Invisible ink is the perfect solution for when you want to write someone else a note without the risk of anyone else reading it. This is because invisible ink makes it look like you are giving someone a blank piece of paper until the recipient reveals the hidden message. Invisible ink is easy to make and is made with common household ingredients.
The lemon juice method
Step 1: Squeeze the juice of half a lemon into a bowl. If you don’t have fresh lemons at home, prepackaged lemon juice can be used instead.
Step 2: Add half a tablespoon of water to the bowl. And stir until thoroughly combined.
Step 3: Dip a cotton swab in the mixture and write a message on the paper. It is also possible to use a toothpick, paintbrush, or calligraphy pen instead of a cotton swab.
Step 4: Let the invisible ink dry for 15 minutes. As the ink dries, the message will disappear. If you want to make your message even more invisible, crumple up the piece of paper. The creases will hide where you wrote down the message.
Step 5: Heat the paper using a light bulb or candle to make the hidden message appear. Be careful when following this step, and ask an adult to supervise. Do not hold the piece of paper too close to a flame, as it could catch fire. You could also put the piece of paper between two thin towels and iron while on a low heat setting. When the piece of paper comes into contact with heat, a chemical reaction occurs. After a few minutes, the lemon juice will darken and the hidden message will be revealed.
The baking soda method
Step 1: In a small bowl, mix equal parts baking soda and water. Mix this mixture until the baking soda is completely dissolved in the water.
Step 2: Write a message using a cotton swap. Then let the invisible ink completely dry, which takes approximately 15–30 minutes. It is also possible to use a toothpick or paintbrush instead of a cotton swab.
Step 3: To reveal the hidden message, paint over the paper using grape juice. Grape juice has a natural pigment called anthocyanin. This will darken the color of the baking soda because of the pH change. You could also mix 1 teaspoon of turmeric with 120 ml of rubbing alcohol. This will turn the invisible ink red.
The laundry detergent method
Step 1: Write a message with laundry detergent using a paintbrush or cotton swab.
Step 2: Let the laundry detergent completely dry, which will take approximately 15–30 minutes. Once the paper is dry, the message will be invisible, and you can proceed to the next step.
Step 3: You can reveal the hidden message using a black light. Because of the whitening agents in the laundry detergent, your message will glow under ultraviolet light.
The white crayon method
Step 1: Use a white crayon to write a message on a piece of paper. Reframe using other paper colors as the crayon will be visible.
Step 2: Use watercolor paint to paint a thin, even layer over the message. The paint will stick to the paper but not to the crayon wax. Because of this, the message will stand out and be read by the recipient. Additionally, you can also use a dark-colored marker to color over the paper.
Sources:
https://www.wikihow.com/Make-an-Invisible-Ink-Message