Невидимые чернила: история и методы применения

Substance used for writing which is invisible and can later be 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 can later be made visible by some means, such as heat or ultraviolet light. Invisible ink is one form of steganography.

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. 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 steganography 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 and World War II. They used invisible ink and microdots instead of pinpricks. 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. Lemons were also used as organic inks by Arabs around 600AD, and during the 16th century in Europe. 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, Magia Naturalis (1558, 1589). It is colorless but turns pink when exposed to alkali such as ammonia and bicarbonate soda.

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 their 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 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.

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.

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.

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 and a basic substance such as sodium hydroxide. 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.

Former MI6 agent Richard Tomlinson 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.

Invisible ink is not commonly used in art. Some artists, however, have incorporated invisible ink into their work, either alone or in conjunction with more conventional media. Jean Michel Basquiat is known to have worked with invisible ink. In 2012, Sotheby's London discovered Basquiat’s signature painted in invisible ink on the 1982 work, Orange Sports Figure. In 2018, analysis by an art conservator revealed invisible ink markings on an untitled Basquiat painting from 1981. In 2012, the Hayward Gallery exhibition, Invisible: Art about the Unseen, 1957 2012, included the 1989 work, Magic Ink, by Gianni Motti. It consisted of two drawings created with undeveloped invisible ink. In 2015, Aowen Jin exhibited artwork drawn in invisible ink at the Horniman Museum in London. The illustrations, drawn on the walls and floor of the Music Gallery Performance Space, were only visible under UV light.