Writing secret notes with lemon juice was one of my favorite pastimes as a child. All it took was a small flame to lightly scorch the paper and reveal the hidden message. Now that I'm tall and pay bills, lemon ink just isn't exciting enough anymore. Luckily, we can use another kind of invisible ink to write in fire! By using the saltpeter, we can whip up invisible fire ink in no time.
- Saltpeter (aka potassium nitrate)
- Thick paper
- Cotton swab or paintbrush
- Match or lighter
You can buy potassium nitrate as stump remover at most home improvement stores. You can also make your own from instant cold packs and salt substitute. Make sure to properly label your jars of chemicals!
Step 1 Mix the Ink
Mix your saltpeter with a tiny amount of water until it has the consistency of a thin paint. It's okay if the mixture is clear; we want it to be invisible ink. Thinner mixtures are less visible on paper; thicker mixtures burn more ferociously.
The dark area is just a rust stain.
Step 2 Write the Secret Message
Write your message on the thick piece of paper with your cotton swab (or paintbrush). Start the message on the very edge of the paper; you need an easy starting point for the fire. Apply the ink generously to ensure good results. All parts of your message must be connected in one continuous line of ink. Cursive works well for this.
Thick papers like card stock and construction paper smolder the best.
Go over your design several times to ensure an even coat.
Step 3 Dry the Message
Allow the ink to dry thoroughly. Be sure to remember which edge your writing started on. It can be helpful to make a small mark with a pencil. This mixture was too thick and left a white crystal film when it dried.
This spiral was almost completely invisible.
Make sure to mark where your design meets the edge of the paper.
This is where you will light the ink.
Step 4 Light the Message
In a nonflammable environment, outdoors away from sentient creatures, bring a flame to the edge of the paper where your secret message begins.
The potassium nitrate embedded in the paper will smoulder and burn away your ink in the pattern of your secret message. Longer messages take a while to reveal themselves, so be patient. No one said it was instantaneous.
What secret messages will you write? There is a lot of potential for fast motion video here. If you try out this experiment, take some pictures or videos and post them up on the corkboard. If you need assistance, start a thread in the forum or message me personally. I am happy to help!