Who doesn’t like to send secret messages?
People have been doing it for ages. In fact Giovanni Battista Della Porta, an Italian guy who lived about 500 years ago invented what I believe is the most creative way to send a secret message.
Giovanni wrote messages on the shell of a boiled egg using a mixture of alum and plant pigments. The message seeps through the shell and can only be read by peeling the egg. How crazy is that? Imagine peeling your egg for breakfast and finding a secret message!
While I can’t tell you how to send a message via hard-boiled egg, I do have several simple, foolproof recipes for invisible ink. All of these can be written on ordinary white paper.
Milk and Pencils
Write it: Use a cotton swab or toothpick to write your message with whole (must be whole or at least 2% – won’t work with skim) milk. Let the paper dry completely.
Read it: Rub the graphite (black part) of a sharpened pencil on some sand paper over the message so that a graphite powder falls onto the paper. Gently rub the graphite over the paper until your message appears.
How it works: When your message dries on the paper, most of the milk evaporates but the fat stays on the paper. Graphite sticks to the fat more than the paper to reveal the message.
Write it: Use a cotton swab or toothpick to write your message with vinegar, milk, lemon juice – or really any fruit juice will work. Let the paper dry completely.
Read it: Heat the paper using a hair dryer, hot iron, holding it near a light bulb or warming it in the oven at 300oF for 10 minutes.
How it works: All of the liquids are mild acids that weaken the paper wherever you write your message. The acid also stays on the paper after the invisible ink has dried. When you heat the paper, the acidic message burns or turns brown before the rest of the paper.
Write it: Write your message with a white crayon.
Read it: Paint over the message with watercolors – darker colors like red, green or blue work best.
How it works: Wax repels water. This is why wax paper is so useful – it can’t get wet. Here the watercolor paints will not stick to the white crayon drawing and your message is easily visible.
Acids & Bases
Write it: Mix 2 tablespoons of baking soda with 2 tablespoons of water. Use a cotton swab or toothpick dipped in the mixture to write your message. Let the paper dry completely.
Read it: Use a cotton swab, sponge or paint brush dipped in grape juice to paint over the message.
How it works: The baking soda, a base, reacts with the grape juice, an acid, to produce a different color on the paper.
Write it: Use a cotton swab or toothpick to write your message with clear or white liquid laundry detergent. Let the paper dry completely.
Read it: Hold the paper under a black light to see the message.
How it works: Laundry detergent contains phosphors which fluoresce or glow in the ultraviolet light given off by the sun (and black lights) and make your clothes (and secret message) look bright.