Since I have been collecting several of the vintage cels from the 1930's/40's, I have seen some absolutely amazing things, and some horrific counterfeits that passed off for $10,000 items.
So, here are the criteria I use in order to ID and authenticate an original Vintage cel from < 1960's, particularly Disney.
1. Color variations. Line and color fading - Age wear
I know that all vintage Disney cels especially those from the mid-1950's and earlier are made of a material called Nitrate rather than Acetate. With that said, all original, unrestored cels get this yellowish tint to them and gets worse when exposed to warm or humid temperatures.
So, many of the paint colors in a cel, specifically white tend to have a yellowish hue when compared to the original screen cap. A great example of this coloration would be my 1935 Band Concert cel which has all paint in un-restored condition:
http://starphoenix.rubberslug.com/galle ... mID=243800
When compared to the video, his gloves int he cel are yellower. The other thing you can see is there is a "halo" ring around the image. This was a conservation technique used during the 40's and 50's where the conservationist would apply a clear "nail polish" material over the paint in order to prevent the paint from cracking as easily.
The other huge thing to note for vintage cels with nitrate based material (and this includes Warner Bros, Max Fleischer, and Disney cels) is since nitrate is very volatile, they immediately begin "crinkling" to warmer temperatures. In many cases, they crinkle and deteriorate so badly, the cel eventually gets destroyed on its own.
So, for conservationists attempt to salvage the art by trimming the cels and reapplying them to another cel (later on using acetate based cels) in order to prevent further deterioration.
I actually use this phenomenon to determine if a cel REALLY is authentic 1930's-1950's, if it has been trimmed and reapplied to another cel (you could tell this by rubbing your hand over the image or just looking for the trim area if they did not do a great job on it). If a cel is completely fake, it would be made by materials such as acetate cels, which do not have the crinkle or discoloration effect and this is how I KNOW it is a counterfeit.
When I saw the black and white Mickey cel the other day (the one they advertiSed as REPRODUCTION, but could easily be played off as original if someone else decides to be dishonest), it had absolutely no yellowing or wrinkling effect on the cel.
If you look closely at the Band Concert cel, you will be able to see some of the wrinkling of the cel near the bottom of the image and this is how you know it was not reproduced or cut and applied to a acetate based cel.
3. Production information on the cel (layers, numbers, scene numbers)
It is EXTREMELY rare to find an original vintage cel that was not cut and still has its production information (especially the 1930s/40s and earlier). But, this is what I found out yesterday when i was inspecting my Flowers and Trees Cel and my Cookie Carnival cel.
http://starphoenix.rubberslug.com/galle ... esID=42380
Both of these cels are made using layers. The Flowers and Trees cel is 3 layer cel (really 4 in this scan) on a key master background. Animators use a 3 digit number with either a 1 for the first layer and 2 for the 2nd layer, etc. So, for example he bottom layer of this cel would be the tree labeled at 104 while the 2nd layer would be the 2 bats labeled at 201 while the black bird was 304. I have seen this type of numbering system in the earlier works while the post Vintage cels i have seen (ie Rescuers down under and Anime cels) use a letter + number system:
example: A-1 for bottom layer, B-# for 2nd layer, C-3 for the 3rd layer on top.
So, generally speaking, the later Disney cels i have seen and all my Anime cels have this labeling system.
4. Line and image smoothness - Up until 101 Dalmatians (1961), all cels were hand inked completely. It was not until 1961 when they used the first xerox machine. With that said, hand inked cels' black lines tend to deteriorate a bit less than xeroxed lines because they use a different type of ink. The lines on early cels also tend to be slightly more jagged and I have seen some spots where the lines were "gone over" a couple times because the first stroke was not solid enough for the cel.
5. Episode/Film identification. Does it match up?
This can be hard to do for older films since a lot of the editing is done even after they produce the film, and then go back and re-edit it when the films/shows are re-released onto VHS or DVD. Sometimes, they even cut out certain scenes completely. But it is always a good thing to locate the screen cap and use that as a reference. But, even if you can't find it, does not mean you have a fake or counterfeit cel.
6. Any factors that can contribute to the appearance of a cel and what to consider as being a determining factor. This deals with lighting effects, analog vs. digital effects, digital re-mastering and studio edits, etc.
This is a topic of great discussion recently. Studios still go through the editing process after the cel images are snapped and I have seen variations in lighting and color shades when a cel is supposed to be snapped during a lighted scene or a night time scene. Or, for example, in Tinkerbell's case from Peter Pan, they never made the cels for her wings and just used a special lighting effect to make the wings and the lights. If you look at this cel of Tinkerbell, you can see she has wings, which is actually painted onto a separate cel after the production. The actual original production cel is of tinkerbell alone without her wings.
http://starphoenix.rubberslug.com/galle ... mID=267215
So, if you ever buy a Tink cel, keep that in mind.
7. Providence. Do you know who owned the cels before and how to find out?
To me, this is EXTREMELY important especially when you know that counterfeits are common in certain types of art. In particular, I was most concerned about my Eyvind Earle background.
http://starphoenix.rubberslug.com/galle ... mID=324618
About 3-5 years ago, some Earle piece surfaced that were actually really good counterfeits and were sold for $10,000-50,000 on the market. Knowing this, it is very necessary to do a lot of research on where the cel or art came from.
As JWR mentioned, knowing the place you buy it from and knowing where they get their arts, and keeping connections with trusted people are very very good ways to protect yourself.
For places like Ebay, it is always a buyer's beware market unless you know the seller as being a Beta member or what-not and they can provide youw ith details of where they got the piece.
Also, one other advice I recently gave to someone. Do not display your entire art piece on your gallery. Always crop it and hide the registration holes and production information. This is what museums do with paintings by hiding the edges and many of the "flaws" that authenticate the piece beneath the frames. If a question ever comes up where a counterfeit pops up, they can never fully copy an original cel/art piece whose parts are hidden.
I have seen MANY fan cels and counterfeit cels on Ebay, but when dealing with a fake, I always look at the screen cap or the cel it was made from and I have often found that the fan cel was produced just big enought o cover the screen capture. If you look at your Anime cels, they generally cover more area than a TV screen. They generally cut off 1-1.5" from top/bottom and 1-2" from the sides when snapping the image.
So, these are the criteria I use for early art pieces. I will try to post some more pictures when I get a chance, specifically close up shots to show specific age wear so many peeps (especially the new collectors) can use this as reference for age and production wear.