Development

Development or Processing is a photographic process in which the latent image contained in film or paper after exposure is converted into a visible and permanent image.

Photography, since its invention, relied on metallic salts (silver, iron, etc) that were affected by light. In the 17th century Angelo Sala, an Italian scientist, reported that silver nitrate became black when exposed to light. Silver salts react to light in a predictable manner, the more exposure, the darker they become. This is applied into film or glass plates, which after exposure need to be developed.

Daguerreotypes, the first commercial photographic processes, were made with silver-coated copper plates; which were "sensitized" with iodine crystals (creating Ag-I crystals). After exposing to light the plate was exposed to Mercury vapors until the image was visible and then washed in a Sodium Thiosulfate solution.

Further processes evolved and abandoned the use of mercury. However, development of images, both in film and paper is mostly a chemical process. The processing of film and papers consist normally of a few steps: Developer, Stop Bath, Fixing, Washing, and Stabilization. The initial results is generally a negative, with the exception of Reversal Processing which ends in a direct positive. Some processes also include toning, either for color change or for improved permanence.

Chemicals

Chemical development is the most common method, and involves chemical agents that are mixed to produce the working solutions. Many chemicals can be bought in pre-mixed packages, either solid or liquid for use by the photographer. Several of these formulas have been published, and can be found online or in books, but most are proprietary.

Chemicals must be handled with caution, the use of eye and skin protection is very important, as well as good ventilation in the space.

Water is the most important chemical used in development, impurities present in water might interfere with development and must be compensated for. Among others, Iron and other metallic impurities interfere with chemicals, hardness can cause water marks during drying, and suspended solids might deposit on the image creating spots. Potable water is generally acceptable for processing; however for the preparation of concentrated solutions distilled or deionized water is sometimes recommended.

Black and White Development

Black and white images are mad by converting the exposed silver halide crystals to metallic silver. Each of the small crystals will become a grain.

Chemistry is simple, and many factors affect development such as temperature, time, agitation, water quality, etc. The chemicals that re used for developing are listed below

Developer

Developer is a reducing agent that converts the latent image into metallic silver. The composition of the developer affects the final quality of the image, by affecting the size of the grain, accutance, definition, etc.

Developers are made of four basic components:

• Developing agent: a chemical compound that reduces silver halides that have been exposed to light to metallic silver. Several developing agents have ben used through history and the most common are:
  • Hydroquinone
  • Metol or Elon, sulfate salt of N-methylaminophenol
  • Phenidone
  • Pyrogallic Acid (pyro)
  • para-amino-phenol
  • Pyrocathechin (Catechol)
  • para-Phenylenediamine (ppd)
  • Ascorbic acid (vitamin C)

Some of this agents have unique characteristics and mixtures of them have been tested for increased development activity (super additivity).

• Preservative: to reduce the oxidation of the developing agent and extend the life of the solutions. These agents prevent oxygen in the air from making the developer useless too quickly.
  • Sodium sulfite
  • Sodium bisulfite

Potassium salts can also be used but that is less common

• Accelerator: A compound that increases the activity of the development by changing pH or REDOX potential without being a developer itself.

Alkaline pH helps in increasing the development activity and reducing the time needed from hours to minutes. Depending on the final pH there are mild (pH 8-10), moderate (pH 10-11) or strong alkalis (pH>11). The strongest the alkali the faster the developer action, and the contrast that is attained. In many cases buffer solutions are used to ensure that the pH is fairly constant during the life of the developer.

• • Mild Alkalis: Borax, balanced alkali (Kodak)
  • Moderate Alkalis: Sodium (potassium) carbonate
  • Strong alkalis: Sodium (potassium) hydroxide

• Restrainer: compounds that prevent the formation of "fog" in the image.
  • Potassium bromide
  • Potassium iodide
  • Benzotriazole (BZT)

Some other chemicals might be used in the formulation of developers, such as water softeners and chelating agents to prevent the ions present in water to interfere with development or wetting agents to help with elimination of anti-halation layers or dyes.

