Add Yahoo as a preferred source to see more of our stories on Google. William Abel, a mail censor at Iowa's State Prison, Fort Madison, using a hot iron to bring out a secret message written in lemon ...

Prisoners, Lovers, & Spies is the first history of invisible ink and secret communications revealed through thrilling stories about scoundrels and heroes and their ingenious methods for concealing ...

A letter from the Revolutionary War with lines written in invisible ink. Credit: Wikimedia Commons Last year, the US Central Intelligence Agency released documents detailing recipes for invisible ink ...