One form of secret writing used by both the British and American armies was invisible ink. Invisible ink, at the time of the Revolutionary War, usually consisted of a mixture of ferrous sulfate and water. The secret writing was placed between the lines of an innocent letter, in case they were intercepted by the enemy army, and could be discerned by treating the letter with heat by placing the paper over the flame of a candle or by treating it with a chemical reagent such as sodium carbonate. John André gave instructions to British spies to mark their letters written in invisible ink with a ‘F’ for fire and ‘A’ for acid, so that the reader knew whether to use heat or a chemical solution to read the letter. Note the ‘A’ at the top of John Andre’s letter to Henry Clinton of 1780. Benjamin Tallmadge also added invisible ink to his resources after the British captured some of his letters from the Culper Gang in 1779.
Letters written in invisible ink needed special care; water or other liquids could smear the invisible ink and make it impossible to read. The Rebel Army used invisible ink frequently to report to George Washington. There are some examples in the Library of Congress’ collection of Washington’s papers. Sir Henry Clinton’s spy network did not use invisible ink as frequently, but we have included one example in our exhibit. The letter written by Benjamin Thompson to an unidentified person was written in invisible ink. The paper was either heated by flame or treated with acid, therefore the paper is very brittle and dark.