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.

Daguerrotypes, the first commercial photographic processes, were made with silver-coated copper plates; which were "sensitized" with iodine crystals (creting 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 development abandoned the use of mercury and as photography advanced, development of images, both in film and paper.....

The processing of film and papers consist normally of a few steps: Development, Stop Bath, Fixing, Washing, and Stabilization. Some processes also include toning.

Chemicals

Developer

Developer is a reducing agent that converts the latent image into metallic silver. 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
  • 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. 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.

Stop Bath

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

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. Acid can be added to the fixer to extend its life, but carefully so as not to precipitate the sulfur (cloud the fixer). Ammonium Thiosulphate is much faster and efficient for modern emulsions. They 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)
  • Berg Bath (trademark or Berg chemicals)