Excerpted with permission from Prisoners, Lovers, & Spies: The Story of Invisible Ink from Herodotus to al Qaeda, by Kristie Macrakis. Available from Yale University Press. Copyright © 2014.



In the fall of 1916 British censors opened a letter by a most intriguing spy. His name was George Vaux Bacon and he was an American journalist sent to Britain in September 1916 by German Secret Service officers based in New York City. Bacon was a lanky Minnesota-born young man, with light reddish hair, pale blue eyes, a fair complexion, and a weak receding chin. He wore a pair of wire-rimmed round glasses that made him look intellectual. The journalism occupation was no cover story, though. Bacon really was a writer and journalist who worked for magazines like the film tabloid Photoplay and served as the New York City publicity representative of a film production company before the German Secret Service recruited him to become a spy.

British counterintelligence wondered why the Germans hadn’t sent any journalists into England yet. Most of the spies they captured had commercial cover. Soon after Bacon arrived in England, censors intercepted his mail—with suspicious underlining—to a contact with a cover address in Holland. Unfortunately for Bacon, the Rotterdam address had recently been placed under censorship surveillance because MI5 had received a tip from MI6 that German intelligence was using it as a cover address. Since Bacon planned to travel to Rotterdam like some of the other



German spies who had been apprehended, counterintelligence decided to shadow him during his stay there. The Bacon investigation in turn led to an “important gang of American spies,” as MI5 called them, sent to England from the United States by two German master spies based in New York City—Albert O. Sander and Charles Wunnenberg (Karl Wünnenberg in German). Both of these spy runners had cover jobs in journalism and allegedly worked at the Central Powers Film Company.



Although the United States was still neutral when Wunnenberg and Sander were sent over, it had been supplying the Allies with munitions, war materials, and provisions. The Germans had launched a massive sabotage campaign in the United States focused on the East Coast because they wanted to block delivery of materials to the Allies. They even had chemical specialists based in Hoboken, New Jersey, like Dr. Walter Scheele, who created a cigar bomb. Scheele and his spy ring were implicated in the huge Black Tom explosion in July 1916 that woke up half of New York City and destroyed a major munitions warehouse along the New Jersey shoreline, but Scheele escaped to Cuba.



Soon after the Black Tom incident, Wunnenberg approached Bacon in New York City under the alias “Davis.” Wunnenberg asked whether Bacon would be willing to go to England to collect information useful for the German government, including antiaircraft defense, troop movements and morale, and information on new battleships. At first Bacon was concerned about the fate of spies in England, but after the spymasters offered him a liberal expense account and twenty-five pounds a week, Bacon agreed.



Bacon then asked how he would get the messages back to the spymasters in New York City. “Surely the British censor will see them,” he said nervously.



“No, no,” replied Sander. “I will give you the secret of fooling the censor.” That secret turned out to be a new invisible ink German chemists had developed in order to increase security after the lemon juice spy losses.



After Bacon secured a passport with the pretense that he was going to collect war pictures for the Central Press, he went to see Wunnenberg again, who asked, “Have you got a pair of black woolen socks?”



Bacon looked at Wunnenberg in disbelief and told him he had plenty of socks but nothing in black, so he went down to the store and bought some. When he returned Wunnenberg took out a toothpaste-type tube and said, “Give me your socks.” As Bacon watched, Wunnenberg smeared a thick brown paste on the top of the socks.



“There . . . that is a secret ink which the English will never discover,” Wunnenberg confidently told Bacon. He then instructed Bacon to soak the top of the socks in water, squeeze them out and use the liquid as secret ink when he wrote letters to Holland. He also told him to use a ballpoint pen—to avoid scratches—and rough paper so that the paper would absorb the ink.



Bacon was now ready and outfitted for his spy mission in England. The day after arriving in London he decided to try out his invisible ink. He “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.” With the secret ink he wrote a long letter about conditions in England to his contact in The Hague. After several weeks he traveled to Holland to meet his spy contacts. He spent the rest of the fall writing several legitimate articles for magazines in New York. During November and December he spent quite a bit of time in Ireland, collecting information on the Irish independence movement for his handlers while eluding the watchers. It was during Bacon’s time in Ireland that British counterintelligence made its move. He received a letter asking him to appear at Scotland Yard on a “confidential matter.” Originally officials planned only to interview him “in the hope that he could be ‘frightened out of the country.’” Sensing that port authorities would have been alerted to prevent any escape attempt, Bacon showed up for the interview on December 9.



Grilled about the people he was writing to in Holland, Bacon claimed not to know that he was dealing with Secret Service personnel. He was, he said, simply trying to sell them some films. The investigators did not believe him and detained him on suspicion of being a German spy.



That suspicion soon turned to confident certainty when they searched his belongings and found incriminating secret-writing materials, including the invisible-ink socks. They found ballpoint pens, a bottle of invisible ink, and rough and unglazed paper. Equally incriminating were the checks and hotel bills from his stay in Holland. They even found the code name “Denis,” the Dutch master spy, in one of his notebooks.



The strain began to wear on Bacon during his detention at Brixton Prison, and this produced an unexpected turn of events for the British. In February 1917 Bacon wrote the authorities asking to make a statement. When an officer from Scotland Yard arrived, Bacon told him he would feel better if he were able to make a full confession.

Bacon told Scotland Yard about his recruitment by Sander and Wunnenberg and the method by which he had been instructed to communicate to evade the censors. He insisted, like many spies, that he never intended to pass on any damaging information; he was, he claimed, cooperating with the Germans to get an interesting story on espionage.



