Eleven Russians were arrested this week and accused of spying in the United States. The group is said to have employed seemingly outdated instruments for their espionage. According to the Justice Department's criminal complaint, one suspect told another that he would transmit a message to her "in invisible." Do spies use the same invisible ink you can get at toy stores?
Pretty much. There are three main techniques for making invisible ink, all of which have been used by toy manufacturers and spies alike. The best known method is to write a message on a sheet of paper using an acidic substance like lemon juice. The acid weakens the fibers of the paper such that the writing darkens when exposed to heat. The second technique develops the ink via chemical reaction. For example, the chemical thymolphthalein can be used to make a blue ink that quickly turns colorless. It turns visible again if you submerge it in a chemical with high pH. The third technique is to use liquid that glows when exposed to UV light. That could be anything from organic substances like milk or semen to fluorescent chemicals like laundry detergent.
The CIA has acknowledged in court papers that American spies do use invisible ink, but the agency won't disclose which types are in play. Unless the government has made some secret breakthroughs in invisible-ink technology, however, the kind used in spycraft won't be too far off from what you'd find at a novelty store. In 1998, the CIA refused to declassify World War I-era records describing the manufacture of invisible ink and has been similarly skittish about releasing a 1945 report called the "Secret Ink Technical Manual." American researchers have discovered techniques used by some other countries. The East German Stasi, for example, used a method similar to carbon copying: Place a piece of chemical paper between two blank sheets, write a message on the top sheet, and the chemical (cerium oxalate) would be transferred onto the bottom sheet. Dip it in a solution of manganese sulfate and hydrogen peroxide and the writing turns orange.
While coded messages date back to ancient civilizations—Herodotus described one ruler tattooing a message onto his slave's shaved head, letting the hair grow back, and sending him off for the recipient to reshave—invisible ink was probably first used sometime in the 17th century. During the Revolutionary War, Gen. George Washington used disappearing ink made from oak galls—plant growths that were also used to treat hemorrhoids—to transmit messages. Few letters remain, however, because the developing process made the paper so brittle and because the messages were often destroyed for privacy reasons. During the Civil War, the Union spy Elizabeth Van Lew sent dispatches to Maj. Gen. Benjamin "Beast" Butler using a cipher often written in invisible ink. During WWII, the American singer and actress Josephine Baker carried invisible messages for the French resistance written on her sheet music.
Invisible ink has since faded, so to speak, as other, more sophisticated coding technologies have become more common. Microdot technology allows the messenger to shrink a text to one-four-hundredths of its original size. Steganography, the practice of concealing a hidden message in other data, often an image, was also favored by the Russian suspects, who exchanged more than 100 digital photos with embedded text, according to the FBI.
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Explainer thanks Joseph Finder, author of Vanished; Tony Mendez of the International Spy Museum; and John A. Nagy, author of Invisible Ink: Spycraft of the American Revolution.