, metal and pla ...

Cotton swab Cotton is a soft, fluffy Staple (textiles), staple fiber that grows in a wikt:boll, boll, or protective case, around the seeds of the cotton plants of the genus ''Gossypium'' in the mallow family Malvaceae. The fiber is almost pure cellulose ...

, or even a finger dipped in the liquid. Once dry, the written surface looks as if it were blank, with a similar texture and reflectivity as the surrounding surface. The ink can be later made visible by different methods according to the type of invisible ink used. The ink may be revealed by heat or by application of an appropriate chemical, or it may be made visible by viewing under

ultraviolet light Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...

. Inks which are developed by a

chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

may depend on an acid-base reaction (like

litmus paper Litmus is a water-soluble mixture of different dyes extracted from lichens. It is often absorbed onto filter paper to produce one of the oldest forms of pH indicator, used to test materials for acidity. It is a purple dye that is extracted fro ...

), reactions similar to the

blueprint A blueprint is a reproduction of a technical drawing or engineering drawing using a contact print process on light-sensitive sheets. Introduced by Sir John Herschel in 1842, the process allowed rapid and accurate production of an unlimited number ...

process, or any of hundreds of others. Developer fluids may be applied using a spray bottle, but some developers are in the form of vapor, e.g.

fumes used to develop

ink. There are also toy invisible ink pens which have two tips—one tip for invisible ink writing, and another tip for developing the ink. Invisible ink is sometimes used to print parts of pictures or text in

book A book is a medium for recording information in the form of writing or images, typically composed of many pages (made of papyrus, parchment, vellum, or paper) bound together and protected by a cover. The technical term for this physical arr ...

s for children to play with, always including a "

decoder pen 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 sp ...

" 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 marker An ultraviolet (UV) marker is a pen whose marks are fluorescent but transparent: the marks can be seen only under an ultraviolet light. They are commonly used in security situations to identify belongings or to prevent the reproduction of unautho ...

s with fluorescent ink that glows when illuminated with a UV light is often used to invisibly mark valuable household items in case of

burglary Burglary, also called breaking and entering and sometimes housebreaking, is the act of entering a building or other areas without permission, with the intention of committing a criminal offence. Usually that offence is theft, robbery or murder ...

. There are specialty security maker pens formulated for writing on non-porous surfaces such as

glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...

plastic Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

, etc. The mark can be read by using a

blacklight A blacklight, also called a UV-A light, Wood's lamp, or ultraviolet light, is a lamp that emits long-wave (UV-A) ultraviolet light and very little visible light. One type of lamp has a violet filter material, either on the bulb or in a separat ...

or other UV light source. Security marker pens can be obtained commercially and are widely used as a crime countermeasure. Some commercially available invisible inks glow very brightly, in a variety of colors, under

light. This makes them suitable for use in readmission such as 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 printer Inkjet printing is a type of computer printing that recreates a digital image by propelling droplets of ink onto paper and plastic substrates. Inkjet printers were the most commonly used type of printer in 2008, and range from small inexpensi ...

s. Such inks are usually visible under

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 The United States Postal Service (USPS), also known as the Post Office, U.S. Mail, or Postal Service, is an independent agency of the executive branch of the United States federal government responsible for providing postal service in the U ...

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. 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. An E2E voting system called

Scantegrity II Scantegrity is a security enhancement for optical scan voting systems, providing such systems with end-to-end (E2E) verifiability of election results. It uses confirmation codes to allow a voter to prove to themselves that their ballot is included ...

uses invisible ink to enable the voter to obtain a confirmation code only for the voted selection.

Properties of an "ideal" invisible ink

What an "ideal" invisible ink is depends on its intended use. For example, property marking should ideally be done with ink easily read under ultraviolet light, whereas in espionage such an ink would be considered too easily detectable since a large number of letters may be 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 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: # Mixes with water. # Non-volatile, i.e. no pronounced smell. # Not depositing crystals on paper, i.e. not easily seen in glancing light. # Invisible under ultraviolet light. # Does not decompose or discolor the paper e.g.

silver nitrate Silver nitrate is an inorganic compound with chemical formula . It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called ''lunar caustic' ...

