First of all, always be safe. If you don’t have to, don’t write on the photo. One way to do this is to place the valuable photo in an archival clear envelope and write on the envelope. But most likely that won’t happen, so as an alternative, make sure you write along the edges in the margins or on the back, not in the middle. Too many photos have come through my office with embossed ball point pen and pencil dents, coming right through the photo, not good! Remember to write on a hard surface, glass, smooth formica, granite, etc., are the best. Some wood surfaces or card tables are usually not the best.
Do not use writable Scotch tape or the glue might migrate through and or become messy, a lot depends on the photo paper type. A soft pencil or ball point pen will smear easily and can dent the paper if not done carefully. A fine “Sharpie” or “gel based pen” should do. The ink will dry quickly and not bleed through. Also pens identified as “photo-safe” will not migrate through the photopaper.
There are several factors to take into consideration:
- Acid free ink.
- Bleed free ink that won’t leach into the paper.
- No pressure required pens. (Ballpoints require pressure to get the ink to flow, damaging paper fibers and allowing the ink to get into the very pores of the paper).
You can obtain acid free ink pens of the correct type through archival supply houses and scrap-booking/craft stores.