I've got some educational film strips and Tru-vue stereo rolls from the 40s. The pictures were taken late 30s - early 40s. They have a thick film base which is yellow towards amber. It's marked Kodak safety film. Is that color normal for film of that age? Perhaps it was toned that way deliberately? I wonder if they used hardener or some special thick film base? Or some kind of lamination even. Now that I do a lot of reversal I'd like to make my own half frame strips so I'm especially interested in hardeners or ways to make the film resilient to handling.