Did you know?
You can even write a secret message on a hard-boiled egg, using invisible ink. Simply write your message using a mixture of alum and vinegar on the shell. To reveal the message, just peel the egg.
Invisible ink is a substance that, when used to write on paper, is not visible initially, but gets uncovered after a certain process is completed. Invisible ink can be divided into different types based on the revealing process or what the source of the ink is. Based on the revealing process, invisible ink can be divided into those that are revealed by heating, those that are uncovered by chemical interactions, and those that are visible under ultraviolet light. Depending on the source, invisible inks can be categorized into organic inks, such as lemon juice, vinegar, body fluids, etc., and sympathetic inks that require addition of specific chemical reagents to be visible. An example of a sympathetic ink is iron sulfate, which needs a subsequent coat of potassium cyanate to be uncovered.
- Take equal amounts of water and baking soda, and mix them into a paste.
- Dip a cotton swab into this, and write the message on a sheet of paper.
- Let it dry for a while.
- You can then reveal the message in two ways:
- Directly hold the paper over a candle flame (be careful not to burn it) or a light bulb, and the message will appear in brown color.
- Take grape juice concentrate and using a cotton swab, paintbrush, or a sponge, spread it over the paper. The acidic grape juice and alkaline soda react, and the message appears in purple color.
- Squeeze a lemon in a bowl. You can add a few drops of water if you want.
- Dip a feather tip, stick, or cotton swab into the bowl, and write the message on a sheet of paper.
- Allow the paper to dry.
- Hold the paper near a candle flame, bulb, or some other heat source, and the message will appear brown in color.
- Alternatively, you can sprinkle salt all over the writing, allow it to dry, and then color the whole page with a wax crayon. The message will then be visible.
- To make invisible ink that glows in UV light, take any of the above substances. Prepare a consistency that can be applied to paper.
- Using a paintbrush or cotton swab, write any message on the sheet of paper. Words written on dark paper will have better contrast.
- Allow the paper to dry.
- Turn of the lights or go to a dark room, and hold a black light near the paper. The message will glow.
So now you know how to make homemade invisible ink using common household ingredients. If you are wondering why the ink reveals itself after heating, it's because the ink weakens the paper fibers, and these weak fibers are the first ones to darken on the application of heat. In case of ink that is uncovered by UV light, it's because the ink absorbs the ultraviolet wavelengths and exudes visible light, which can be easily seen by an observer in a dark room.