We have a couple of conservators who are members of
RP. Hopefully, one of them will see the thread, and offer suggestions on cleaning and historical value. In the meantime, be aware that there were different processes, and certain types of images are affected by different things. The first thing you need to do is to identify the process, then do a little research before putting any chemical or cleaning solution on the image. There are many things that can be put on antique images, that won't show adverse effects immediately. But they might completely destroy the image in time. So before you do any cleaning, it is my suggestion that you know what you are about to do before you do it. You might want to check out the historical processes threads, which were submitted by Thomasgeorge. I'm not sure if there is any information about cleaning artifacts, but the reading is worth the effort. I think a soft gum eraser can be used carefully to remove some dirt from emulsions, but be very careful.
Ed