Spying during the American Revolution consisted of a complicated system of hidden networks, interpersonal relationships, scientific knowledge, personal cunning, guile and risk taking. They used ciphers and codes in letters so that if the letter was intercepted, the British or the colonists, (whomever intercepted the letter) would have to try to decipher it which would take forever. The ciphers would represent and replace existing letters in the alphabet. Sometimes they would use invisible ink, which is ink that you can’t see, and it was made from ferrous sulfate and water at the time because those were the most abundant supplies they had. 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.
That’s just some about the spy system in the Revolutionary war, but there is a lot more!