People have always wanted to communicate privately. There once was a time when regular ink was secret enough, because not much of the world knew how to read. Then literacy ruined it for everyone, and drastic steps had to be taken. Enter invisible ink, the way to keep what was written secret, sometimes by unsavory…
The oldest secret documents in the CIA's treasure-trove have been cracked open finally. Believed to be the only remaining classified WWI documents, the six files have revealed all manner of espionage message tricks—including the recipe for German invisible ink.
While I doubt the super serious stiff in this DIY video is actually a super spy, he does have the DIY know-how necessary to hack apart an inkjet printer and make it into an invisible ink-spitting espionage machine. Enjoy! [MAKE]