Invisible writing is a fun trick to show kids. The classic is with lemon juice. You can take a Q-tip, dip it in lemon juice, and then write a message. Wait a while for it to completely dry. The message is now hidden. Then with a lighter, or candle, you heat up the paper from either side and revel the message. Tada! You're a spy who can send messages. It's probably better to use a toaster, at least for kids, as the paper can easily get burned or catch fire.
The way it works is that the carbon-based compounds in the juice was absorbed in the paper fibers. The weak acid of the lemon juice also softened the fibers. Heating the paper causes some of the chemical bonds to break down and the carbon is cut loose. When the carbon comes into contact with air, it burns or oxidizes. The oxidizing turns a darker color.
During World War I, there were real "lemon juice spies". No kidding. But it didn't work out too well for them. As the British stepped up censorship of letter, some German agents operating in England used citrus to hide their communications. A British agent named Mabel Beatrice Elliot flagged letters written by three German spies. After heating them the letters, the secret messages were revealed.
It wasn't the best way to conceal messages. Many spies were caught with this indeed childish method. Some were even caught with lemon on them, or with lemon pulp stuck in the nibs of pens. In 1915, 11 spies were caught and sent to the Tower of London to be executed, 4 of whom had used the lemon juice technique.
Invisible ink played a crucial role in past war times. After the lemon juice spies, the Germans got more creative and invented better ways. But some weren't so well carried out. The CIA hadn't been created yet at that point either.
It turns out there are many ways to transmit invisible messages, and these have been listed in a confidential file named CIA-RDP11X00001R000100010003-7 released by the CIA in 2011 and published online in 2016 about various methods of concealing information. The document titled "Invisible Photography and Writing, Sympathetic Ink, Etc." apparently dates to around 1930, and lists about 50 methods of creating invisible writing or photos.
One method doesn't appear to be listed. It was used by another spy named George Vaux Bacon who was caught smuggling messages in socks during WWI. Bacon was recruited by New York based handlers Albert O. Sander and Charles Wunnenberg (Karl Wünnenberg in German), who were a master spies of the German Secret Service.
Bacon was asked if he would go to England to collect information for the German government. This included "antiaircraft defense, troop movements and morale, and information on new battleships". He was reluctant to do it at first, but when offered an expense account and 25 pounds per week, he agreed.
Skeptical of being caught, as the lemon juice spies were and face a similar fate, Wunnenberg showed Bacon the new method. Bacon was asked to buy some blacks socks. Wunnenberg then applies a toothpaste-like thick brown paste on the socks and told him "that is a secret ink which the English will never discover". But he was wrong.
Once in England, Bacon "soaked the end of the sock containing the ink in a glass of water, producing a light brown liquid about the color of scotch whiskey" to then write the messages with a ballpoint pen on rough paper that would absorb it. He sent mail to Holland soon after arriving. But the address was known by MI5 as a cover address for German Intelligence.
Bacon was in Ireland when the British counterintelligence sent him a letter to appear at at Scotland Yard on a “confidential matter.” He thought it was just about selling some films. Once there, suspicions become certainty for the British, as "they found secret-writing materials, including the invisible-ink sock. They found ballpoint pens, a bottle of invisible ink, and rough and unglazed paper." He confessed in February 1917 and implicated his handlers Sander and Wunnenberg as the ones who recruited him.
He was condemned to death by the British court for his spying activities. But U.S. officials convinced the Brits to commute his sentence to a life in prison in order for him to testify against his New York handlers and spymasters. He was transfered to U.S. custody, and one spent one year in jail as the judge said he "disliked it very much to send such a bright young man to the penitentiary".
This #deepdives challenge was a challenge for me to find something to write about. But I must admit it was interesting to look into some of the original spycraft of invisible ink. I didn't know there were so many ways to do it, nor did I know about it's history and importance in WWI.
I don't know what new invisible writing techniques there are, as they might just be classified as well. Or even if they are still in use given the advent of computer cryptography to send concealed messages. Maybe physical messages still need to be used as more secure offline methods in some cases. The need for encrypting messages with invisible writing surely led to the modern cryptography of the computer age, which includes blockchain technology.
References:
- INVISIBLE PHOTOGRAPHY AND WRITING, SYMPATHETIC INK, ETC.
- 11 Historical Uses for Invisible Ink
- Invisible Ink War: How Chemists Revealed Germany’s Secret WW I Writing [Excerpt]
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