Doesn't always work. I've got old Agfa negatives I developed from my grandpa in Korea in the 50s. Developed them after finding them in his attic maybe 10 years ago now. They sat between two panes of glass for 5 years with volumes of books on top, not a single change toward flat. I finally gave up and just put them in the archival sleeves and in the binder with the curve.
Professional drum scanners would immerse the film in mineral oil. Epson used to ship a kit for their flatbeds. Popular with 8x10 photographers.