Celluloid film is made up of a flexible, transparent base that is coated with a gelatin layer (the emulsion), which contains millions of tiny, light-sensitive grains. When the film is exposed by the shutter in the lens, the grains absorb light, creating a latent image that is not visible to the naked eye. The film is then treated with developing chemicals, which cause the exposed portions of the film to become visible in a negative image of the original scene: light and dark areas in a scene are reversed. The film is then "fixed," which removes the developing chemicals, and the undeveloped grains are washed away to prevent further exposure of the film. The negative film is then printed by allowing light to pass through it onto a second strip of film, creating a positive film for projection.
Early film stock was made of cellulose nitrate, an extremely flammable plastic. Nitrate film burns rapidly, even without a supply of air, and gives off poisonous and explosive gases. It has even been known to ignite spontaneously. Cameramen had to be extremely careful when using and storing nitrate film; one spark from a cigarette could cause an entire day's work to go up in flames. In 1897, a fire broke out in a French movie theater that was projecting a nitrate-based film, killing over 180 people. In 1914, a fire began in a California film-finishing house, destroying ten buildings. Kodak introduced a flame-resistant, cellulose triacetate film stock, also known as Safety Acetate, in 1909. But the film industry resisted Safety Acetate, which was less flexible, harder to splice, and wore out more quickly than nitrate film; studios continued to use the more flammable celluloid until Kodak introduced Improved Safety Base Motion Picture Film in 1948.
A few early film cameras used paper film stock. Evidence suggests that around 1883, French photography enthusiast Louis Le Prince (1842–1890) built and experimented with a single-lens camera that used a paper negative film. Prior to 1912, the Kinora Film Company offered an amateur camera and viewing device that utilized paper film stock in a flip-book format.