Developer
From Silvergrain Research
In developed-out processes, exposed material is first processed in developer. In a black-and-white developer, the developing agent chemically reduces the silver halide to metallic silver, thereby forming visible image. Thus developer is a solution of a certain reducing property.
In developed-out processes, light exposure makes only invisible latent image, which is greatly magnified in the developer to make visible image. In the case of silver-gelatin photography, the size of latent image can be as small as 4 silver atoms, while a fully developed grain in a high speed emulsion may contain about 20 billion silver atoms. This translates to a chemical amplification with the gain factor of up to 5 billions.
Development process is one of the most important technologies in photography, which helped to greatly enhance the photographic speed in the 19th century. Another technology that contributed to rapid exposure is optical designs with large apertures.
On the other hand, printing-out process makes visible image during exposure, and the material does not use a developer. Such materials are usually washed in plain water or acid solution to remove unexposed, unreacted sensitizer. Some people call these rinse baths developer, but it is a misnomer.
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Negative v. Print
In practice, different types of developers are used for negatives (film and plate) and print, although there is some overlap in the chemical formulation of these. Negative developers are used for in-camera material to obtain high quality negative images. They are usually designed for fine grain and good definition and tonality. On the other hand, print developer is designed for good tonality, image hue, contrast, density, and processing consistency. Unlike in negative materials, the fine grain property is irrelevant.
There are occasionally "universal" developers that can be used for both negatives and prints, but they are too much of compromise and they do not provide the best of either world for continuous tone pictorial applications.
Classification of Negative Developers
Fine grain
Accutance
Speed enhancing
Lith effect
A booklet on Kodak Fine Line developer by T. R. Hearsum, 1975.
Classification of Print Developers
Image Tone
Image Contrast
Processing Style
More academic classifications
Chemical v. Physical
Solvent v. Nonsolvent
Organic v. Inorganic
Developer Constituents
Solvent
Developer solution is usually an aquaous solution, but the solvent system may incorporate other solvents. Alcohols and glycols are common, but for commercial products, nonvolatile solvents are preferred. These nonwater solvents are often used when highly concentrated liquid developer is prepared.
Developing Agent
Developer usually contains one or more developing agents. Most modern developers use two developing agents. One developing agent is the main reducing agent, and the other is the electron transfer agent (ETA), and the developer is made so that they give superadditive development. Common combinations include metol-hydroquinone (MQ), phenidone-hydroquinone (PQ), metol-ascorbate, phenidone-ascorbate.
Alkaline Agent
Most organic developers are mild acids but they develop in alkaline solution (the dissociated form is the active spiecie). Therefore, the developer usually contains one or more alkaline agent to raise the solution [pH] to a desired level. The developer activity varies with pH, but the way it varies is agent-specific, and it is not very generalizable. For the case of typical Metol developers, a extremely fine grain developer would commonly use the pH range between 7 and 8. A general purpose film developer would use 8 to 9. Accutance developers, push developers, and other developers usually use pH greater than 8.5, and more commonly greater than 9.5.
Suitable alkaline agent depends on the target pH. It is important to select an agent that exhibits good buffering property at the target pH, so that the developer solution is reliable. For low pH, triethanolamine (7.4 to 8.3) and borate (8.2 to 9.5, in any form of boric acid, borax, metaborate) are useful. For higher pH (9.5 to 11), carbonate is useful. There are other useful bases. Alkanolamines and substituted alkanolamines are useful, and useful compound can be selected depending on the target pH. Alkanolamines have some interaction with silver halide, especially in the case of primary and secondary amines. Tertiary amines are less reactive with silver. Alkanolamines can also act as a chelating agent, solvent, radical scavenger, preservative, in addition to the pH buffer, and when used effectively, especially in blends of multiple amines, they can be very useful in formulating ascorbate developers.
Preservative
Sulfites are by far the most common developer preservative in the history of photography. It is thought to slowly react with oxygen to form sulfate in developer solution, thereby protecting the developing agent. It also sulfonates quinone forms of aromatic developing agents, depriving of the quinone's oxidation acceleration properties. However, in certain conditions, aerated sulfite can produce potent oxidizing agent and close attention must be paid to avoid this condition, especially in low pH ascorbate developers.
Sulfite is occasionally added as metabisulfite or bisulfite (they are really the same thing---see the link), and then it is converted to sulfite by addition of one or more alkaline agents.
Sulfite may be present in a developer in the form of amine adducts. Kodak HC-110, for example, contains sulfite as amine adducts.
Other preservatives include hydroxylamine and other mildly reducing agents, but they are not very common in b&w developers.
Ascorbate is often said to act as a developer preservative, but this author questions such a role in many situations. Ascorbate is a very effective developing agent, and reacts very rapidly with mild oxidation condition. This is not a very desirable property for a preservative.
Silver halide solvent
Restrainer
Chelating Agent
Developing Agents
glycin Ascorbate Developers D-76 Dilution Film Developer Recommendations Japanese Developers Lith Printing pH Print Developer Recommendation Silver-gelatin photography Surfactant Survival Chemicals Ilfosol S
