Some moments happen only once in a lifetime. Treating your photographs correctly can help preserve those moments. To best maintain your photograph, you should only write on it with an archival pen or marker, especially if you wish to write on the front. Other types of ink may not stick to glossy surfaces or may fade or smear or contain chemicals that will shorten the life of the photograph.
All paper naturally deteriorates over time, including photo paper, but acid accelerates this process. When choosing a pen, make sure it says "acid-free" somewhere on the barrel. It may also say "archival-safe" or "photo-safe." These three phrases all mean that the ink does not include any harmful acids. These pens can be found in the scrapbook section of most hobby stores.
Inks are made from either dyes or pigments. Dyes are dissolved in water, oil or alcohol and tend to fade over time. Pigments do not dissolve or rely on a solvent for color intensity. Because of this, they resist fading better than dyes. For ultimate fade resistance, however, a pigment must also be lightfast, which means that it resists fading upon exposure to light, especially ultraviolet light. For the longest lasting results, write on your photograph with a pen based on a lightfast pigment.
Some inks are sucked through the paper's pores away from the original contact point. This creates an undesirable effect called "bleeding." Bleeding may make handwriting illegible and diminishes its aesthetic quality. For best results, choose a bleedproof pen.
If a pen is not waterproof, it can smear or run if it comes into contact with moisture. Even the sweat of someone's hand may be enough to lift the ink from the photo's surface. For this reason, always write with a waterproof pen.
The Photographic Activity Test, or PAT, tests the interaction between a substance, such as an ink, and a photograph. Pens or markers that have passed the PAT are guaranteed to be of archival quality and are safe to use on photographs. Just because a pen claims to be archival does not mean that it has been subjected to the PAT, however, so when choosing a pen or marker, check the back of the package. If the package indicates that it is PAT approved, it is guaranteed to be of genuine archival quality.