Abstract: A thermal marking device for surfaces of binder-containing webs of mineral wool includes, mounted on a raisable and lowerable mounting frame, a roller which has on its periphery electrically heatable marking elements. The marking elements carry image panels, the marking sections of which take the form of letters, symbols, pictorial represenations etc. and project at least 5 mm beyond the surface of the image panel. Furthermore, the envelope curve of the surface of the marking sections corresponds substantially to the outer surface of an annular cylinder, the axis of which coincides with the axis with the axis of the roller. Furthermore, the marking device is equipped with heating elements, the heating output from which can be locally and variously adjusted. As a result, it is possible to produce homogeneous marked patterns by a suitably reduced or increased supply of energy at locations having need for heat output while creating non-homogeneous decomposition patterns.