unmounting old b&w prints

When looking at an old family photo album with pictures from the 1920s
and 1930s I noticed that many photos have writing on the back that I
would like to read. I was able to slowly unmount some photos that were
glued to a the heavy black paper sheets of the album, and the writing
identified the people in the photo and sometimes the date of the photo.
Unfortunately, some photos cannot be removed without damaging the
photo. I previously scanned all photos at 300 dpi to preserve the images
but attempts to unmount some photos leaves remants of the back of the
photo attached to the sheets of the album. I have tried heating the
underside of the album sheets with a hair dryer but the glue does not
soften when heated. The commercial product "un-du" doesn't soften the
glue that was used to mount these photos. I would appreciate receiving
any advice about how to soften the glue that holds the photos to the
album sheets, so that I can gently unmount the photos from the album
sheets. Info about any relevant web pages or books would be particularly
helpful to me. Thanks.