The Secret History of Invisible Ink, Part 2: Invisible Writing Made Visible

In my last post, I mentioned that the Government Printing Office (GPO) played an important role during World War II in preventing prisoners of war from using invisible inks to send intelligence back to the Axis powers. Here’s the story of that secret battle fought by the paper experts at GPO.

From late 1942 to mid-1946, the United States experienced an unprecedented influx of almost a half-million German, Italian, and Japanese prisoners of war. They were in a position to damage the war effort through attempts at escape and, based on prior experiences in World War I with German prisoners of war, the possibility of espionage.

Since the Geneva Convention mandated that prisoners of war could write home, significant information on prisoners’ locations and labor activities could be transmitted through the use of invisible or sympathetic inks made from such common substances as lemon juice, milk, washing soda, baking soda, starch solution, and even human urine. The Convention also severely limit­ed the use of heat and chemicals to detect secret messages on such letters.  The War Department turned to GPO’s  chemists (see photo above) for an answer.

After extensive tests, GPO’s experts devel­oped a paper base with a silicate or clay coating. The coating contained a powder or dyestuff that would react to moisture or any acid water solution by turning green. The paper was called Sensicoat. This paper’s heavy 56-pound weight, high cost, were negative factors, so GPO then developed a lighter, uncoated, and more economical paper, Analith. After this paper went into production, secret messages to the Axis were greatly reduced. It was a reduction noticed and acted upon by German intelligence.

American censors noticed something very interesting about packages of food and clothing addressed to German prisoners as 1944 passed the halfway mark. A small amount of putty-like material about size of a kitchen match head began to turn up in various places of concealment. Repeated tests showed that the putty-like material was a “dry ink.” After several conferences with the wartime Bureau of Censorship regarding this problem, GPO’s chemists began work on a new paper, bearing in mind that it also would have to retain its sensitivity to fluid invisible inks. The result was a coated sheet processed with a water-sensitive formula and with great sensitivity to the detection of all types of dry inks

By 1945, more than 29 million sheets of the new stationery had been ordered at $1.04 per thousand and GPO had blocked a potentially dangerous flow of in­formation toAmerica’s enemies. It was an achievement shrouded in wartime secrecy, but one gratefully acknowledged by those who knew about the technical difficulties involved.

For more GPO history, go here.

12 Responses to The Secret History of Invisible Ink, Part 2: Invisible Writing Made Visible

  1. joelZ says:

    good luck! GPO

    Like

  2. Learn Italian Online Free says:

    I never thought that invisible ink is very important before for sending intelligence and information secretly, I never would know that with out this.

    Like

  3. video blog recording software says:

    G’Day! Govbooktalk,
    I know what you mean, Is it Mars, Science, Human Psychology, Medicine, Background, Interstellar Cuisine, or Tremendous-Key Invisible Widgets?
    Good Job!
    False

    Like

  4. Tom says:

    Three cheers for GPO; anything on the invisible spy-dust the KGB used to detect agents and double agents? When placed on doors, desks, cars, etc they could track our people.

    Like

  5. irmaojb41@hotmail.com says:

    Great… The GPO group is the best… God bless all them!
    But, we are at a third war or not? I presume that we are but it is very slow like a Hollywood film and goes on a crescendo like a bad sinphony! We have to prevent this event end help our people, Latin and Nort Americans!JBA

    Like

  6. zannias vasilis says:

    CONGRATULATIONS!I LOVE ARTICLE LIKE THIS!ACTUALLY , THIS STORY SHOWS TO US THAT THE ROLE OF GPO DURING WORLD WAR II WAS VERY IMPORTANT!IT IS VERY SIGNIFICANT
    FOR ME , THAT THE GPO HAD BEEN ACTING LIKE A SOLDIER IN THIS WAR , AGAINST THE ENEMY OF THE U.S.A.IN MY PREVIOUS MESSAGE I HAD MENTIONED , THAT IN ANY WAR EVERY KIND OF GUNS ARE ALLOWED(SINCE INVISIBLE INK …UNTIL ESPIONAGE VIA UAV , MICRO-CAMERA , SATELITES …HAARP)!THIS IS THE MAIN TARGET:TO PREVENT
    THE ENEMY TO MAKE DAMAGE IN OUR NATION!!!AND THE GPO HAD TAKEN PART IN THIS ROLE WITH HIGH EFFECTIVELLY AND SUCCESS!!!THAT IS WHY I ADMIRE GPO!

    Like

  7. alejandro says:

    Awesome!
    I was impressed with your story, and I’ve seen this video:

    http://www.gpo.gov/newsroom-media/gpohis_small.htm

    Like

  8. Chuck says:

    UNSUBSCRIBE ME !!!!!!!!!!!!!!!!!!

    Like

    • govbooktalk says:

      I wasn’t able to find your email address on the blog susbcriber list and my email to you bounced back. Could you be subscribed under another email address? if not, you may have received a link from one of our New Titles by Topic Email Alerts — we try to let subscribers to those know abbout blog posts in their areas of interest. If that’s the case, there is an Unsubscribe link at the bottom of that email. If you have any problems finding it, send whatever you received to me at jcameron@gpo.gov and I’ll take care of it.

      Like

  9. frank bertrand says:

    I wonder if our prisoners did the same thing as POWs in Germany. I doubt if any POWs were able to communicate as Japanese internees

    Like

  10. Fran says:

    Cool tidbit of American History … waaay cool.

    Like

  11. Wow, the things we didn’t learn in high school and college. Thanks for shedding light on this important part of our history.

    Jeff

    Like

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