You might not be drafting bomb instructions on the back of a cocktail napkin in a German bar anytime soon, but being skilled with invisible inks could definitely pay off.
Photo by Kristen D.
While you may remember invisible inks mainly from the back of comic books, wedged between X-ray specs and other questionable goods, it’s probably the most legitimate “spy tool” around. At The New in Print, a blog devoted to printing and ink technology, they’ve rounded up a variety of different kinds of invisible inks revealed by special lights, liquids, or other methods. Lemon juice, vinegar, and even water can be used to create basic “invisible” messages. How can you use lemon juice?
Lemon juice is a great example of a ‘heat-fixed’ invisible ink. An invisible lemon juice message, scrawled onto a piece of paper, can be developed by exposure to any heat source such as a radiator, an iron or a 100W light bulb.
Many acidic household liquids (Coca Cola, wine, apple, orange and onion juice included) behave in quite the same way, as do a number of bodily fluids. During the Nazi occupation of Poland, people frequently sent postcards containing hidden messages written in saliva and even urine.
While you may not be scripting in bodily fluids, it’s quite the trick to have filed away. For more substances that can be used and techniques to apply and develop them, check out the full guide below. Don’t forget to sound off in the comments if you have your own experience with nifty covert tools like invisible inks.