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Photographic film contains silver halide in a layer of emulsion covering the surface of the film. When exposed to light the halide breaks down and produces metallic silver. When the film is developed the silver remains as black stains while the halide is dissolved and ends up in the fixer and wash water. Commercial labs uses filter canisters to remove trace silver from the wash water.
As the silver in a black and white picture is black, the darker a picture is the more silver it contains. Highest silver content is in unexposed film.
For exposed film the amount of silver is directly proportional to the darkness of the picture.
Common ways to recover the silver from film includes :
- Burning the film in a furnace and smelting the ash.
- Sodium cyanide to leach the silver
- Sodium thiosulfate
- Stripping the emulsion with sodium hydroxide
- Incineration and chemically leaching the ash
Incineration can have quite high losses as the halides are volatile at higher temperatures. A filter is required in the chimney to recover the silver going up in smoke.
- US5064466 Non-toxic process for recovery of photographic silver. Uses hypochlorite solution.