Invisible ink

Invisible ink is a substance which can be used to write with, which is either invisible on application or disappears quickly, and can be subsequently restored by some means. The use of invisible ink is a form of steganography, and has been used in espionage.

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".

suggestions for more on this topic

• how it works chemically
• was invisible ink ever used for actual espionage (yes: see Kahn's The Codebreakers)

Table of contents

1 List of invisible inks 1.1 Revealed by heat 1.2 Developed by chemical reaction 1.3 Fluorescent inks

List of invisible inks

Revealed by heat

Some of these are organic substances that oxidize when heated, which usually turns them brown.

• Milk
• Lemon juice
• Onion juice
• Apple juice
• Sugar solution
• Diluted honey
• Diluted cola drink
• Vinegar

Inorganic inks:

• Sodium bicarbonate solution
• Sodium carbonate (washing soda) solution
• Magnesium sulphate (Epsom salts)
• Salt solution
• Copper sulphate solution
• Ammonium chloride solution
• Iron sulphate solution
• Sodium nitrate solution
• Cobalt chloride solution

Developed by chemical reaction

In most cases, one substance changes colour when mixed with an acid or base.

• Phenolphthalein, developed by ammonia or sodium carbonate.
  • Phenolphthalein (the main ingredient in laxative pills) is a pH indicator that turns fuchsia pink in the presence of a base such as ammonia.
• Vinegar, revealed by red cabbage water.
  • Vinegar is an acid (ethanoic acid) that affects the pH indicator in red cabbage water.
• Ammonia, developed by red cabbage water.
• Copper sulphate developed by sodium iodide.
• Copper sulphate developed by sodium carbonate.
• Copper sulphate developed by ammonium hydroxide.
• Lead nitrate developed by sodium iodide.
• Iron sulphate developed by sodium carbonate.
• Iron sulphate developed by potassium ferricyanate.
• Cobalt chloride developed by potassium ferricyanide.
• Copper sulphate developed by potassium ferricyanide.
• Iron sulphate developed by sodium sulphide.
• Starch, developed by iodine solution (ink turns dark blue, paper turns light blue).
• Lemon juice, developed by iodine solution (ink turns white, paper turns light blue).

Fluorescent inks

These are chemicals that glow (see fluorescence) when illuminated by ultraviolet light from a lamp.

See also: cryptography