Hi! I also collect old cels, as my favorite anime and the 
one I primarily look for cels from has been running for
about 30 years, in some form or fashion.

I think light exposure is mainly what causes line fading,
which is why you have to be careful framing cels you like
and to keep them out of direct sunlight. I think also what
it is, is that the paint they use for the lines is different
than the other paint and succeptable to fading, although
a lot of certain kinds of paint fade too. Have you seen
first or even second season cels of Sailor Moon where her
red bow is faded a yucky brown color? That's sort of what
I was thinking about.

I don't think you can totally prevent fading. I do think that
storing cels out of direct light or humidity will slow the
fading process, but they weren't painted to last a long time
in the first place, so they shouldn't be indestructable.

Hmmm....I think my Pilaf cel is probably the oldest cel I 
have in good shape. It's from 1986 I believe, and there is
almost no line-fading at all, although the cel is permanently
adhered to the sketch. I was afraid it was a fake at first
because it was in such good condition, but the paper is 
yellowed a little bit on the sketch, so I think it's real.
The oldest cel I own is from 1979...some of the airbrush
effect on it has rubbed off. I have another Lupin III cel
that I can't place so I have no idea how old it is.

valerie