Theory of Development (How developing fluids affect B&W film)
To be practical, a developing agent must:
- clearly differentiate between exposed and unexposed silver halide; i.e., be free from fogging;
- reasonably stable in solution;
- sufficiently soluble in water;
- must not soften the gelatin layer; and
- should be non-toxic.
Components of a Practical Developer
The most successful and the most commonly used developers are organic compounds. The mechanism of development is a rate process depending upon time and temperature as well as concentration.
- Developing agent(s) - to reduce silver halide to metallic silver
- Alkali - to keep the developing solution at the proper pH (a base) for the developer activity.
- Preservative - sodium sulfite) as an anti-oxidant agent that combines with developer by products that affect the rate of development.
- Restrainer - potassium bromide to lower fog (from unexposed but chemically reduced halide crystals.
Characteristics of the Principal Film Developing Agents.
Hydroquinone (p-dihfdroxybenzine) Hydroquinone is a slow, powerful developer which is used alone where high density and contrast are required, as in (litho) process work, but more often with metol (see below). Alone it is greatly retarded by low temperature being practically inert below 55°F. Above 70°F excessive fog and stain may be encountered.
Metol (p-methylaminophenol) Metol is an extremely energetic, soft working developer not greatly affected by temperature or by the restrainers. Metol with sulfite alone is a useful negative developer where low contrast is desired. With the addition of an alkali it forms a more energetic but soft working developer which is useful for thinly-coated, fine-grain negative emulsions. It is usually used with hydroquinone. The speed of development (as measured by the growth of density and contrast) is greater for the combination than with the sum of both metol and hydroquinone separately. This is called “additivity.” Kodak called the stuff "Elon" in the event you find a can of it in a time capsule.
Paraminophenol This is a rapid soft-working developer similar to metol but is exhausted more rapidly. It is useful for high-temperature development and in the preparation of highly concentrated developers of excellent keeping properties. (Rodinal, Azol, etc.)
Phenylenediamine Para- and ortho-phenylenediamine are the only truly fine-grain developing agents but the low reducing energy and the solvent action on silver halide require a considerable increase in exposure. To avoid this, phenylenediamine is often combined with glycine, metol , or other more energetic developers. Although the grain is not as fine as with phnylenediamine alone, the speed loss is much less. The phenylenediamines are strongly toxic and contact with the solution should be avoided.
Pyro (pyrogallol) (1,2,3-trihydroxyhenzine) Pyro, has been almost completely superseded by metol-hydroquinone (MQ) and other non-staining developers. Pyro produces two images, one of silver and the other a dye image which is an oxidation by-product of the developer. The density of the stain image depends on the amount of sulfite but is influenced by the temperature, dilution, and other factors that affect the rate of oxidation. Pyro is often combined with metol, less frequently with glycin. With diaminophenol, it develops without alkali.
Pyrocatechin (catechol) (o-dihydroxybenzene) Pyrocatechin resebles pyro in many ways. Its stongly marked tanning action has been used in compensating developers. The fine-grain developer, Meritol, is a compound of pyrocatechin and paraphenylenediamine.
Phenidone (1-phenyl-3-pyrazolidone) Alone Phenidone is an active developer but of exceptionally low contrast. It is used chiefly with hydroquinone, being much more efficient than metol.