Invisible ink

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Invisible ink also known as security ink, is a substance used for writing, which is invisible either on application or soon thereafter, and which later on can be made visible by some means. Invisible ink is one form of steganography and it has been used in espionage. Other uses include anti-counterfeiting, property marking, hand stamping for readmission, children's games, and marking for the purpose of identification in manufacturing.

Contents

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 written surface should appear blank, and of a similar texture and reflectivity as the surrounding surface. With letters, a cover message should be written over the invisible message, as 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 ink may 'run' when it crosses a line of invisible ink, thus betraying the presence of invisible ink. Invisible ink should not be used on ruled paper as it may similarly 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. 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.

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