Developers can be a single solution, or a double solution which is called Divided developers.

Stop Bath

In order to stop development, a stop bath of a mild acid is used. Acetic or citric acid solutions are used, however this must be weak to avoid the formation of CO2 bubbles which can create pinholes in the emulsion. Water can also be sued as a stop bath

Fixer

Fixation is a critical step in making images permanent. Fixation must remove the unexposed light sensitive compounds while not damaging the image that is already formed.

There are 3 types of fixer, depending on the pH of the bath: neutral, acid and alkaline.

The most common fixing agents are thiosulphate (Hypo or hyposulfite) compounds, plain Sodium Thiosulphate was used for many decades, but its action is relatively slow and it is affected by the alkali that is carried over from the developer and exhausted quickly by iodine salts used in modern films. Ammonium Thiosulphate is much faster and efficient for modern emulsions (1980s onwards) that use iodine compounds. Acid can be added to the fixer to extend its life, but carefully so as not to precipitate the sulfur (cloud the fixer).

Alkaline fixers are easier to wash from film and paper and can't over fix.

Acid that is added to fixer must be a weak acid, such as acetic mixed with sulfite or bisulfite as preservatives.

Hardener compounds were added to fixer, to make the wet emulsion harder and prevent scratching.. These are not needed with modern emulsions.

Stabilizers

Any remaining fixing salts will damage the photographic emulsion, and several stabilizing solutions exist. These stabilizers are used either before washing the print (HCA or Berg Bath) or as final no-rinse steps (Systan)

• • Hypo Clearing Agent (HCA)
  • Systan (trademark of AGFA), currently and improved versin is made by Adox as Adostab
  • Ag-Guard (trademark of FujiFilm)
  • Berg Bath (trademark or Berg chemicals)

Washing

After fixing it is very important to wash away all remaining fixer, by-products and silver compounds from the emulsion, this ensures longevity of the images. Washing also carries away the sensitizing dyes that are used in films to make them sensitive to the whole spectrum of light.

Several methods of washing exist, either using running water or by changing water in the small developing tanks. Thorough washing is highly recommended.

As final step of washing a wetting agent (Kodak Photo-Flo, Edwal LPN or similar) is used to prevent the formation of drop marks while drying. In lieu of a wetting agent, deionized water can be used for the final rinse.

Other Chemicals

Specialty chemical solutions, such as intensifier, reducers, and bleachers are used occasionally.

Darkroom

Darkroom is a room isolated from sunlight, where the development process is carried out. Depending on the type of process it can be as small as a changing bag, or as large as a whole newspaper floor. It must be kept clean and have good ventilation. Commercial film processing machines essentially act as self-contained darkrooms, where the whole process is carried out in a small space.

Paper positives are made in the darkroom, by enlarging and development of the paper. During enlargement of Black and White paper a safelight can be used, due to paper being sensitive only to blue and green light. Safelights are units which filter the blue and green parts of the spectrum, traditionally they used RED filters; but amber has replaced them. Safelights can also be used for orthocromatic plates and film and non-silver based processes such as cyanotypes

Enlarging

Photographies are captured in film, which can be a small image. Those images can be printed using an Enlarger by projecting the image onto paper.

Color Development

Color film has been available since the 1930s, and it is based on different light sensitive couplers and silver. Development of these films follows similar steps to Black and White. However the chemicals used help develop the chromogenic layers, leaving only dyes behind. A step to remove the silver is also incorporated in the development of color images. This bleaching step is in many cases part of the fix step, thus called Blix

Reversal Development

Reversal development produces a positive directly from the exposed film. While this is common in color slides, with Kodachrome introduced in 1934.

Black and white reversal is less common; and only a few films are suitable for this process. Reversal development process requires that film has more silver than normal films. After the first developer, a bleach step is performed to wash away the negative silver image. The silver halide grains left behind are then exposed to light or receive a chemical fogging process which produces a negative of a negative (i..e a positive); a second developer stage is then applied followed by fixing.

Color film reversal is described in detail the E-6 process page of camera-wiki.org.