Invisible ink allows secret messages to be written that no one else can read. The ink is only visible on paper after special treatment. Some types of invisible ink are easily developed using common household items, while others require more advanced science techniques. Writing invisible messages and revealing them can be a fun way to pass along information that only certain people should know, such as roommates’ phone numbers or social network passwords.
Depending on how they are used, a variety of liquids may act as invisible ink. Some inks, for example, are only visible under certain types of light or following a chemical reaction. Other inks are visible only when exposed to heat, while still others appear only under a black light or ultraviolet light. During WWII, allies and Axis scientists alike worked on developing invisible inks for use by spies. The inks often mimicked household products like headache and fever remedies or even laxatives to pass them off as medicine.
During the time of this war, it was important for spies to be able to quickly and accurately write and develop their invisible ink. Inks should be odorless and developable with minimal chemicals. They should also be able be revealed by a specific type of light.
Some of the most popular invisible pens are organic fluids which oxidize (turn a brown color) when heated. Other types of invisible paint are made from non-organic substances, such as nitrocellulose or methyl ethylketone. These can be revealed with a solution of water and iodine.