I don't know if anyone's come up with this method 
or not, but I thought I'd like to share it.

I have a few cels that are slightly stuck to 
other layers, and a few cels that were stuck to 
their sketches.

To unstick them, I put them in a cool, dry place 
for two or three days, and then gently tried to 
unstick them. The acetate separated very easily, 
without any threat of chipping.

In all, acetate-stuck-to-acetate seemed to 
separate more easily than sketch-to-acetate. And 
it worked best for cels that were only stuck 
together in limited areas. For some cels with 
major stickage, I was able to unstick them to a 
certain point, and then had to give up or risk 
damaging the cel. I think I might try to re-cool 
them and then attempt again later.

"Cool" was about 20 F, I think. "Dry" means just 
that--- dry. Not humid. So that means don't go 
piling your cels in the freezer. ;o) Since it's 
just a one-time thing, I don't think the major 
temperature change had much of an adverse affect 
on the cels, and at least it let me get them 
separated.

I thought I would share that, if anyone else has 
mildly-stuck cels that they'd like to attempt to 
unstick.

Take care,
-Cres