Invisible ink facts for kids

Invisible ink is a substance used for writing, which is either invisible on application, or soon thereafter, and which later on can be made visible by some means. The use of invisible ink is a form of steganography, and has been used in espionage. Other uses may include anticounterfeiting, property marking, hand stamping for readmission, children's games, and marking for the purpose of identification in manufacturing.

Application and use

Invisible ink is applied to a writing surface with a specialty purpose stylus, 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.

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.

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.

Invisible ink types

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.

• Milk
• Lemon, apple or orange juice
• Onion juice
• Sugar solution
• Diluted honey
• Diluted cola drink
• Vinegar or wine
• Soap water

The writing is made visible by heating the paper, either on a radiator, by ironing it, or by placing it in an oven. A 100W light bulb is less likely to damage the paper.

Images for kids

• 

  A coded letter from Benedict Arnold, originally written in invisible ink. Handwriting by Peggy Shippen Arnold is interspersed with coded communication in Arnold's hand.

• 

  A US$20 bill showing a visible stripe under an ultraviolet light

See also

In Spanish: Tinta invisible para niños