Cels Forum

Nowaday, most of these types of folders are using
polypropylene for the transparent plastic pockets to store
photos and artworks.  However, the papers between can
be acid-free or non-acid-free, it's really depending on
the brands and years they made.  The older ones might use
the non-acid-free papers.  If you don't want the cel to
stick with the paper, you can just go to your local comic
store and buy some comic bags (they also made by 
polypropylene).  Put the cel in the bag, then put it 
inside the folder.  Unless you store the cel under sunlight
or hot temperature; otherwise, the cel won't stick to 
this polyproylene material.
The way you store cel is up to you, you can store it by
series or size or alphabetical order or whatever.  By the
way, it is good to store the cel and pencil sketch 
separtely.  If you put them together and store, they tend
to stick together easily.  Plus your cel folder can get
pretty thick and heavy with both cels and sketches :)