Marked Cards are cards that have had their backs marked in a subtle way so that it’s possible to know their values without turning them over. They’re often used by cheats at card games or by magicians to perform tricks. In the past, people would physically alter the cards in order to mark them, but today, technology has greatly advanced the process. Marking cards is possible in many ways, including using an invisible ink pen or adding juice. Only a trained eye will be able to see luminous markings.
There are three main types of marks that can be made on the backs of a deck of cards: Scratching, blocking and cut-out work. Scratching is the most common method of marking. This involves either using your fingernail or a device like a lighter to remove a small amount of ink or leave a scratch on the card. This can be very difficult to detect, but it’s not impossible.
Blocking and cut-out work use the same method, but rather than simply scratching the card, they apply ink to specific areas on the backs. It could be as simple a coloring in a petal of a flower detail or more complex, such as adding a bird (blocking it out) to the back design. Cards marked with juice or in a luminous color are also used. However, they can only be detected by trained eyes or sunglasses.