Some of the older animation cels are acetate based and have yellowed with time, others are brittle, some have the paint flaking off.
They need to stabilize the acetate on some, others have turned to almost a translucent putty and those are unrecoverable at this time.
As I were, nitrate film.
Used up until the 1950’s, flammable, acidic because it is nitrocellulose/nitric acid.
The list of lost films, not at all comprehensive, has quite a few entries from the 1930’s.
Interesting subject, I’m not well versed on it as you can see by my acetate instead of nitrate film goof.