These Dixon China Markers can mark on almost any porous or non-porous surface. This includes glass, plastic, film, paper, metal, and rubber. On porous surfaces, marks wipe away clean. These markers do not need a sharpener as they are paper wrapped with a tear string. Markers are available in a variety of different colors. They are also PMA certified non-toxic. If you are an environmental scientist or just someone who needs a dependable instrument to make your mark, these markers won't let you down.