One form of secret writing used by both the British and American armies was invisible ink. 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 which would reveal the letter’s hidden contents.
Invisible ink dates all the way back to the ancient Romans and Greeks. Secret writing was first done using liquids such as vinegar, milk, or lemon juice, which weaken the fibers of paper. When gentle heat is later applied to the page, the weakened fibers darken first, and the secret writing is revealed.
In addition to writing secret messages between the lines of letters about pedestrian topics, Washington also advised his agents to write on the pages of common items such as almanacs, books, and pamphlets. The high quality of the paper in such items supported secret writing especially well, and the items themselves could pass through military checkpoints without suspicion.
Most invisible inks are really very simple and designed so that when the component parts are discovered, they raise no suspicion. For example, the one very common invisible ink was to remove the tannin (oak gall) from ordinary ink rendering it colorless. Typically, the writer would pen his message in tea (what could be more ordinary in “British America?”) which would dry the same color as parchment then the recipient would brush the message with a solution of green vitriol (FeSO4) a common medication for anemia to make it develop into black ink. Having either of these in your possession should raise minimal suspicion.
Simple Recipes for “Invisible” ink
Lemon Juice as Ink
- Write you message in lemon juice and allow to dry. ß Should be invisible
- Reveal the writing by holding the paper over a source of heat (like a candle or dry iron). Writing will be revealed as light brown.
Milk and Graphite
- Write your message in milk and allow to dry. ß Should be invisible
- To reveal the writing, rub with powdered (not solid pencil) graphite. The writing will cling to the graphite so you can read the message.
These methods have the disadvantage of being easily revealed by subjecting the document to heat. When suspicious messages were intercepted by the enemy, this would often be done to screen for hidden messages (both the British and Americans used invisible inks). To avoid this detection, some messages were written in “reagent” inks that required two compounds to reveal the message.
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.”
Here are two simple example reagent inks:
Culper Reagent Ink
- Write your message in tea and allow to dry. This should be invisible
- Reveal the writing by brushing with a solution of ferrous sulfate (FeSO4). Writing will be revealed as dark brown.
Turmeric Ink
- Write you message with a solution of baking soda (NaHCO3). This should be invisible
- Reveal the writing by brushing a solution of turmeric dissolved in alcohol. Writing will be revealed as blue on a background of orange.
Another approach to hiding a message is to literally keep in in a shell.
Message in an egg
- Mix alum (a chemical which used to be used in pickling) with vinegar.
- Write with this mixture on the shell of a hard-boiled egg.
- Heat the egg gently to “fix” the message.
- Reveal the message by simply peeling the egg. Writing will be dark brown on the white albumen of the egg.
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Michael Carver -- Historical Interpreter/Reenactor 