For this you will need: Normal drinking soda, a little warm water, a spoon, a toothpick with cotton wool (you can also use a cotton swab or a refillable fountain pen), a sheet of paper and a candle.
Step 1. Drinking soda must be diluted in water so that a concentrated solution is obtained. It is necessary to allow the mixture to cool slightly. It is not necessary to achieve complete dissolution of soda.
Step 2. If there is a fountain pen that can be seasoned with freshly prepared solution.
Then we use it. If you don’t have such a device at hand, you can use a cotton swab or a regular toothpick with a piece of cotton at the end.
It is necessary to dip it in an aqueous solution of soda, squeeze slightly. After that - you can safely write a message on a blank sheet of paper.
Step 3. It is advisable to let the ink dry a little before proceeding with the development of the letter. In order for the secret record to become visible, the sheet with it must be held above the heat. It can be an iron, gas or electric stove, but in our case an ordinary candle was used. To prevent the paper from catching fire, you need to lightly drive it above the flame.
Step 4. If everything is done correctly, then after about 30-40 seconds an inscription of a yellowish-brownish tint will appear. Done! The inscription is displayed. You can also heat a sheet with a secret recording on the battery, but it will take much longer, and the recording itself will not be so bright.
It is also possible to use lemon juice or concentrated vinegar instead of a soda solution, however, these substances leave slightly visible traces on the paper, which no longer corresponds to the "secret" stamp.