Secret ink

Secret inks are inks that are either invisible or change their properties after some time.

Secret inks were already known about 2000 years ago, which only became visible after treatment with heat. Later, inks were added that disappeared after some time or that changed color.

As old as the ink is its counterpart - the secret ink. It is also called "magic ink" or "sympathetic ink" and is used to transmit secret messages. In this case, the message is not encrypted, as is usual for spies, but written with an invisible liquid. The recipient receives a white sheet which, when heated, releases the message.

The simplest and non-toxic ingredients for secret inks can be found in any kitchen: apple juice, vinegar, milk (buttermilk), orange juice, lemon juice, sugar water or onion juice. For application on paper, feathers, wooden or cotton swabs are suitable. If you later hold the sheet in front of a lamp or heat it with an iron, the ink turns brownish.

The reason for this is the high acid content in the liquids. Acid attacks the paper, and the warmer it gets, the faster the paper ages and turns yellow. Added sugar further supports the coloring, and it turns brownish with heat.