to back cels with cotton rag when framing. ::

Edited from CELMAIL archive:

Acid free, will, over time (yes, a long time) burn your paper - and yes, 
ultimately damage it. As to cels - the more contemporary ones - I think, 
that because they are a polymer (a plastic), you could probably bury them 
in the ground for a thousand years, dig them up, and they'd still be fine! 
All kidding aside, the paint on the back of the cel is probably what you 
should be concerned about preserving. So again, use 100% cotton rag back 
directly against the cel if you have no background, or if you have a 
background (laser or otherwise), then put the foam core (or rag backed 
foam core if your framer carries it).

Finally, we don't recommend buffered mats at all. The bevel (the inner 
edge of the mat) is a tan or yellow color and will get brown with age. The 
acid free mat is also an "off" color. Rag is bright white!

-ADLER & Co.