If the sketch is non-matching and not something 
of interest, what I do is: 

o  Soak a damp towel with water
o  Pat the back of the sketch down so it's wet
o  Let sit for a minute or two
o  Peel them apart carefully. 

The cel paint is oil based, so it's relatively 
waterproof. But, the sketch will probably be 
ripped to pieces if you're not careful, but 
fairly wrinkled even if you are.  

I use tap water myself, but distilled water is 
probably better.  

Caveat: If this is a multi-layer cel, don't let 
the water get between the layers. It's amazing 
how long water takes to evaporate between two 
pieces of stuck acetate and it's almost 
impossible to remove any residue that's left 
behind. 

Chi