I recently bought several animation cels of the Mother Goose and Grimm series that CBS aired back in 1991, so I thought now would be a good time to write about them.
Until the early 2000s, most hand-drawn cartoons were painted on cels, which are clear plastic sheets, usually about 10" x 12" big.
After the animation drawings are cleaned up, they are either inked directly onto cels or xeroxed. Then they flip the cel and add paint to it.
Here's the front:
And the back, where paint is applied:
After they are painted, the cels are then placed over the background art and photographed to create one frame of animation.