Cinema has been enriched by stories and characters brought to life in animated films, which evoke memories and emotions of our past and are time capsules of the prevailing attitudes of their eras. Although animation today is ubiquitous, it has a relatively brief history. In the 1930s, short animated films, shown in theaters before a feature film, emerged as a popular form of entertainment. The enjoyment of watching animated characters was a much-needed diversion for the public during socioeconomically difficult times. Arguably, animation advanced as an art form with the 1937 release of the first American fulllength animated feature, Walt Disney's Snow White and the Seven Dwarfs. The characters in that film were imbued with real voices, emotions, and thoughts within a well-crafted and fanciful story. Since then, audiences of all ages have been captivated by the stories and characters of animated films.

In traditional hand-drawn animation, each moment in time was captured on an individual sheet of thin, transparent plastic. Outlines of the characters were meticulously drawn in ink on the fronts of the sheets, while the colors and details of the characters were painted on the reverse sides. The plastic sheets, known as cels, were then placed face up over painted backgrounds and photographed by a movie camera one cel at a time. Playing back the sequence of images on a movie projector produced the illusion of motion. An enormous amount of time, effort, and resources was expended by artists and craftspeople to create full-length animated features.

Without advances in the plastics industry, animation as we know it could never have developed. Although many types of industrial polymers were manufactured in the twentieth century for a multitude of purposes, only a few possessed the physical properties necessary for animation cels—colorlessness, transparency, and flexibility. Colorlessness and transparency were essential for the painted background to show through clearly, free of distortion, allowing the characters to be placed in their proper positions within the scene. Flexibility was important because cels needed to be manipulated easily during inking, painting, and photography, and later stored with minimal risk of damage. Flexibility was imparted to polymers during manufacturing by the addition of chemicals known as plasticizers, which separate the long polymer chains and cause them to slide smoothly past one another, thus transforming otherwise rigid polymers into flexible plastics.

The earliest plastic used for cels was cellulose nitrate, plasticized with camphor and triphenyl phosphate, a white, waxy solid that reduced flammability. Over time, this plastic proved highly unsuitable for cels and films because of its inherent flammability and its tendency to yellow, wrinkle, and generate hazardous gases with age. Eventually, cellulose nitrate was replaced by the safer and more chemically stable cellulose acetate, plasticized with a variety of phthalates (typically colorless oily liquids) and triphenyl phosphate. Cellulose acetate comes in two formulations—diacetate and triacetate—that vary chemically and have different mechanical behaviors and plasticizer requirements. Unfortunately, cellulose acetate was also found to degrade (albeit much more slowly than cellulose nitrate) by a chemical reaction called hydrolysis, which releases acetic acid, a pungent chemical commonly known as vinegar. In fact, the deterioration of cels and other objects made from cellulose acetate was known colloquially as vinegar syndrome. Like cellulose nitrate, cellulose acetate is also susceptible to wrinkling and discoloration as it ages, as well as oxidation from light exposure. Eventually, cellulose acetate was replaced in animation by polyester, a film that does not require plasticizers. Today, animation is done almost entirely on computers, thus severing the connection between animation and the world of plastics.

The Disney Animation Art Collection

Originally, the collection of animation cels, drawings, and other materials related to Disney animated films was housed in what Walt Disney called the "morgue"— a term from the newspaper business for the place where old articles and files were kept. The art morgue on the Disney Studio lot was located in a basement where all artwork from completed animated films and shorts, as well as unproduced animated projects, was stored. As years passed, some production cels were given away or sold at Disneyland Park. Other cels from early productions were cut up and glued to monochromatic backgrounds for sale by the Courvoisier Galleries. Nonetheless, many cels in the collection remained intact. In the early 1990s, all animation artwork was relocated from the studio lot morgue to the Disney Animation Research Library (ARL), a state-of-the-art, climate-controlled facility (the cels are currently stored between 62°F and 65°F and at 50 percent relative humidity, in vaults equipped with carbon filtration). In this new location, the collection could be spread out, organized by film and year, and stored in archival containers. Today, the ARL collection comprises sixty-five million pieces of art, including more than two hundred thousand animation cels from the 1920s to 1989, when Walt Disney Studios released The Little Mermaid, its final full-length animated film made with handinked and painted cels. The collection also contains a small number of hand-drawn replica cels (some created after 1989) produced on polyester for commercial purposes or traveling exhibitions.

Preserving such a large collection presents the ARL conservation staff with a variety of challenges. One challenge is identifying the type of plastic used for each cel. Previously, visual and tactile clues had been the only methods available to the ARL. In addition, although the vast majority of the cels in the collection remain in good condition, some show evidence of buckling, yellowing, and off-gassing from hydrolysis and oxidation. Moreover, some paints (made with plant gum binding media) are prone to cracking, flaking, and delamination. A key preservation concern for cels is finding the optimum storage temperature and relative humidity that preserve the plastic with minimal negative impact on the paints.

