My brother's apartment building burned down about a year ago. (Some drunk a$$hole in another apartment fell asleep with a cigarette in bed.)
It was a strange scene. The place was literally reduced to ashes but there were pockets of things that were barely scorched or even in perfect condition. We went in to salvage anything we could find. Inside the bottom drawer of a charred bedroom dresser was his photo collection. The boxes were scorched but many of the photos inside were intact.

We were able to recover 80% of his photo collection. Many of the ones that were damaged got scanned, digitally restored and reprinted.
I soaked a lot of the photos apart in clean water with a drop of PhotoFlo added.

Mind you, these photos were all traditional gelatin/silver photos, no older than about 1980 or 1970.
If they are alternative process (Kalitype, etc.) all bets are off.

If your photos are no older than about 1930 and they are in otherwise good condition with no mold or discoloration, careful treatment with water should do it.

Alternatively, you could try carefully steaming them off the glass.