Application and use
Invisible ink is applied to a writing surface with a specialty purpose stylus, stamp, 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 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 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 for developing 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.
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.
Former MI-6 agent Richard Tomlinson alleges that Pentel Rolling Writer rollerball pens were extensively used by SIS agents to produce secret writing (invisible messages) while on missions.
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.
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 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.
- Cola drink (diluted)
- Honey (diluted)
- Lemon, apple or orange juice
- Milk
- Onion juice
- Semen - The aptly-named Sir George Mansfield Smith-Cumming, head of the British Secret Intelligence Service (SSB) noted of his agents that "Every man carries his own stylo".
- Soap water
- Sugar solution
- Urine
- Vinegar or wine
- Cobalt chloride turns blue when heated and becomes invisible again after a while (if not overly heated)
