Greybeard,
Thank you for your reply. Actually, I have taken a sample roll from the group I mention, put it into a long Printfile sleeve (the length of the roll). I then took the sleeve and wound it "backwards", emulsion out against the curl, around a small tube (like a paper towel core), left it for months, and when removing it, it returned almost immediately to the curled condition. I assumed flat would do no better.
I'll try the heat.