Marking cards with invisible ink is a technique used by magicians and con artists to cheat in card games. These markings are often invisible to the naked eye, but can be detected by specialized gadgets and cameras.
Several types of invisible ink can be used to mark cards, including heat-activated inks that reveal their messages when heated or irradiated. Other types of ink are chemically based, such as sodium carbonate solutions, which react with acid to reveal the message.
Some common organic fluids can be used to create a message, including clear soda, lemon juice, apple juice, onion juice, wine or vinegar, milk, soapy water, sugar water, and even bodily fluids like blood, saliva, and urine. Many of these inks can be activated by heat, so they are a popular choice for people who want to write a secret message on paper.
Invisible inks that can be revealed by heat are easier to use than those based on acids or bases, but they are more difficult to make work and require extra care. They must be kept a low temperature, such as a hot plate, and they should not be ironed or burned, because this could cause the ink to break down and make the message visible.
Another type of ink is a liquid that is formulated to change its color when exposed to a certain kind of light. These inks are sometimes called UV inks because they glow under ultraviolet (UV) light sources. These inks are commonly used for security purposes and can be found in pens and other devices that contain them.
The chemical composition of inks can vary, so they are best suited to specific tasks and applications. Some inks are easily developed by adding iodine or other typical developers, while others require a more complex mixture of chemicals and can take several days to develop.
Choosing a combination of ingredients is the key to creating a good invisible ink. The ideal ink should be nonreactive with iodine, and it should also be easy to find.
One way to get started is to ask students to research the materials that they can find in their kitchens or around their homes. They can use these as a springboard to explore other kinds of inks that may be more practical for their projects.
They can then discuss the differences between different kinds of inks and choose an ink that works best for their project. They can then combine different types of inks to form a blend that is both luminous and invisible.
These blends can be incorporated into a deck of marked cards and used to create the secret messages on the back of the cards. This is the most common method for marking cards and is very practical for magicians, who can make their magic much more effective.
Other methods include block-out and cutting. Block-out is a popular method for marking cards, which involves blocking out the back of the card with colored ink. This method has limitations, however, because the traces of coloring can be easily seen by cheaters at close range. Cutting is a less-practical method, since it requires more skill and can be easily detected.