The ink is later made visible by different methods according to the type of invisible ink used. The ink may be developed by heat or by application of an appropriate chemical, or it may be made visible by viewing under ultraviolet light. Inks which are developed by a chemical reaction may depend on an acid-base reaction (like litmus paper), reactions similar to the blueprint process, or any of hundreds of others. Developer fluids may be applied using a spray bottle, but some developers are in the form of vapours, e.g. ammonia fumes used to develop phenolphthalein ink.
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.
Security marker pens or UV Markers with fluorescent ink that glows when illuminated with a UV light may be used to invisibly mark valuable household items in case of burglary. They may be especially formulated for writing on non-porous surfaces such as glass, plastics, etc. The inks are applied and then identified using a black light or other UV light source. The owner of a recovered, stolen item which has been marked in this way can be traced simply by using an ultraviolet lamp. 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 colours, under UV light. This makes them suitable for use in readmissions such as hand stamping.
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. 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.
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.
A non-toxic, full color invisible ink jet ink set has been developed by Vermont PhotoInkjet, LLC which is marketed under the Firefly™ brand. It has been pre-packaged for Epson printers. A creativity software has been developed by Vermont PhotoInkjet which allows both experienced and inexperienced computer artists to create artistic expressions which can only be seen under black light.
An E2E voting system called Scantegrity II uses invisible ink to enable the voter to obtain a confirmation code only for the voted selection.
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 World War II SOE training manual identified the following properties of an "ideal" invisible ink:
# Very water soluble, i.e. non-greasy. # 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 discolour the paper e.g. not silver nitrate. # Unreactive 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 #7.
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 of World War I vintage. In general, SOE used invisible inks as a back-up method of communication, for when 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 to dilute their invisible ink as much as possible to reduce chances of detection.
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 presence 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 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, 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 the infrared light spectrum.
* Cola drink
The writing is rendered visible by heating the paper, either on a radiator, by ironing it, or by placing it in an oven. A 100-watt light bulb is less likely to damage the paper.
* Phenolphthalein, commonly used as a pH indicator, turns pink in the presence of a base such as ammonia fumes or sodium carbonate.
Other inks work in a near opposite way by absorbing ultraviolet light but without fluoresceing. When these are used on fluorescent 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 detergents containing optical brighteners.
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.
Former MI-6 agent Richard Tomlinson alleges that Pentel Rolling Writer rollerball pens were extensively used by MI-6 agents to produce secret writing (invisible messages) while on missions.
In 2002, a gang was indicted for spreading a riot between federal penitentaries using coded telephone messages, and messages in invisible ink.
In 2008 a British Muslim, Rangzieb Ahmed, was alleged to have a contact book with Al-Qaeda telephone numbers, written in invisible ink.
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