To address these preservation issues, the Getty Conservation Institute (GCI) and the ARL initiated a collaborative project in 2009 whose aim was to study production and replica animation cels from the ARL collections dating from 1929 to 2000 using various scientific techniques and to relate the test results to the purported age of the cels. Knowledge obtained from studying the ARL cels would be relevant not only to animation cels from other studios who bought materials from the same suppliers but also to modern sculptures and design-art objects made from the same plastics. Thus, the ARL cel collection provided an ideal reference set and case study for broader museum conservation studies.

On a study group of more than a hundred cels, noninvasive analyses of color and gloss were performed with UV-Vis spectrophotometry, while the plastic types were identified using Fouriertransform infrared spectrometry. Other analytical methods were invasive and thus could be employed only on minute samples. These methods included gas chromatography–mass spectrometry for measuring the extent of hydrolysis of the polymers and, with a pyrolyzer added to this instrument, identifying cel plasticizers. Thermomechanical analysis and dynamic mechanical analysis measured the mechanical response of the cels to changing temperature. For the invasive tests, at least two film productions from each decade were studied, yielding a total of eighty-one physical samples removed from the cel edges.

Research Findings

The results from the GCI-ARL research were revealing. For instance, the premise that cellulose acetate replaced cellulose nitrate in the mid-1950s was exposed as a misconception. In the cels analyzed, it was found that cellulose diacetate use began as early as 1929, whereas cellulose nitrate was last used in 1942. Also significant was that not every cel in a production was made from the same type of plastic. For example, both cellulose diacetate and cellulose nitrate cels were used for different scenes in Fantasia (1940), Snow White and the Seven Dwarfs (1937), and Bambi (1942). Perhaps the animation industry practice of scrubbing and reusing cels from older productions when blank cel supplies ran low explains this finding. More unexpectedly, there appeared to be a period of transition from cellulose diacetate usage to cellulose triacetate between The Fox and the Hound (1981) and Mickey's Christmas Carol (1983). The two types of plastic were found to differ in their acetyl contents, thermomechanical properties, and plasticizer contents and compositions. Undoubtedly, these differences will be reflected in the long-term behavior of the cels during storage.

From the standpoint of chemical composition, the test results suggest that the cels remain in generally good condition. Only the most volatile phthalate plasticizer (diethyl phthalate) showed any evidence of slow evaporative loss over time, reflecting overall stability of the plasticizer composition in most cels. Overall, there appeared to be a slight reduction in acetyl content with age, although the exact extent of the hydrolysis reaction in any single cel was difficult to gauge because of batch-to-batch variations in the original acetyl content of the cellulose diacetate and triacetate sheets.

The study of the ARL animation cels altered some perceptions of the collection. It is now clear that appearance, tactile behavior, and production date are insufficient means of differentiating. This inadequacy illustrates the types of problems curators, registrars, and conservators face when cataloguing a collection without more sophisticated analytical tools. No simple relationship existed between the type of plastic and production date. Furthermore, the collection actually contained four types of plastic (cellulose nitrate, cellulose diacetate, cellulose triacetate, and polyester)—not three as once thought. The lessons learned from this study are relevant to animation cels from other studios, museums, and private collectors.

As is often the case in conservation research, the findings raised further questions, which we will pursue in the next phase of our project. To what extent are deterioration rates for the four plastics affected by environmental conditions present in storage? A vast amount of scientific research, most notably by the Image Permanence Institute, has shown that the life span of cellulose nitrate and cellulose acetate used as film supports for photographs and motion pictures could be greatly prolonged by lowering the temperature and controlling the relative humidity in storage. But are these conditions optimum for conserving animation cels for which extremely cold and dry conditions could exacerbate paint damage? In addition, are the cels off-gassing vapors that might impact their stability? If so, are there sorbents that could effectively remove them from the storage environment? Exploration of minimally invasive methods for reattaching cracking and flaking paints is another much-needed area of research.

Clearly, many of these issues extend to the display and storage of museum objects made from cellulose acetate and cellulose nitrate, such as sculptures by Naum Gabo and Antoine Pevsner, which face the additional environmental risk factor of light exposure. Because light levels are extremely low in the ARL storage vaults, photo-oxidation does not appear to be a major hazard.

Animation art archives like the Disney Animation Research Library strive to preserve a unique cultural legacy that is in danger of being lost forever. With the passing of time and the advent of computer animation, the medium itself is becoming obsolete, the remaining artists and technicians have fewer newcomers to train, and the art itself is slowly deteriorating. Yet there is hope that knowledge gained by this research will inspire other collaborations that lead to advances in storage conditions and conservation treatments. Preserving animation art for future generations is a worthwhile goal. Who could fault us for dreaming of a world in which characters like Snow White, beautifully rendered on animation cels, live happily ever after?

Kristen McCormick is the manager of Art Collection and Exhibitions at the Disney Animation Research Library. Michael R. Schilling is a senior scientist at the GCI.