- Karl Muller was a German spy using lemon juice as invisible ink in Britain during WWI
- MI5 uncovered hidden messages by heating letters addressed to a Rotterdam post office box
- A Deptford baker, John Hann, was Muller's assistant and possessed pierced lemons with ink
Could a lemon get someone a death sentence? Over a century ago, it did.
In 1915, during the height of the First World War, the fruit became the undoing of a German spy. Blackened with age and preserved in cotton wool, that same lemon, once used to conceal invisible ink, was among the most curious relics displayed earlier this year in MI5's first public exhibition at The National Archives in London's Kew.
Karl Muller
The man behind the lemon was Karl Muller, a German who slipped into Britain in January 1915 posing as a Russian shipping broker. He arrived with forged papers and a walrus moustache, passing himself off as just another refugee from Belgium.
In reality, Muller was an agent for German intelligence, tasked with reporting on British troop movements.
His chosen weapon was not a pistol or a bomb, but lemon juice. By pricking the fruit with a pen nib, Muller extracted liquid to write hidden messages between the lines of innocent-looking correspondence. The words were invisible until warmed over a flat iron. It is a centuries-old method of invisible ink.
How He Was Caught
Britain's postal censorship office, already alert to the threat of enemy spies, became suspicious of a letter addressed to a Rotterdam post office box. When MI5 officers tested it with heat, they revealed coded notes about troop drills in Epsom and departures from southern ports.
The trail led detectives to John Hann, a Deptford baker of German origin working as Muller's assistant. A search of Hann's home uncovered a lemon pierced with pen pricks and blotting paper with traces of secret writing.
From there, investigators closed in on Muller's Bloomsbury lodgings, where they found one lemon in his overcoat pocket and another cut into segments, carefully wrapped in cotton wool in a drawer.
Asked why he carried lemons, Muller told officers they were for "cleaning his teeth." It was not a convincing explanation.
The Giveaway Fruit
Forensic experts tested the items and discovered cellular traces from lemons on Muller's pen nib. It was direct evidence linking him to invisible ink letters.
By June 1915, both Muller and Hann were tried in secret at the Old Bailey. Hann was sentenced to seven years' imprisonment.
Muller, found guilty of espionage, faced the death penalty. On 23 June 1915, at the Tower of London, he walked calmly to the firing squad, shaking each soldier's hand before being blindfolded and executed.
MI5's Double Game
The story didn't end with his death. MI5 continued to send fabricated reports to German intelligence in Antwerp under Muller's name. The ruse convinced Berlin that their spy was still active, and money kept flowing across the Channel. The British security service even used the funds to purchase a two-seater Morris car, cheekily named "The Muller," which they drove on surveillance jobs.
