Cards are marked with invisible marks on the back to help poker cheaters and magicians identify suits or numbers easily. Besides marking cards with luminous and juice methods, poker card backs can also be marked by alcohol-based solutions or invisible ink marks. These marked cards can only be seen with a special poker device and cannot be detected by human eyes.
In the past, card marking was a laborious process. Cheaters could mark a deck of cards by making visible crimps and bends on the cards, or by using engravings that resemble Braille script. They would even add tiny bumps, called blisters, on the cards. These are still used by some players to gain an advantage over their opponents. Moreover, they can use certain inks or pigments to change the designs on the card backs and add or remove lines, as well as scratch and tear the cards.
Nowadays, the production of card marking technology has greatly improved. Many manufacturers are now producing highly concealed marking systems for cards. The most sophisticated systems are embedded into the card’s back pattern design, and only the cheater knows about them. Other less-obvious ways to mark cards include the addition of a specific odor, or the application of a chemical that makes the cards glow in the dark.
A very practical and popular method of marking playing cards is the block-out work. This technique involves using the same color ink as the card’s back color and coloring in small white areas of the card. This technique is extremely practical for magic shows and card games that are not affected by the riffle test. However, it is a dangerous way to mark cards in a real poker game because the traces of coloring can be detected by players at close range.