Invisible Ink Techniques for Card Marking: A Closer Look

Card marking is a form steganography where invisible marks are written onto the surface of documents or other objects. These marks can then be made visible through a process, usually using heat or ultraviolet light. The invisible ink is a liquid that is applied with a special pen, stamp, fountain pens, toothpicks, cotton swabs, or similar items dipped in the liquid. This ink then dries.

After the invisible ink is written on the paper, a developer is applied to reveal the message. This may be a simple solution such as lemon juice, or it can be more complex chemical reagents. In order to use invisible ink, the writer must take care not to raise suspicion. Telltale signs can alert a censor, such as scratches caused by a pen, roughness in the paper or a difference in reflectivity due to the ink.

When you need to send a secret message, invisible ink is a useful tool. It is easy to make, and you can use ingredients that are probably already in your house. You will need a pen and a shallow bowl or dish, white paper, and an acidic liquid such as vinegar, lemon juice or milk to make your own invisible pens. You will also require Q-tips, and a heat source such as an iron or radiator.