Hard rubber contains a high sulphur content activated by water or UV light and depositing as superficial discolouration; in ultrasonic baths the effect occurs within seconds. Re-colouring the material is not possible, but the sulphur film can be removed mechanically by careful polishing, although here too slightly material and thus possibly surface structures or fine engravings are removed. Older Montblanc pens can also be of hard rubber or celluloid and react similarly; celluloid can develop cracks after drying with too long water contact.