I work in the photo dept of a state history museum & we print old negs and plates from both our collection and sometimes the larger one housed within the state archives (they have their own labs as well). My advice is pretty simple--leave the negs alone. Don't rewet them or attempt to rewash them in any way. You run a high risk of making things worse. handle them with cotton gloves to protect them from your skin oils, and if neccessary use a gentle air or a brush to clean the dust off them.
there are three types of film bases besides glass plates, that you'll most likely encounter. two of them--nitrate and acetate--have a host of problems as they get older and start to naturally deteriorate. There's not much that you can do about this besides duping and/or making prints and shooting copynegs. you can't reverse this deterioration, but you can slow it down by the right storage methods. As they go--they exhibit all sorts of weird surface blemishes and stains-- the acetate (safety film) will shrink--buckles and warps the emulsion. The nitrate becomes brittle, turns sticky and then eventually turns to an acidic dust, and can be hazardous to everything around it including you.
It is possible to remove some of these stains, but I'm not a conservator, and I would never really attempt it, much less be allowed to do it at work. The acetate negs can be salvaged by stripping the emulsion off in a chemical bath and then floating it onto a new base--but from what I understand from the archives, it's an expensive procedure and there's no guarantee either. The nitrate can become water soluable at a certain point--so just like any of these old photos or negs--rewetting them and/or reprocessing them is asking for trouble if you don't know what you're doing.
fwiw--they way I print them is on a coldhead. we use glass carriers quite a bit, because few of these negs will lay flat even in a neg-a-flat carrier. you quite often have to use split filter printing and flash the paper to handle the densities and the contrasts of these old negs to match modern papers.
If you have glass plates, these have a whole set of special handling considerations and they can be very difficult to work with because they become brittle and the emulsion often flakes off. It's almost better, imho, to leave the dirt and dust that's embedded onto the emulsions alone. I really worry about making things worse--but like I said, I'm not allowed to do anything but print them or duplicate them at work.....
Books:
best book, but out of print:
"Collection, Use and Care of Historical Photographs" by Robert A. Weinstein and Larry Booth. published by the AASLH (American Assoc for Sate and Local History), 1977. ISBN-O-910050-21-X.
"The Care of Photographs" Siegfried Rempel, 1987, ISBN-0-941130-48-7
"Conservation of Photographs" Kodak, pub. F-40 (Cat # 193 5725)
ISBN- 0-87985-352-2
links:
NPS museum handbook:
http://www.cr.nps.gov/museum/publications/MHI/AppendM.pdf
CoOL--conservation online. do a search through the archives of the mailing list here titled, Consdistlist. You can also try to dig through the Smithsonian's site for the CAL lab, for handling & storage methods, but this is mostly geared towards paper (I did attend a seminar here several years ago about managing photo collections that was a joint smithsonian, IPI, National Archives thing and it was well worth the money--they do a similar one now at the George Eastman House, if you're really interested).
http://palimpsest.stanford.edu/
The Image Permanence Institute:
http://www.imagepermanenceinstitute.org/sub_pages/8contents.htm
-- Download 2 things to help--the Acetate base Film Guide and the Preservation Calculator software for tracking the temp/rh for your film storage. The whole site is loaded with useful information though.
my advice--every state gov't has an archive that handles it's public records. You should be able to get advice on storage and handling as a patron. In the dept I work for they actually hold a seminar on caring for family photographs, and I'll bet most other states do something similar. You should be able to get this advice for free as a patron.
good luck--
KT