Key stages in production of Ag-based photographs. Two silver halide particles, one of which is impinged with light (h
ν) resulting in the formation of a latent image (step 1). The latent image is amplified using photographic developers, converting the silver halide crystal to an opaque particle of silver metal (step 2). Finally, the remaining silver halide is removed by fixing (step 3).
All photographic processing use a series of chemical baths. Processing, especially the development stages, requires very close control of temperature, agitation and time.
Black and white negative processing
Black and white negative processing is the chemical means by which photographic film and paper is treated after photographic exposure to produce a negative or positive image. Photographic processing transforms the latent image into a visible image, makes this permanent and renders it insensitive to light.
- The film may be soaked in water to swell the gelatin layer, facilitating the action of the subsequent chemical treatments.
- The developer converts the latent image to macroscopic particles of metallic silver.
- A stop bath,† typically a dilute solution of acetic acid or citric acid, halts the action of the developer. A rinse with clean water may be substituted.
- The fixer makes the image permanent and light-resistant by dissolving remaining silver halide. A common fixer is hypo, specifically ammonium thiosulfate.
- Washing in clean water removes any remaining fixer. Residual fixer can corrode the silver image, leading to discolouration, staining and fading.
The washing time can be reduced and the fixer more completely removed if a hypo clearing agent is used after the fixer.
- Film may be rinsed in a dilute solution of a non-ionic wetting agent to assist uniform drying, which eliminates drying marks caused by hard water. (In very hard water areas, a pre-rinse in distilled water may be required - otherwise the final rinse wetting agent can cause residual ionic calcium on the film to drop out of solution, causing spotting on the negative.)
- Film is then dried in a dust-free environment, cut and placed into protective sleeves.
Once the film is processed, it is then referred to as a negative.
The negative may now be printed; the negative is placed in an enlarger and projected onto a sheet of photographic paper. Many different techniques can be used during the enlargement process. Two examples of enlargement techniques are dodging and burning.
Alternatively (or as well), the negative may be scanned for digital printing or web viewing after adjustment, retouching, and/or manipulation.
† In modern automatic processing machines, the stop bath is replaced by mechanical squeegee or pinching rollers. These treatments remove much of the carried-over alkaline developer, and the acid, when used, neutralizes the alkalinity to reduce the contamination of the fixing bath with the developer.
Black and white reversal processing
This process has three additional stages:
- Following the stop bath, the film is bleached to remove the developed negative image. The film then contains a latent positive image formed from unexposed and undeveloped silver halide salts.
- The film is fogged, either chemically or by exposure to light.
- The remaining silver halide salts are developed in the second developer, converting them into a positive image.
- Finally, the film is fixed, washed, dried and cut.
Chromogenic materials use dye couplers to form colour images.
Modern colour negative film is developed with the C-41 process and colour negative print materials with the RA-4 process. These processes are very similar, with differences in the first chemical developer.
The C-41 and RA-4 processes consist of the following steps:
- The colour developer develops the silver negative image, and byproducts activate the dye couplers to form the colour dyes in each emulsion layer.
- A rehalogenising bleach converts the developed silver image into silver halides.
- A fixer removes the silver salts.
- The film is washed, stabilised, dried and cut.
In the RA-4 process, the bleach and fix are combined. This is optional, and reduces the number of processing steps.
Transparency films, except Kodachrome, are developed using the E-6 process, which has the following stages:
- A black and white developer develops the silver in each image layer.
- Development is stopped with a rinse or a stop bath.
- The film is fogged in the reversal step.
- The fogged silver halides are developed and oxidized developing agents couple with the dye couplers in each layer.
- The film is bleached, fixed, stabilised and dried as described above.
In some old processes, the film emulsion was hardened during the process, typically before the bleach. Such a hardening bath often used aldehydes, such as formaldehyde and glutaraldehyde. In modern processing, these hardening steps are unnecessary because the film emulsion is sufficiently hardened to withstand the processing chemicals.