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Does anyone have a basic idea of how fixer works?

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... and once the work of the fixer has been carried out, it is necessary to remove the remains of said fixer that could remain on the film, since they will become harmful contaminants against the optimal conservation of the image and in the long term.

That is why after using the fixer, the film is thoroughly washed.

Edited by jose_angel
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In 1727 T. H. Schulze discovered that a compound called silver chloride darkened when exposed to light. He had discovered one of the most important chemicals used in modern film-based photography. Images of stencils and shadowgrams were now possible, however the images faded quickly. In 1837 J.B. Reade discovered that a chemical called “hypo” mixed with water rendered silver-based images permanent (fixed). 

The light sensitive coat (emulsion) used on modern films consists of silver crystals called silver salts imbedded in purified gelatin and coated on a plastic base. When loaded in a camera and exposed to an image of a vista cast by a lens, the silver salts undergo a change. The change is too subtle to be noticed. Nevertheless, an image of the vista, be it invisible, exists. We call this a latent image. 

We bathe the exposed film in a solution called a developer. The developer is mainly water, however it contains an agent that seeks out those silver salts that have been exposed to light. The developer fractures these exposed crystals into their two component parts (silver and a halogen). Halogen is Swedish for salt maker. The halogens we use are bromine, chlorine, and iodine. 

The job of the developer is to reduce those silver salts that have be exposed and ignore those that have not been exposed. When reduced, the silver component of the crystal remains imbedded in the gelatin binder of the film, and the halogen component dissolves in the waters of the developer. The imbedded silver is a metallic tuft that is opaque -- thus metallic silver forms the image. 

The image that results is beautiful, but the tufts of metallic silver are surrounded by salts of silver that were never exposed, and thus remain undeveloped. These unexposed crystals will in time, self-reduce, liberating metallic silver that appears dark (black). In other words, the beautiful image will fade to black in an hour or so. 

The countermeasure to the self-fading is a hypo solution. This solution is selective. It targets unexposed, thus undeveloped, silver salt crystals. This is the fixer that renders films and papers permanent. In just 10 minutes hypo dissolves away all the unexposed salts of silver. While the hypo will attack metallic silver, this action is not quick; the film or paper will be removed from the fixer long before any detrimental action occurs. 

In summation: The developer is selective in that it works on exposed silver salts ignoring the unexposed. The fixer works on unexposed silver salts ignoring the metallic silver image.   

The traditional fixer is Sodium Thiosulfate. This works fast but during World War II Ammonium Thiosulfate was discovered. This fluid works twice as fast and thus it it’s called rapid fix.

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