In its Freedom of Information Act Electronic Reading Room, the CIA recently made accessible the U.S. Government's six oldest classified documents. The documents are from the World War I era and describe how spies concealed information by secret writing. Method #25 from document 6 is dipping a toothpick in common milk and writing between the lines of an ordinary letter. Ironing the letter with a hot flatiron supposedly reveals the concealed writing.
Via Barco 2.0
Via Barco 2.0