Photographic processing

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Photographic processing is the industrial process by which conventional photographic film is treated after photographic exposure in order to produce the desired negative or positive image. Photographic processing does three things: it transforms the latent image into a visible image that can be seen, it makes the visible image permanent, and it renders the film insensitive to light.

The general process is similar whatever the make of film or paper. Although generally not considered "conventional," exceptions include instant films such as Polaroid and thermally developed films.

Kodachrome cannot be processed except in Kodak laboratories. There are also a small number of relatively uncommon processes using dye decomposition technologies such as Cibachrome.

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[edit] Processing apparatuses

Before processing, the film must be removed from the camera and from its cassette, spool or holder in a light-proof room or container.

[edit] Small scale processing

In amateur processing, the film is removed from the camera and wound onto a reel in complete darkness (usually inside a darkroom (with the safelight turned off) or a lightproof bag with arm holes). The reel holds the film in a spiral shape, with space between each successive loop so the chemicals may flow freely across the film's surfaces. The reel is placed in a specially designed light-proof tank (called daylight processing tank) where it is retained until final washing is complete.

In case of sheet film, they can be processed in trays, in hangers (which are used in deep tanks), or rotary processing drums.

[edit] Commercial processing

In commercial processing, the film is removed automatically or by an operator handling the film in a light proof bag from which it is fed into the processing machine. The processing machinery is generally run on a continuous basis with films spliced together in a continuous line. All the processing steps are carried out within a single processing machine with automatically controlled time, temperature and solution replenishment rate. The film or prints emerge washed and dry and ready to be cut by hand. Some modern machines also cut films and prints automatically, sometimes resulting in negatives cut across the middle of the frame where the space between frames is very thin or the frame edge is indistinct, as in an image taken in low light.

[edit] The process

Whether processing at an amateur or commercial level, the film is treated in chemical baths. Each of these baths is closely monitored and maintained at a specific temperature and treatment time. Developer baths are most sensitive to deviations from the standard processing conditions (e.g., time and temperature); other baths are less sensitive.

  • The developer, which turns the latent image to metallic silver.
  • A stop bath, which stops the action of developer, typically a dilute solution of acetic acid. In modern automatic processing machines, this step is replaced by mechanical squeegee or pinching rollers. In small scale darkrooms, stop bath may use citric acid or other organic acids, or simply plain water. Any of these treatments removes the bulk 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.
  • The fixer makes the image permanent and light-resistant by dissolving any remaining silver halides. The fixer is sometimes referred to as "hypo," a misnomer originating from casually shortened form of the alchemist's name hyposulphite. None of "hyposulphite," "hyposulfite" and "hypo" is used to mean thiosulfate in modern chemistry.
  • Clean water wash to remove any fixer, as residual fixer can deteriorate the silver image, leading to discoloration, staining and fading. The washing time can be reduced and the fixer more completely removed if a washing aid (also known as hypo clearing agent, or HCA) is used after the fixer.
  • An optional final immersion in a film conditioner. This is a solution of a non-ionic wetting agent in water which helps uniform drying and eliminates drying marks from hard water.
  • Film is then dried in a dust-free environment, and finally cut (if roll film) and put into protective sleeves.

[edit] Monochrome (black & white) reversal processing

Black and white reversal processing (to make black and white positives) has two additional stages. Following the first developer, the developing action is stopped by an ordinary stop bath, and the film is then bleached to remove the negative image produced by development. The film then contains a latent positive image in the form of unexposed, undeveloped silver halide. The subsequent steps therefore fog the film, either chemically, or by exposure to light, after which all the remaining silver halide is developed in the second developer. This allows the silver halide crystals that were not originally exposed in the camera to be developed into a positive image. Finally, the film is fixed, washed and dried conventionally.

[edit] Color processing

Color negative developing (C-41 process) and color print developing (process RA-4) are very similar. The principal difference is in the color developer formula in the first step; and the combining of the bleach and fixer steps in process RA-4 with a bleach-fix mixture (blix), which dissolves both the silver halides and the elemental silver leaving only a dye image. Mixing the bleach and fixing agent in process RA-4 is optional; they are often mixed to minimize the number of processing steps.

