If you try taking one of your normal cels and 
holding it up to a light, usually you can even 
see through it in places you didn't realize you 
could have. So the heavy black paint is put on to 
keep light from shining through.

Interestingly, my observation has been that the 
paint on newer cels is thinner than on older 
ones. . . For example, my TV Miyu cels have a lot 
less paint per painted square inch than my Miyu 
OVA cels. (That observatin at least keeps the 
studio the same.) I don't collect from any *very* 
old shows, so I'd love to hear what collectors 
who do collect the very old shows think of this 
notion.

Many Sharp Smiles,
--Drac