Total Amateur question

What is a silver gelatin photograph? How do you tell if it silver gelatin? and does that type affect the value?

 

Silver gelatin are the most common black and white photographs. It's the type of film used for snapshots before color. Of course, it was used professionally as well. Silver salts were embedded in a thin gelatin layer applied to a clear acetate film for negatives, or an opaque paper for positives.

From a purely artistic standpoint, I don't think the medium should be the determining factor, but there were (are) many different media and techniques that can be used to produce photographs. Some of them are more obscure than others and no doubt there are people who collect on that basis.

Some techniques - ambrotypes, tintypes, etc. - may be indicative of antique periods, but I'll bet you could find modern practitioners of any given method.

 

Gelatin emulsions (the layer that carries the silver halides) have been used since the 1880's on both negatives (glass, and later various polymer films) and paper prints, as moreotherstuff has said. Gelatin remains somewhat sensitive to water, unlike the earlier collodion and albumen emulsions. Under magnification, gelatin may be seen to swell slightly if a drop of water is placed on it.

 

The Graphics Atlas (https://www.graphicsatlas.org/) from the Image Permanence Institute is an excellent site with detailed information on identification of all kinds of prints. They even have images as they appear microscopically! (probably more info than you would want, but...) And you can also compare two types of prints using their "compare" feature if you are trying to decide which type you have.

Here is their information on silver gelatin photographic prints:
http://www.graphicsatlas.org/identification/?process_id=266