Card Marking Ink

Card marking is the process of altering playing cards in a method only apparent to marker or conspirator, such as by bending or adding visible marks to a card. This allows different methods for card sharps to cheat or may be used for magic tricks. This comes from Card marking - Wikipedia (https://en.wikipedia.org/wiki/Card_marking).
And for the card marking ink, it refers to the invisible ink that is used to mark the playing cards. The invisible ink may include different kinds of invisible juice that can mark on the back of playing cards. Usually this invisible ink marks are invisible to naked eyes but luminous for special infrared contact lenses to see.  
Many guys have interest in marking playing cards with invisible ink or card marking ink. But not all people can do that successfully. That's because the card marking ink are mixed with several kinds of different ink so as to mark different color playing cards. It's difficult to mark the cards well with only one kind invisible ink or juice.