CONSERVATION
Earlier animation used Cellulose Nitrate, a material that is high flammable with a tendency to yellow and wrinkling. Due to instable nature, such material was replaced by Cellulose Acetate, another cel polymer that was less flammable, although still susceptible to photodegradation, hydrolysis, wrinkling and shrinking. Most Japanese animation cels are in the form of Cellulose Acetate.
Although Cellulose Acetate has a more stable chemical composition that its predecessor, it is still subject to degradation when poorly stored. The degradation of the Cel will cause it to release acetic acid which gives off a strong smell of vinegar. This phenomenon is therefore known as the "Vinegar Syndrome" (VS). High humidity and high temperatures are causes of degradation. Exposure to acidic material will also trigger or accelerate the process.
As such, Cels should always be stored in a dry and low temperature environment. Any storage (e.g. folder, sleeve etc.) should be acid-free.
In addition, fellow collectors have also shared the following tips:
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Cels need to "breath" - do not seal a Cel in an enclosed container to ensure that acid (if any) is not entrapped
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If a frame contains a number of Cels (and they are not stuck to each other), store each Cel in a separate sleeve. VS frequently develops between stuck Cels where no / less fresh air can reach.
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If Cels are stored in Cel bag, change the cel bag periodically. Change the Cel bag when it shows sign of wrinkling.
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Cels with VS should be quarantined. Store Cels with VS in a separate place (another room) from healthy Cels.
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In case a Cel is showing signs of VS, store it together with Microchamber paper which is known to absorb odour and chemical by-products of deterioration.
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Store Cels upright, i.e. do not stack cels on top of each other. Heavy weight / pressure may cause paint on Cels to crack.
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Store Cels separately from their Background / Genga / Douga etc. The paper may not be acid-free and this prevents paint from sticking onto the Cel.
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Avoid exposing Cels to UV, this will cause lines to fade (turns brown then eventually fades away entirely). Cels produced by particular production houses are more prone to line fading than others.
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If a Cel develops VS, consider sending it to professionals for treatment. The odour can be treated although wrinkles in the Cel cannot be reversed.
Further reading on conservation (although mostly related to animation cels used for American animation):
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Karen Hong Saracino, Animation Cel Storage and Preservation: Caring for a Unique American Art Form, (John F. Kennedy, 2006) http://library2.jfku.edu/Museum_Studies/Animation_Cel_Storage.pdf
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Kristenn McCormick, Michael R. Schiling, Miriam T. Giachet, Joy Mazurek, Herant Khanjian and Tom Larner, Animation Cels: Conservation and Storage Issues (The American Institution for Conservation of Historic & Artistic Works, 2014) 251-26 https://29aqcgc1xnh17fykn459grmc-wpengine.netdna-ssl.com/osg-postprints/wp-content/uploads/sites/8/2015/03/osg021-11.pdf