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 ...

AUGUSTA, Ga. (WRDW/WAGT) - When we were kids, “invisible ink” meant lemon juice on notebook paper, write the message, and to see it someone would have to hold it up to the light for the words to ...