Stabilo-All Pencils will allow you to write clearly and legibly on the backs of photographs or film negatives, as well as on glass and metal surfaces. They are water soluble and can be wiped off of smooth surfaces with a damp cloth or erased easily when used on paper.
Applications
Writing on the backs of contemporary photographs can be tricky because they usually have a polyethylene surface. Conservators advise against using marking or ink pens, as they can transfer and are not reversible. Standard #2 graphite pencils work well on paper-backed photos, but not on plastic-coated surfaces.