. # Nonreactive with iodine, or with any of the other usual developers. # Potential developers for the ink should be as few as possible. # Should not develop under heat. # Easily obtainable and has at least one plausible innocent use by the holder. # Not a compound of several chemicals, as this would violate . From practical experience "6" and "9" were usually incompatible. SOE agents were trained not to risk their lives through reliance on insecure inks, most of which were from World War I. In general, SOE used invisible inks as a back-up method of communication when other, more secure communication techniques were unavailable. The agency 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 advised to dilute their invisible ink as much as possible to reduce chances of detection.

Screening letters for 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. 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, usually from using undiluted ink), can be obvious to a careful observer who simply makes use of strong light, a magnifying glass and his or her nose. 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. Invisible ink is not effective with glossy or very smooth paper types, since the

sizing Sizing or size is a substance that is applied to, or incorporated into, other materials—especially papers and textiles—to act as a protective filler or glaze. Sizing is used in papermaking and textile manufacturing to change the absorption ...

of these papers prevents ink from being absorbed deep into the paper and it is easily visible, especially if 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 such surfaces. 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, the letter may then 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" theoretically could 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 to produce optimal security in optimal time.

Invisible ink types

For practical reasons, the inks are listed here according to their method of development. It must be understood however that some inks - particularly those of organic origin or those consisting of a mixture of several chemicals - may be made visible by several methods. 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.

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. The most secure way to use any of the following substances for invisible ink is by dilution, usually with water, close to the point when they become difficult to develop. * Cola drink * Honey solution, sugar (sugar turns into

caramel Caramel ( or ) is an orange-brown confectionery product made by heating a range of sugars. It can be used as a flavoring in puddings and desserts, as a filling in bonbons, or as a topping for ice cream and custard. The process of caramelizatio ...

by dehydration) * Lemon, apple, orange or onion juice (organic acids and the paper forms

ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ar ...

under heat) * Milk (lactose dehydrates) * Bodily fluids such as blood serum. * Soap solution (

carboxylic In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...

partially oxidizes) * Wine, or vinegar *

Cobalt chloride Cobalt chloride (cobalt paper) may refer to: * Cobalt(II) chloride (CoCl2) * Cobalt(III) chloride Cobalt(III) chloride or cobaltic chloride is an unstable and elusive compound of cobalt and chlorine with formula . In this compound, the cobalt at ...

, which turns blue when heated and becomes invisible again after a while (if not overly heated) The writing is rendered visible by

heating A central heating system provides warmth to a number of spaces within a building from one main source of heat. It is a component of heating, ventilation, and air conditioning (short: HVAC) systems, which can both cool and warm interior spaces. ...

the paper, either on a

radiator Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics. A radiator is always a ...

, by

ironing Ironing is the use of a machine, usually a heated tool (an iron), to remove wrinkles and unwanted creases from fabric. The heating is commonly done to a temperature of 180–220 °Celsius (356-428 Fahrenheit), depending on the fabric. Ironing wor ...

it, using a hair dryer, or by placing it in an

oven upA double oven A ceramic oven An oven is a tool which is used to expose materials to a hot environment. Ovens contain a hollow chamber and provide a means of heating the chamber in a controlled way. In use since antiquity, they have been us ...

. A 100-watt

light bulb An electric light, lamp, or light bulb is an electrical component that produces light. It is the most common form of artificial lighting. Lamps usually have a base made of ceramic, metal, glass, or plastic, which secures the lamp in the soc ...

is less likely to damage the paper.

Inks developed by chemical reaction

In most cases, these substance changes color when mixed with an

acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...

or base. *

Phenolphthalein Phenolphthalein ( ) is a chemical compound with the chemical formula, formula carbon, C20hydrogen, H14oxygen, O4 and is often written as "HIn", "HPh", "phph" or simply "Ph" in shorthand notation. Phenolphthalein is often used as an indicator in ...

, commonly used as a pH indicator, turns pink in the presence of a base such as ammonia fumes or

sodium carbonate Sodium carbonate, , (also known as washing soda, soda ash and soda crystals) is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield moderately alkaline solutions ...

