A form of a traditional animation technique is Cel, which is shortened from Celluloid. The technique is named after the compound Celluloid, which is made from nitrocellulose and camphor and other added dyes and agents, that was used in the first half of the 20th century but was replaced by cellulose acetate as it was highly flammable and expensive to produce. Celluloid is still used in some objects like table tennis balls, and guitar picks.
The technique is used by creating a drawing on a Cel and laying it atop a static background. Cels can be layered on top of each other to create a moving animation, for example a characters torso would be on a Cel, while their arms and hands are on multiple separate ones that are replaced to show movement. Images on the Cels are often painted on the reverse side that isn’t shown so the brush strokes aren’t seen.
In most animated features, usually over 100,000 hand-painted cells would be used. Snow White and the Seven Dwarfs by Disney is an example of this technique. In fact, many earlier Disney films were made using this technique, but eventually phased out in 1990 and replaced with Computer Animation Production System.