Warning: this is pretty revolting and not suitable for children, so think carefully about whether you want to read on.
Formed in 1909 as fear of German aggression was reaching its height, the Britain’s Secret Service Bureau (later to become MI6 – the Secret Intelligence Service) came into its own as soon as World War I broke out in 1914.
Relying heavily on agents supplying information on troop movements from both enemy-held countries and Germany’s neutral neighbours (most prominently the coast of Denmark where the German fleet would pass, and the Netherlands which was a vital overland transit point for German soldiers passing through to the Western Front) there was a need to get vital information through hostile censors and back to Blighty.
With wireless radio still in its infancy, invisible ink was the obvious solution.
According to Keith Jeffrey’s superb The Secret History Of MI6 1909-1949, senior Secret Service man Frank Stagg observed “all were anxious to obtain some which came from a natural source of supply.”
After asking around London University to no avail, Secret Service Bureau chief Mansfield Smith-Cumming found his “natural source,” one that his male agents were easily able to knock up at home, deep behind enemy lines, and as a bonus remained hidden to the most common method for German intelligence to test letters for invisible ink – iodine vapour.
It was semen, and the poor chap who discovered it was quickly bullied out of the office.
Stagg remembered that he would “never forget [Cumming’s] delight when the Deputy Chief Censor, FV Worthington, came one day with the announcement that one of his staff came had found out that semen would not respond to iodine vapour and told the man that he had had to remove the discoverer from the office immediately as his colleagues were making life intolerable by accusations of masturbation.”
This new supply of invisible ink was fraught with its own dubious drawbacks however.
“We thought we had solved the problem,” wrote Stagg. “Then our man in Copenhagen… evidently stocked it in a bottle, for his letters stank to high heaven and we had to tell him that a fresh operation was necessary for each letter.”