. *

Vinegar Vinegar is an aqueous solution of acetic acid and trace compounds that may include flavorings. Vinegar typically contains 5–8% acetic acid by volume. Usually, the acetic acid is produced by a double fermentation, converting simple sugars to et ...

, is revealed b
red cabbage water
Vinegar contains

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component ...

that affects the pH indicator in red cabbage water. Vinegar may also be developed by heat, as above. *

Ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...

, developed by red cabbage water. *

Copper sulfate Copper sulfate may refer to: * Copper(II) sulfate, CuSO4, a common compound used as a fungicide and herbicide * Copper(I) sulfate Copper(I) sulfate, also known as cuprous sulfate, is an inorganic compound with the chemical formula Cu2 SO4. It ...

, developed by

sodium iodide Sodium iodide (chemical formula NaI) is an ionic compound formed from the chemical reaction of sodium metal and iodine. Under standard conditions, it is a white, water-soluble solid comprising a 1:1 mix of sodium cations (Na+) and iodide anions (I ...

ammonium hydroxide Ammonia solution, also known as ammonia water, ammonium hydroxide, ammoniacal liquor, ammonia liquor, aqua ammonia, aqueous ammonia, or (inaccurately) ammonia, is a solution of ammonia in water. It can be denoted by the symbols NH3(aq). Although ...

potassium ferricyanide Potassium ferricyanide is the chemical compound with the formula K3 e(CN)6 This bright red salt contains the octahedrally coordinated 3−.html" ;"title="e(CN)6sup>3−">e(CN)6sup>3− ion. It is soluble in water and its solution shows some g ...

. *

Lead(II) nitrate Lead(II) nitrate is an inorganic compound with the chemical formula Pb( NO3)2. It commonly occurs as a colourless crystal or white powder and, unlike most other lead(II) salts, is soluble in water. Known since the Middle Ages by the name plumbum ...

, developed by

. *

Iron(II) sulfate Iron(II) sulfate (British English: iron(II) sulphate) or ferrous sulfate denotes a range of salts with the formula Fe SO4·''x''H2O. These compounds exist most commonly as the heptahydrate (''x'' = 7) but several values for x are know ...

, developed by

or potassium ferricyanate. *

Cobalt(II) chloride Cobalt(II) chloride is an inorganic compound of cobalt and chlorine, with the formula . The compound forms several hydrates ·''n'', for ''n'' = 1, 2, 6, and 9. Claims of the formation of tri- and tetrahydrates have not been confirmed.M. T. Sau ...

, developed by

. *

Iron(III) sulfate Iron(III) sulfate (or ferric sulfate), is a family of inorganic compounds with the formula Fe2(SO4)3(H2O)n. A variety of hydrates are known, including the most commonly encountered form of "ferric sulfate". Solutions are used in dyeing as a morda ...

, developed by

sodium sulfide Sodium sulfide is a chemical compound with the formula Na2 S, or more commonly its hydrate Na2S·9 H2O. Both the anhydrous and the hydrated salts in pure crystalline form are colorless solids, although technical grades of sodium sulfide are gener ...

. *

Starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diets ...

, developed by

iodine Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , and boils to a vi ...

solution which turns starch dark blue and the paper light blue. *

Lemon juice The lemon (''Citrus limon'') is a species of small evergreen trees in the flowering plant family Rutaceae, native to Asia, primarily Northeast India (Assam), Northern Myanmar or China. The tree's ellipsoidal yellow fruit is used for culina ...

, developed by

solution (ink turns white, paper turns light blue). *

Sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g ...

(common table salt), developed by

. *

Cerium oxalate Cerium(III) oxalate (cerous oxalate) is the inorganic cerium salt of oxalic acid. It is a white crystalline solid with the chemical formula of Ce2(C2O4)3. It could be obtained by the reaction of oxalic acid with cerium(III) chloride. Uses Ce ...

developed by

manganese sulfate Manganese(II) sulfate usually refers to the inorganic compound with the formula MnSO4·H2O. This pale pink deliquescent solid is a commercially significant manganese(II) salt. Approximately 260,000 tonnes of manganese(II) sulfate were produced w ...

and

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3%� ...