In color reversal processing the film is developed in an MQ (metol/hydroquinone) developer similar to a black-and-white developer, followed by a rinse or a stop bath. The film is fogged in the reversal step, and is then developed in a color developer. Next, the film is then bleached to remove the black-and-white negative image (metallic silver is removed by the bleach), while keeping the color positive image (dye is not affected by the bleach). The film is optionally washed to minimize the carry-over of the bleach to the next bath, fixer. Film is then fixed, washed and dried conventionally. 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.

[edit] Further processing

For black and white emulsions both negative and positive, further processing may react the silver with other elements such as selenium or sulfur to produce a more permanent image. In these cases the silver in the image is changed to silver selenide or silver sulfide, which are more resistant to oxidising agents (pollutants) in the atmosphere. Successful selenium or sulfide toning requires complete fixation. In other cases, the silver may be chemically bleached using a potassium hexacyanoferrate (III) solution and then re-developed in a range of toning formulations. This two-step technique is collectively called "indirect toning." Depending on the toning solution, sepia, red and blue colors may be obtained. If color negative film is processed in conventional black and white developer, and fixed and then bleached with a bath containing hydrochloric acid and potassium dichromate solution, the resultant film, once exposed to light, can be redeveloped in color developer to produce an unusual pastel color effect.

See also: Photographic print toning

[edit] Environmental and safety issues

Many photographic solutions have high chemical and biological oxygen demand (COD and BOD). These chemical wastes are often treated with ozone, peroxide or aeration to reduce the COD.

Exhausted fixer and to some extent rinse water contain silver-thiosulfate complex ions. They are far less toxic than free silver ion, and they become silver sulfide sludge in the sewer pipes or treatment plant. However, the maximum silver concentration in discharge is very often tightly regulated. Silver is also a somewhat precious resource. Therefore, in most large scale processing establishments, exhausted fixer is collected for silver recovery and disposal.

Many photographic chemicals use non-biodegradable compounds, such as EDTA, DTPA, NTA and borate. EDTA, DTPA, and NTA are very often used as chelating agents in all processing solutions, particularly in developers and washing aid solutions. EDTA and other polyamine polycarboxylic acids are used as iron ligands in color bleach solutions. These are relatively nontoxic, and in particular EDTA is approved as a food additive. However, due to poor biodegradability, these chelating agents are found in alarmingly high concentrations in some water sources from which municipal tap water is taken. Water containing these chelating agents can leach metal from water treatment equipment as well as pipes. This is becoming an issue in Europe and some parts of the world.

Another non-biodegradable compound in common use is surfactant. A common wetting agent for even drying of processed film uses Union Carbide/Dow Triton X-100 or octylphenol ethoxylate. This surfactant is also found to have estrogenic effect and possibly other harms to organisms including mammals.

Development of more biodegradable alternatives to the EDTA and other bleaching agent constituents were sought by major manufacturers, until the industry became less profitable when the digital era began.

In most amateur darkrooms, a popular bleach is potassium hexacyanoferrate (III) (common name potassium ferricyanide). This compound decomposes in the waste water stream to liberate cyanide gas. Other popular bleach solutions use potassium dichromate (a hexavalent chromium) or permanganate. Both ferricyanide and dichromate are tightly regulated for sewer disposal. In order to meet the regulation, the solution must be diluted 20 000 (twenty thousand) times or more. All of these popular black and white bleaches are damaging to the environment, and should be replaced with any of the existing eco-friendly alternatives.

Borates, such as borax (sodium tetraborate), boric acid and sodium metaborate, are toxic to plants, even at a concentration of 100 ppm. Many film developers and fixers contain 1 to 20 g/L of these compounds at working strength. Most non-hardening fixers from major manufacturers are now borate-free, but many film developers still use borate as the buffering agent. Also, some, but not all, alkaline fixer formulas and products contain a large amount of borate. New products should phase out borates, because for most photographic purposes, except in acid hardening fixers, borates can be substituted with a suitable biodegradable compound.

Developing agents are commonly hydroxylated benzene compounds or aminated benzene compounds, and they are harmful to humans and experimental animals. Some are mutagens. They also have a large chemical oxygen demand (COD). Ascorbic acid and its isomers, and other similar sugar derived reductone reducing agents are a viable substitute for many developing agents. Developers using these compounds were actively patented in the US, Europe and Japan, until 1990s but the number of such patents is very low since late-1990s, when the digital era began.

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