Brother George Washington was fascinated with the tools used in espionage and, in fact, there are books that delve into the subject. It's understandable... the man had a revolution to win. Invisible ink was one of Brother Washington's favorite tools, but it was fairly ineffective. Invisible ink of the day was made of a concoction largely comprised of lemon juice. The British were well aware of that type of invisible ink and knew all that had to be done to expose the writing was to subject it to heat. The process of heating the paper made it brittle and not many of the secret documents created that way survive today.
Realizing the fallibility of the lemon-based ink, Washington eventually used a special chemical ink which James Jay, brother of first US Chief Justice John Jay, had developed. Jay's ink was invisible until a second chemical agent revealed it, and was far more effective. This particular formula, known as "sympathetic stain," consisted of ferrous oxide (FeO) granules dissolved in water. A solution of sodium carbonate (more commonly known as baking soda) and water applied to the ink made it visible.
One rare surviving invisible ink specimen Washington sent involves another well-known aspect of his life: his troublesome teeth. It is a letter to his dentist, Dr. John Baker. In it, the General complains of a rough spot on his infamous dentures and requests the dentist send him one of his cleaning tools.
It might at first seem ridiculous Washington would be so careful as to write a such an innocuous letter to his dentist in invisible ink, but Washington knew if the letter fell into the wrong hands it would provide British intelligence with the name of a pro-American dentist, as well as Washington's location — new Windsor, New York — in the return address. The British, in fact, did intercept the letter. Accounts differ as to the ramifications of its capture. Some say they were unable to read it, and that may have saved his dentist's life, some say they decoded it and were amused by its content, others say it provided valuable information as to Washington's whereabouts.
Whatever the case, Washington made ample use of invisible ink in secret messages during the American Revolution and even had an organized spy ring, the Culpers, to, among other duties, deliver those messages. George Washington's most important secrets, it seems, had nothing to do with the Freemasons.