Mystery Writing Appears Eerily In Full View

Here's an exhibition that will appeal to those intrigued by the world of spirit phenomena. If not, then simply attempt to solve the riddle it poses.
A member of the audience is given a free choice in their selection of a playing card.
The magician then introduces another card, this time depicting a small writing slate. By simply holding this card against the side of a tea-cup, the value and suit of the chosen card mysteriously appears on the slate. Just as strangely, the message fades from view, leaving the slate empty once again. Was it an illusion?