Invisible ink

Invisible ink has been used by spies for centuries. At the time of the Revolutionary War, invisible ink made of a mixture of ferrous sulfate and water was commonly used. The secret messages were often written in between the lines of a normal letter. When heat or a special chemical (such as sodium carbonate) were applied, the message that was placed in between the lines would appear. In modern times, inks containing special properties are used and require viewing under ultraviolet (UV) light to see the message.

Invisible ink