Buried on page eight of a post-war issue of THE STARS & STRIPES was this column reporting on the wartime activities of the AEF censors in France - men assigned to not simply censor all outgoing mail from Europe, but to also chemically test each one for traces of invisible ink.
Of the 50,700 letters that the Doughboys wrote, only one was found to have contained information useful to the Prussians and transcribed with invisible ink. No information was given as to who he was or what became of him.
One of the many innovations attributed to the creative frenzy generated by the First World War was the discovery of a low-budget source for invisible ink in the form of seman. The director of British Intelligence during the war, Mansfield Smith-Cumming, KCMG, CB (1859 – 1923), is credited with this discovery; he is remembered for standing before his agents and declaring: "Every man [is] his own stylo."
Today, the merchants at Amazon.com offer an Invisible Ink Pen that Sir Cumming would have found a bit more tidy, to say nothing of classy.
Read about American censorship in Occupied-Japan...