Ciphers and secret codes were used to ensure that the contents of a letter could not be understood if correspondence was captured. In ciphers, letters were used to represent and replace other letters to mask the true message of the missive. The letter’s recipient utilized a key--which referenced corresponding pages and letters from a well-known book, such as Entick's Dictionary--to decode the document’s true message. Some spy groups even created their own pocket guide to serve as a cipher’s key. Similarly, some letters were written in intricate secret codes where numbers and special characters replaced letters, a method most notably practiced by the Culper Spy Ring.
One form of secret writing used by both the British and American armies was invisible ink. During the Revolutionary War invisible ink usually consisted of a mixture of ferrous sulfate and water. The secret writing was placed between the lines of an innocent letter and could be discerned by treating the letter with heat or a chemical substance. The recipient placed the paper over the flame of a candle or treated it with a chemical reagent, such as sodium carbonate, which would reveal the letter’s hidden contents.
Acquiring intelligence about troop movements, supplies, and battle plans was General Washington's highest priority. Because such field reports could not be overtly communicated to him, placing his agents at great risk, Washington used an 18th-century form of invisible ink known as “sympathetic stain.”.
Learn more about invisible ink and read a secret message in this letter from Benjamin Thompson.
James Jay, the brother of John Jay and a physician practicing in England at the time, created a chemical solution out of tannic acid to be used as an invisible ink, and supplied quantities of the stain to the colonists. George Washington himself instructed his agents in the use of what was referred to as the "sympathetic stain," noting that the ink "will not only render. . .communications less exposed to detection, but relieve the fears of such persons as may be entrusted in its conveyance." Washington suggested that reports could be written in the invisible ink "on the blank leaves of a pamphlet. . . a common pocket book, or on the blank leaves at each end of registers, almanacks, or any publication or book of small value.”
British spies placed rolled up letters and small notes into a variety of holsters to hide potentially sensitive information. The hollowed out quills of large feathers that were used as writing utensils, for example, could hide a tightly rolled up letter. Other materials were used to hide messages, ranging from buttons on a textile to hollowed out small, silver balls. One particularly unlucky British spy named Daniel Taylor was caught in New Windsor, New York with a message sent from Henry Clinton to John Burgoyne hidden inside one of these small silver balls. In haste, the spy swallowed the silver ball to avoid detection. However, Patriot soldiers forced the spy to drink a purgative and vomit up the ball. Momentarily undeterred, Taylor grabbed the ball and swallowed it again. Under the threat of being hanged and having the ball cut out of his stomach, Taylor relented. However, Taylor would eventually meet the cruel fate of the gallows, executed on October 16, 1777.
The true contents of letters were also hidden through the use of mask letters. These documents were intended to be viewed by a recipient who would place a shaped template over the full letter. The true message of the letter would then appear within the boundaries of the “mask.” The letter and the “mask” were usually delivered by separate couriers to ensure that the trick would go undetected.
This mask letter from Sir Henry Clinton utilized the Cardan system. Read the primary source (Clements Library, University of Michigan)