The simplest forms of invisible ink are lemon juice, and milk. For this type of 'heat fixed' ink, any acidic fluid will work. Write on paper with a fountain pen, toothpick or a finger dipped in the liquid. Once dry, the paper appears blank. The writing is made to appear by heating the paper, on a radiator, iron or oven for example.
Other types of invisible ink use different chemical reactions, usually an acid-base reaction (like litmus paper) similar to the blueprint process. These dual chemical ink/decoder pairs use a spray bottle for the decoding liquid or vapor (e.g. for ammonia fumes to decode phenolphthalein ink), or an invisible ink pen with two tips, one the encoding tip, and one the decoding. A cover message should be written over the invisible message, since a blank sheet of paper arouses suspicion.
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 decoding pen is included with these books so that the children may rub the decoding pen over the invisible part of the text or picture, revealing the answer to a question printed in regular ink, the missing part of a picture, or the like.
Very rarely, invisible ink has been used in art. It is usually decoded, though when it is not, it makes a mockery of the concept of "visual art".
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List of invisible inks
Revealed by heat
Some of these are organic substances that oxidize when heated, which usually turns them brown.
Developed by chemical reaction
In most cases, one substance changes colour when mixed with an acid or base.
These are chemicals that glow (see fluorescence) when illuminated by ultraviolet light from a lamp.
See also: cryptography