Invisible ink is usually invisible to naked eyes and luminous to invisible ink reader.
Luminous ink is not an easy product to create. It's liquid but not only one kind of liquid. There are several compounds that blend together to create the invisibility.
Yet the luminous ink is green in color, once the chemical reacts with the playing cards the agent in the luminous ink "burns" through the card. Once you wipe the access surface ink away with a napkin or paper towel, the ink has done what it has intended to do. It has turned invisible. A ruby red filter is the only way to see the markings made.
We have invisible ink to sell. Our technicians try lots kinds of powder and ink to make it. It can mark the cards well especial by machine. It is OK to mark cards in a pen with such ink, though it won't be as good as that by machine. This way the consumer gets to experience the creation part and have left over ink to create even more decks (perhaps sell them). For more information on luminous ink check out our other articles.