There was one thing Bacon did not know: what was needed to develop the secret ink he used. He was simply told how to obtain the ink by wringing out the socks. The British set to work using spectroscopic methods to identify the secret-ink substance. Once they did this, they could try to find an appropriate reagent. Through this analysis, chemists determined that Bacon, as well as other spies, was using Argyrol, the commercial name for a silver salt of a protein mixture sold as a light brown powder soluble in water. Argyrol can also be used as an antibacterial and antiseptic; it was used to combat gonorrhea before sulfa drugs and antibiotics were developed in the 1940s. When investigators confronted Bacon with the bottle of medicine, he claimed he was using it as an antiseptic but did not know that it was impregnated in the socks.



The case of the impregnated socks marked a big change in the Germans’ methods. The British and French had found reagents for the simpler secret inks, like revealing iron chloride with potassium ferrocyanide concealed in soap, and lead acetate in perfume that they had come across by searching suspects at border crossings.



But now the Germans had begun to develop methods that did not respond to a single reagent. And the seesaw battle of wits reached a climax. German chemists began to make very dilute solutions, with concentrations ranging from 1:50,000 to 1:500,000. Not only is it hard to detect the secret ink in dilute solutions, but chemical analysis doesn’t help because the metal molecule doesn’t appear with ordinary developers.



In Britain, the celebrated physicist Thomas Ralph Merton was hired to join the Secret Service after he was rejected for active service because of poor health. Collins provided him with a small amount of liquid, and, using spectroscopy, Merton was able to detect minute amounts of silver from the secret ink impregnated in clothing like Bacon’s socks.



Meanwhile, the French also came across cases of clothing impregnated with secret ink—socks and shoelaces—using organic compounds of silver like protargol (similar in composition to the argyrols). It appears as though the French and the British worked together to find a way to develop this extremely dilute secret-ink substance. While Collins headed up the work in the British Postal Censorship chemistry department, the Parisian Department of Judicial Identity, led by Gaston Edmond Bayle (1879–1929), called the “Grand Inquisitor,” made major breakthroughs in France.



Bayle studied chemistry and worked at the Pasteur Institute in Paris and for the French railway service before joining the French police in January 1915 as a forensic chemist. And just as Collins’s investigations sent numerous Germans to the Tower of London to be shot, so too did Bayle and his team have a hand in sending German agents to the dungeons at Vincennes. The Germans blamed treason within their own ranks for these losses, but French Secret Service man Charles Lucieto reprimanded them long after the war for their “clumsiness as chemists.” He admonished them for sending their spies to a foreign country equipped with “soap made of potassium ferrocyanide or toilet water that contains lead acetate.”



French chemists used electrolysis—a chemical separation method using electricity—to reveal the dilute writing. Apparently, a sheet of paper qualifies as insulating material. If paper with tiny traces of certain metals (the secret-ink substance) is placed in a nascent metal medium—the French used silver nitrate and a reducing agent—the medium is deposited by electrolysis on the trace particles and makes them visible.



Bayle and his team spent months analyzing German secret inks. In addition to working on the dilute silver protein mystery, they spent three months analyzing a dilute secret-inkimpregnated handkerchief officials had seized at the border. They finally discovered the single developer using a catalytic operation (still not declassified) to reveal the secrets.



Bayle was a forensic science hero in France. He was considered brilliant and he solved case after case using science. Since French intelligence officers were worried about the security of their own secret correspondence, Bayle developed a special secret ink requiring four different reagents used in a set order. He communicated this special secret process to the French War Department in May 1918. No one succeeded in deciphering it, nor has it been made public.

Bayle loved “to unravel a really clever fraud.” Even though he was highly regarded internationally because of his skill in unmasking frauds, his victims felt bitter that he found scientific ways to catch them. Some denied any wrongdoing. In 1929, Bayle found that a traveling salesman, a Joseph Philipponet, had produced a fraudulent document to deceive his landlord and obtain money. Seeking revenge, the salesman hung around the police department one morning waiting for Bayle’s arrival. As Bayle walked up the steps to his laboratory, Philipponet shot him in the back three times. Bayle swayed, rolled down the stairs, and lay sprawling at the bottom. One last gasp and a mouthful of blood on the floor and he was dead. When the police in the department came to cuff Philipponet, he said: “My document was genuine! What I have done was worth the death of a father of five children!”

Although Bacon failed as a spy, the British thought his secret ink was “first-rate” and the best developed during World War I. Solving the mystery was such a coup that the Postal Censorship department suggested that the king and queen write a secret letter with the substance when they visited the department to thank them for their good work. When the royal couple wrote their names with the celebrated ink, the signatures became brown when developed and chemists fixed the paper with a solution of sodium thiosulfate so the writing would not fade. The framed paper now hangs on a wall in the British counterespionage department.



Collins’s testimony about Bacon’s secret ink was “directly responsible” for the death sentence handed down. Bacon was found guilty of charges including collecting information “with the intent to assist the enemy” and “having been in communication with a spy.” He was sentenced “to suffer death by hanging,” but U.S. officials persuaded the British to commute his sentence to life imprisonment so that he could testify against his New York spymasters. The king approved the commuted sentence.



Meanwhile, in the New York court, Wunnenberg and Sander were each sentenced to two years of penal servitude in March 1917 because of the British tips. Although they initially protested their innocence, they changed their pleas when Assistant District Attorney John C. Knox produced an unexpected piece of evidence. Department of Justice officials had seized all of Sander’s papers, including about a dozen blank sheets. Knox began to experiment with the German secret ink in front of the jury. When he dipped the blank sheets of paper in the developing solution, letters began to appear. In a few minutes he had a complete report of Sander communicating with the German master spy in Holland.



Bacon received only a one-year sentence in the United States because the judge “disliked it very much to send such a bright young man to the penitentiary.” All three spies served their sentences at the federal penitentiary in Atlanta. Bacon’s mug shot as inmate 7097 shows a man who has aged and lost his innocence since his trip to England six months earlier.