A printing method which comprises the steps of placing a book of blank sheets for slip forms (1), the sheets being coated with ink (6) containing a pigment C1 or a pigment precursor C2 and a reaction initiator F; placing on the book a plate (9) having radiation-transparent areas (12) corresponding to a pattern (7) to be printed; and applying X-rays B to the plate (9) so as to print the letter pattern (7) on a predetermined region (5) of the slip forms (1).
