Macaca Mulatta

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Nearly every young child is fascinated with the idea of invisible ink. The thought of being able to write messages that can’t be read by anyone except the intended recipient inspires the imagination of the child who plays at being a spy, a hunter of secret treasures, or other such fantastic games. One of the most effective methods for children to create and use invisible ink is also one of the easiest to accomplish.

The simplest form of invisible ink is simply lemon juice. You can use a fine paintbrush as your writing implement. Just dip the brush into the lemon juice and write away on ordinary paper. Lemon juice dries almost completely clear or invisible, but it you use bright white paper, like typing paper, the writing might still be just visible enough to make it out. Using off-white or colored paper can help hide the invisible writing more completely.

Another trick is to write your secret message in lemon juice invisible ink, while writing some ordinary message in regular pen or pencil over the top of the invisible ink message. Then, if the message is intercepted, anyone who sees it will think the regular ordinary ink message is the only one. If they still think it’s a secret message, they’ll probably spend hours trying to rearrange the letters to find your secret message never realizing that the pen or pencil message is just camouflage for the invisible ink message.

To reveal the secret message, a thermal reaction is required. The paper must be heated. The application of heat causes the lemon juice to turn dark and visible. Although, it may be tempting to use a candle or a match to heat the paper, getting the paper hot enough this way without setting it on fire can be tricky. For safety’s sake, and to eliminate the chance of burning the message, the thermal reaction in the invisible ink can be triggered with the heat from a 100 watt light bulb. Since heat rises, just place the paper over the top of the light bulb and move it slowly until the entire message is revealed. It may take a few seconds for the paper to heat up enough to reveal the message so be patient.

This kind of primitive invisible ink is a lot like modern thermal paper. Today, thermal paper contains ink already on the paper and in order to make it appear, heat is applied in the pattern of the letters that are desired. Thermal paper rolls are used for such things as cash register receipts, credit card slips, and the tiny receipts from gas pumps. Unlike the lemon juice invisible ink, however, the ink on thermal paper covers the entire sheet so anything can be printed by applying heat in the form of a new message. This way, the thermal printer does not need to have an ink cartridge at all, making it cheaper to produce and longer lasting since it never needs to be refilled with a ink cartridge and has fewer moving parts than a conventional printer.