I had requested some information from Vintage Ink 
& Paint about cel restoration.  They just sent me 
the article and it's incredible.  Its quite long 
and goes into the history of cels and how they 
were created, along with the various types of 
paint and materials used, etc.  It explains in 
detail how cels are made, and how restoration 
affects them, and their worth/authenticity.  If 
anyone is interested in the article, just email 
me and I will forward the letter to you ^_~