Carbon film is a residue of organic matter marked upon stone via a special fossilization process. This process is also called carbonization or distillation. Carbonization is most common in fossils of plants and invertebrates.
The process involves compression of a dead organism between heavy layers of sediment. As the sediment builds up, the organism experiences tremendous heat and pressure. Volatile chemicals within the organism distill or vaporize and condense. Lipids within the organism's body also polymerize, forming long chains of carbon. These chemical processes break down the body of the organism into an ashy film that preserves many fine details.