Muller used lemon juice as invisible ink to inform on British troop movements. A warm iron was passed over a letter to reveal the secret messages. The lemon was found in his overcoat when he was ...

Steve: That's John Nagy, author of the new book Invisible Ink ... messages that's more sophisticated than the vinegar, lemon juice and add heat, and would they actually then have something ...

From invisible ink in lemons to Cold War betrayals, MI5’s first public exhibition reveals spy stories and gadgets spanning 115 years of British intelligence.

The lemon Karl Muller used for writing in invisible ink was an exhibit at his trial ... He pierced lemons with a pen nib to extract the juice and insert secret writing in innocent-looking letters ...