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Photographic papers contains silver in the emulsion covering the surface. In the photographic copying process from negative film to paper light is used to transfer the negative image on the film onto the silver halide on the surface, converting some into metallic silver. The following development process first amplifies the silver content chemically and then in the fixation step, removes any non transformed silver halide. The excess silver 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 papers.
For exposed papers the amount of silver is directly proportional to the darkness of the picture.
Common ways to recover the silver from paper includes :
- Burning the paper in a furnace and smelting the ash.
- Sodium cyanide to leach the silver
- Sodium thiosulfate
- Incineration and chemically leaching the ash
- US5064466 Non-toxic process for recovery of photographic silver. Uses hypochlorite solution.