Inks visible under ultraviolet light

Some inks glow faintly (

fluoresce Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...

) when under an

lamp. This is a property of many substances, particularly organic substances and body fluids. Other inks work in a near opposite way by ''absorbing'' ultraviolet light but without fluorescing. When these are used on

fluorescent Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...

paper, the inked areas fluoresce less than the surrounding paper area when under an ultraviolet lamp. This is especially a property of inks with a yellow tint. Some UV-visible inks may be detected on a photocopy, due to the relatively strong ultraviolet component in light from the photocopier scanning head. Examples of inks revealed by ultraviolet light are: *

Laundry detergent Laundry detergent is a type of detergent (cleaning agent) used for cleaning dirty laundry (clothes). Laundry detergent is manufactured in powder (washing powder) and liquid form. While powdered and liquid detergents hold roughly equal share o ...

s containing

optical brightener Optical brighteners, optical brightening agents (OBAs), fluorescent brightening agents (FBAs), or fluorescent whitening agents (FWAs), are chemical compounds that absorb light in the ultraviolet and violet region (usually 340-370 nm) of the ...

s *

Soap Soap is a salt of a fatty acid used in a variety of cleansing and lubricating products. In a domestic setting, soaps are surfactants usually used for washing, bathing, and other types of housekeeping. In industrial settings, soaps are use ...

* Body fluids, serum,

saliva Saliva (commonly referred to as spit) is an extracellular fluid produced and secreted by salivary glands in the mouth. In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells (from which DNA can be ...

* Sunscreen * Lemon juice

Inks which alter 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 alter 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 altered 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

thymolphthalein Thymolphthalein is a phthalein dye used as an acid– base ( pH) indicator. Its transition range is around pH 9.3–10.5. Below this pH, it is colorless; above, it is blue. The molar extinction coefficient for the blue thymolphthalein dianion is ...

and a basic substance such as

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkali ...

. Thymolphthalein, which is normally colorless, turns blue in solution with the base. As the base reacts with carbon dioxide (always present in the air), the pH drops below 10.5 and the color disappears. Pens are now also available that can be erased simply by swiping a special pen over the original text. Disappearing inks have been used in gag squirtguns, for limited-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.

Modern usage

As an indication of security, most invisible inks were already known by the end of

. However, in 1999, the U.S.

Central Intelligence Agency The Central Intelligence Agency (CIA ), known informally as the Agency and historically as the Company, is a civilian foreign intelligence service of the federal government of the United States, officially tasked with gathering, processing, ...

successfully requested that World War I invisible ink remain exempt from mandatory

declassification Declassification is the process of ceasing a protective classification, often under the principle of freedom of information. Procedures for declassification vary by country. Papers may be withheld without being classified as secret, and even ...

, based on the premise that invisible ink was still relevant to

national security National security, or national defence, is the security and defence of a sovereign state, including its citizens, economy, and institutions, which is regarded as a duty of government. Originally conceived as protection against military atta ...

. The World War I documents remained classified until 2011. Former

MI6 The Secret Intelligence Service (SIS), commonly known as MI6 ( Military Intelligence, Section 6), is the foreign intelligence service of the United Kingdom, tasked mainly with the covert overseas collection and analysis of human intelligenc ...

agent

Richard Tomlinson Richard John Charles Tomlinson (born 13 January 1963) is a former officer of the British Secret Intelligence Service (MI6). He argued that he was subjected to unfair dismissal from MI6 in 1995, and attempted to take his former employer to a ...

stated that Pentel Rolling Writer rollerball pens were extensively used by MI-6 agents to produce secret writing in the form of invisible messages while on missions. In 2002, a gang was indicted for spreading a riot between federal penitentiaries using coded telephone messages, and messages in invisible ink.

References

Notes Further reading * *

External links

Secret Writing
- CIA Freedom of Information Act (FOIA) Electronic Reading Room {{Espionage Espionage Steganography