Thought this might be interesting,

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Extracted from: Anime Shop Vi

As you know, the primary purpose of cel 
production is to use them for film making. Apart 
from those specially produced for sale, cels are 
not considered as merchandises, and discarded as 
industrial wastes when the film making is 
finished. Naturally, it is rare for collectors to 
find cels in an ideal condition. Instead, most 
after-use cels are damaged; for instance, the 
colors are chipped and mottled. Airbrushed colors 
are easily damaged with one or two camera takes. 
(Airbrushing is one of the special effect 
techniques used to depict luster of metal and 
reflects on glass to show the presence of glass 
in the scene.) Also, backgrounds are apparently 
damaged after a few times of use. It is almost 
impossible to find large quantities of cels 
undamaged. 

"The paper-sticks-to-the-cel problem" - one of 
the major concerns among collectors - is 
attributable to the post-filming handling. 
Because of the tight schedule and least costs 
allowed, cels are usually half-wet when delivered 
to the camera studio. Cels and backgrounds before 
photographing are handled with care, not to cause 
bending and other damage, and never piled on top 
of another in large heaps. At this point, with 
the colors half-wet, it is easier to remove the 
cels from the paper sheets. The pencil sketches, 
after used for the cel production, are placed as 
insulators between cels to prevent them from 
sticking together, while waiting for being 
photographed. Although some sketches may be 
reused for retakes, most of them are destined to 
the use as stick-prevention sheets. 

After the film making and the test run is done, 
the cels are no longer required for the film 
making and regarded as industrial wastes. From 
this time on, the treatment of cels at the 
production site drastically changes. Cels are 
piled in huge heaps and pushed to the corners of 
the studio, with the colors still half-wet. 
Although the pencil sketch sheets are effective 
in insulating cels from one another and 
preventing them from sticking to one another, it 
is often the case that the sketch sheets stick to 
the cels and cannot be removed off the cels. 

Whether cels become stuck to the sketch sheets or 
not depends on the length of time left uncared in 
the production's storage or the weight load 
placed over the cels. Further, different colors 
seem to have different effects, for dark colors 
such as black and brown seem to easily stick to 
the sheets. 

If the cels are left for about one year after the 
film making, most of them stick to the sketch 
sheets and no longer are easily removed. The cel-
sticking-to-sketch problem is most unlikely to be 
attributable to the handling by cel shops or 
collectors, because the after-filming cels are 
not "wastes" for them but merchandises and 
collectibles.