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It's a classic line from a spy movie: "Burn after reading." For German agent Karl Muller, that advice would have really helped.
Muller, a spy who masqueraded as a Russian shipbroker and covertly traveled to England among a crowd of refugees in 1915, used pen nibs dipped in lemon to write invisible messages to his counterparts. He relayed his secrets in between lines of regular ink in what appeared to be otherwise unremarkable letters.
But Muller's cover was blown when an agent from Britain's Secret Service Bureau - known today as MI5 - ran a flat iron over a letter he sent in 1915, causing the invisible lemon ink to show. He was executed by firing squad at the Tower of London.
Now, his trusty lemon - 110 years old and shriveled black - will be on display starting Saturday in a new exhibit called "MI5: Official Secrets," which showcases relics used by spies and spy-catchers, at the National Archives in Britain.
The fruit was found in Muller's coat pocket at the time of his arrest and became a key piece of evidence in the bitter case against him.
"Lemon juice had been used for centuries as a form of 'invisible ink,' since it cannot be seen on paper until it carbonizes when heated," the National Archives says on its website. "Other methods were available by the time of the First World War, but Muller had not realized the risks involved with using such a time-honored approach."
The practice dates at least as far back as the Renaissance, when artist-inventor Leonardo da Vinci combined it with milk to write invisible messages, according to a research paper. Some have experimented throughout history with other acidic or alkaline substances, including urine, vinegar, onion juice, juice from other citrus fruits and semen.
Lemon and grapefruit juice were also sometimes used during the American Revolutionary War.
Invisible ink made from orange juice was a key part of the infamous 1605 Gunpowder Plot, when a handful of English Roman Catholics, angered by religious persecution, conspired to kill King James I and blow up Parliament. They did not succeed, but letters swapped among members of the underground Catholic movement from that time revealed numerous uses of orange ink.
As for Muller, the German spy, his trial and execution were kept secret from the public so that British agents could continue to use his methods to pose as him and send false information to German intelligence units. They collected payments for his purported services and eventually pooled it together to purchase a Morris automobile, an iconic brand of car at the time.
They named the car Muller, according to the National Archives.