Invisible ink may seem like an outdated concept – but even in the 21st century it is still being used to spread secret messages.
As recently as 2008, a British man was found guilty of possessing invisible ink diaries for the purposes of terrorism, and back in 2002, a gang was indicted for spreading a riot between American prisons using coded phone messages, and messages in invisible ink.
Long used in espionage, invisible ink was a favourite tool of European and American spies, and their secret formulae and strategies for using it a closely guarded secret.
Former MI-6 agent Richard Tomlinson has claimed that Pentel Rolling Writer rollerball pens were extensively used by MI-6 agents to produce secret writing during missions.
In 1999, the United States’ Central Intelligence Agency requested that a 1940s technical report on invisible ink remained exempt from mandatory declassification, allegedly because the ink was still important to national security. The information was kept secret for a further 12 years.
It is easier to monitor electronic mail on a mass scale than to monitor communications in snail mail – so invisible ink lives on.