There are basically three types of invisible ink. Each type is defined by how the message can be developed in order to be seen.
1. Heat: The fluid used oxidizes when heated (that is, the compounds that make up the liquid lose some electrons therefore changing their chemical makeup), and this oxidation turns the compounds brown. Acids work well for this type of ink, because they not only may oxidize themselves, but they change the chemical makeup of the paper also, causing it to burn and char more easily.
Paper is made up of a compound called cellulose, which comes from wood and consists of a long chain of linked sugar molecules. The cellulose fibers are pressed together and dried, leaving a thin flexible sheet behind. This flexible sheet is your sheet of paper.
Here’s what you’ll need:
1. Lemon juice, at least one tablespoon
2. Cotton-tipped applicators or a small paint brush
3. White sheets of paper
4. A cloths iron, one with a high setting like linen
5. An ironing board or a bath towel
Here’s what to do:
1. Dip the cotton applicator in the lemon juice.
2. Write a message on a piece of paper using the juice. Don’t use too much juice, or the paper will get wrinkled spots!
3. Allow the paper to dry completely. The message should not be visible.
4. Place your paper with the message face up on your ironing board, or on the bath towel on top of your kitchen table.
4. Once the paper is dry, use your iron on the linen setting to heat up the paper. The message should turn darker and become readable!
How long did it take to make your message appear? Is it easy to read? How could you improve your message? How could you make it harder for others to find your message? Remember to write down all your observations!
HERE ARE SOME OTHER PROJECTS TO TRY:
What other things could make good invisible ink? Sugar water (remember that sugar caramelizes)? Milk? There are records of war prisoners even using their own saliva and sweat as invisible ink!
If you have a black light, try using laundry detergent as invisible ink by mixing it with some water. Write your message, and view the message under the black light. The laundry detergent should fluoresce and show the message!
What else could you write messages on? How about light colored fabric? Be sure to use fabric from an old T-shirt or dish towel or handkerchief—something nobody wants to keep any more!
Reference for further reading:
Macrakis, K; Bell, E. B.; Perry, D. L.; Sweeder, R. D. (2012) “Invisible Ink Revealed: Concept, Context and Principles of “Cold War” Writing”, Journal of Chemical Education, 89(4) 529-532.