Darkroom Digest: A Formal Description of the Silver Gelatin Photographic Process

September 29, 2016  •  Leave a Comment

As the world of photography continues at breakneck speed into the digital realm, I thought it would be fun to publish a high-level article on the black and white chemical-based photographic process.  Modern day digital photographers may not know this information, so I thought it would be good to share it and hopefully inspire a few photographers to explore the wonders of classic darkroom-based black and white photography.  I have to pick a medium to help describe the process, so I will use film, but it could easily be a glass plate or paper too.   

The image at the top left of this article was created with my vintage 8x10 Kodak 2D camera with the stock Wollensak lens.  I used RC glossy darkroom paper as my negative and I hand developed the negative by inspection in a bath of dilute Dektol.  

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Film consists of a substrate which is in turn coated with a light sensitive emulsion.  Film emulsion has miniature crystals of a silver halide suspended in gelatin.  When a photographer exposes the film to light, a latent image is formed on the film.  The latent image is the result of a chemical reaction of the emulsion to the light.  When the film is placed into a developer, the developer acts on the light and dark areas of the silver halide grains.  The developer frees the silver salts from the compound and deposits tiny grains of irregular metallic silver.  

As you may have already deduced, the more light received at the time of exposure, the denser the metallic silver.  Once development is complete, the unexposed and undeveloped silver halide crystals that remains on the film must be removed or they will develop out over time and ruin the negative.  The same principle applies to printing too.   The unwanted crystals are removed by using a fixer.  Historically, fixer has been a bath of ammonium or sodium thiosulfate, which is commonly called hypo.  

The hypo works by forming a soluble compound of the unexposed and undeveloped silver halides which frees it from the gelatin and leaves the developed silver metallic behind.  

After the fixing bath is completed, the emulsion is still saturated with the fixing chemicals and some of dissolved and unwanted silver salts.  If we did not wash our negatives and prints, this would ruin our films and prints over time because the byproducts would stain and slowly decompose our films and prints.  

Washing helps us produce a stable negative/print.  

That is about as simple as I think I can break it down while including important details, but not going down any rabbitt holes too deeply on the science.  I hope this overview helps black and white photographers understand the underpinnings of what happens at a scientific level when developing negatives and prints in the darkroom.  Knowing this information can open up an entirely new world of creative possibilities such as redevelopment techniques and printing methods such as lith and others.  

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