Invisible inks are colorless liquids that require heat, light, or a special chemical to change their colors and make them visible. The basics behind reactions that do not use heat involve an acid or a base and a pH indicator. The colorless liquid, which is either an acid or base, is applied to paper and dried, making it invisible. When a pH indicator is introduced, it reacts with the acid or base properties of the dried liquid and changes color. There have been several types of liquids used throughout history that work well as invisible inks: milk, vinegar, lemon juice, and even urine. Often, invisible inks were not as easy as milk and lemon juice; they sometimes required complex procedures to prevent enemy censors from discovering them.
Following are some examples of invisible inks that have been used by spies in the past:
Cobalt oxide dissolved in hydrochloric or nitric acid produces a liquid that is invisible until it is held up to a flame, at which point it glows blue. The blue will then disappear by blowing on the sheet.
Eggs have been used to hide secret messages. A message is written on the shell of a clean egg and the ink diffuses through the porous surface of the shell. When the egg is boiled thoroughly, the shell is carefully peeled off, revealing the message.