Base
The celluloid base of the film is created first by mixing acetate with either linseed fibers or wood pulp. This mixture precipitates pellets of celluloid acetate, which are then washed, dried and dissolved in an acetone solvent. This forms the "dope," which is then spun out on a chromium plated wheel to the correct thickness. As this happens, the solvents in the dope mixture evaporate, leaving behind a thin strip of plastic. This becomes the film base.
Emulsion
The emulsion is the part of the film which captures the light and records it onto the film strip. Color and black and white film have different varieties of emulsion. For black and white film, an emulsion of silver nitrate crystals and gelatin is prepared by dissolving pure silver in nitric acid. This mixture is stirred and cooled. As it cools, the water separates out from the emulsion, leaving behind crystals of silver nitrate. These crystals are then added to a gelatin mixture that has been treated with potassium bromide and potassium iodide. This creates a suspension of the silver nitrate crystals.
For color film, the process is similar, only three varieties of silver nitrate are used, each sensitive to one-third of the color spectrum - red, green, and blue. They contain dyes known as "linked" dyes, which when washed with developer produce a negative image, leaving behind cyan, yellow, and magenta.
Sensitizing
For black and white film, the coating process begins by sending the film base strip to the coating alley, where it is sprayed with the silver halide and gelatin emulsion. For color film, the emulsion coating process may be done as few as three times to apply red, green, and blue sensitive layers of emulsion. The film passes through the coating alley three times, being sprayed first with the red emulsion, then with the green, then with the blue. This arrangement of emulsions corresponds to the intensity of light waves of those particular colors. Modern color film may have as many as twenty layers of emulsion, each sprayed on separately. As the emulsions are light sensitive, this process must be completed in the dark and the film kept in darkness until it is securely packaged.
Finishing
The finishing process begins with the act of "slitting," which is the process of mechanically cutting the film to the correct width. The film then passes on to a perforating machine, which creates the perforations that will align with the sprockets of the camera. Both of these processes are performed in total darkness, as any light will expose the film. Once the film is cut into the correct size and perforated it is wound onto a small plastic spool. This spool is then inserted into a light-proof metal or plastic canister, and the canister is sealed. The film is now ready to be boxed and shipped.
Tags: silver nitrate, black white, black white film, color film, leaving behind, white film, coating alley