One piece of advice that I always give to my researchers at the Lambton County Archives is to identify the individuals in their photographs. In your personal photograph collection, you may know the names of all the aunts, uncles, and cousins, but unless you document this information, that knowledge is lost to anyone else accessing your material. You should record as much information as possible, including the names of the individuals in the paper, the date the picture was taken, where the picture was taken, and if applicable, the special occasion.
There are established strategies in the archival community for safely recording information on photographs. If you follow these rules, you can minimize the impact of recording important information on your photographs.
My preferred method is to not write on photographs at all, but rather place them in an envelope or sleeve that contains the relevant information. This way, you can label your photographs with the detailed information that you need without physically inscribing on the photograph itself. Through vendors such as Carr McLean, Brodart, and Gaylord, you can purchase transparent photographic sleeves or acid-free envelopes that will safely hold your photographs. By applying a label to the sleeve or writing on the envelope, you can record your information and avoid marking the original photograph. This is the best case scenario and a win-win for your priceless photographs!
There are reasonable options if you do want to inscribe information directly on the photograph. For many older photographs, you can use a soft lead pencil to record your information. Writing on the back of the photograph, keep your letters small and neat to minimize the impact, and make your markings lightly. Pencil can be erased, so by using pencil you are not permanently altering the photograph.
For modern, glossy photographs, pencil often will not stick or mark clearly on the back of the photographs, and it is not a good option. There are many products on the market advertised as “photo safe archival pens” that will write clearly on these types of photographs, but I do not recommend ever permanently marking a photograph. In many cases, the inks will smear, and sometimes bleed through to the front of the photograph. Additionally, you cannot change the information after you have marked it down in permanent ink, unless you want to scribble out what you wrote previously! The Stabilo-All pencil is a good option that will write on the glossy photographs but can be dry erased or wiped off of smooth surfaces. The Stabilo-All pencils are similar to a pencil crayon, and you can find them for purchase online. I have found that in some instances even the Stabilo-All pencils will not write on glossy photographs, and in those cases I use the method of putting the photograph in a sleeve or envelope and applying my labeling to this protective cover.
I cannot overemphasize the importance of documenting your photographs, and minimizing the impact of that documentation by following these guidelines.