The marking pens serve to mark the desired position, and the mark thus applied is then pressed or ironed with a warm iron, so that the wax melts and diffuses into the cloth. After this stage, the mark is not visible to the eye. Naturally, such UV-cartridges or marking pens can be used only with woollen or wool/mixture fabrics. The wax must be absorbed by the fibres and wool does this best of all.