Developer

[dweenteen]

What happens to my film if the developer I apply is too cold in temperature ? ( Kodak Tri X) 

[Alan Marcus]

Modern photo films like Tri X consist of salts of silver imbedded in purified gelatin. The silver salts we use are silver bromine, silver chloride, and silver iodine. These compounds of silver are light sensitive. In other words, when light plays on these silver salt crystals, a chemical reaction occurs. Given sufficient light exposure, the two component parts of these crystals fracture. The silver portion forms a tiny fluff ball of silver, the other portion, called a halogen, revers to its elemental form. The tuft of metallic silver is opaque thus it looks black on backlighted film. The word halogen is Swedish for salt maker. Unexposed silver halogens resemble ordinary table salt however the crystals are off white (cream) and they are super tiny.

Gelatin is the glue (binder) that holds the silver halogens onto the transparent plastic film base. In the camera, we only allow them to receive a teeny exposure to light. The light energy they receive is insufficient to cause a visible change. Nevertheless, a change has taken place and an image of the vista being photographed is formed. This is an invisible image we call this a latent image.

After exposure in the camera, the film with its latent image is submerged in a developer solution. This solution is mainly water with 4 other ingredients. Water is the solvent that holds the developing agent. This is a reducing agent that is able to differentiate between exposed and unexposed silver halogens. When it encounters an exposed crystal, it fractures (reduces) it to its two component parts. Unexposed crystals will be fractured if the film is submerged in the developer solution for a super long time however we will pull the film out before this can happen. The developer solution contains an alkaline that sets the pH basic. This accelerates the developer’s ability to fracture exposed crystals. Additionally, there is a restrainer additive that retards crystal fracturing unless the crystal has been exposed. Lastly, there is a preservative that reduces the likelihood that the developing agent will take on oxygen. If it oxidizes it reverts to coal-tar and loses its ability to develop plus the coal-tar will stain the film.

The alkaline accelerator also causes the gelatin binder to swell like a dry sponge plunged into water. This action opens ups the pours of the gelatin allowing the developer solution and other following solutions to enter the structure, percolate about and then exit with agitation.

Now chemical activity is also accelerated by higher temperatures. Most developers are formulated to work at 20⁰C = 68⁰F. This is considered room temperature by chemists. Higher temperatures cause the gelatin binder to swell more thus the gelatin will be more porous and this hastens fluid flow inside the emulsion (what we call the gelatin silver halide coat). A colder developer takes longer to do its work.

One last point -- film is multi layered. A over-coat to protect the emulsion, one or more emulsion coats with different light sensitivities, a primer coat to hold the emulsion to the film, a back coat called antihalation coat. All expand and contract with temperature at different rates. If the developer solution is too hot or too cold, the tempo of the developing action changes. Additionally, the finished film will likely take on an unwanted curl. If the developer solution temperature is extremely different from the other fluids of the process (incudes wash water), there is the possibility the gelatin with shatter and look like a splintered glass window (reticulation). Best we keep all the fluid temperatures nearly the same.   

All the fluids of the process will work OK within a moderate range. Developer immersion times change with temperature. The developer instructions publish revised developing times based on temperature and dilution strength. We consult these published tables for revised developer time for temperature, dilution, and number of rolls previously treated.

Edited November 9, 2022 by alan_marcus2