Chemistry: Developers
For both film and paper there are a wide variety of available developers, some very similar to each other, some very specific one-trick ponies. Each is designed with a specific goal in mind, however, even if that goal is to be the most general, all-around developer on the market. With this in mind, developers all generally work in the same way, with generally the same components. The differences lie in what those components actually are chemically, and what the mixture is overall.
What Developers Do
In short, developers reduce the exposed silver halide to metallic silver. This forms a visible image from the latent image. The latent image is what is recorded on the film or paper when it was exposed. In a sense it is the possibility of an imge. The developer makes that possibility a reality.
What Developers Are
- Reducing Agent
- The first of three major components in developers is the reducing, or developing agent. It is the primary ingredient, but is not very active chemically. Common reducers are hydroquinone, metol, and phenidone.
- Accelerator
- The accelerator is added to the reducing agent to speed development. It also increases contrast. Excessive accelerator can result in chemical fog, excessive grain, soft emulsions, and short shelf life. Accelerants are an akali, commonly sodium or potassium hydroxide, sodium carbonate, kodalk, or borax. Yes, that is the same stuff in the box at the grocery with that twenty-mule team on it.
- Restrainer
- A restrainer is added to prevent the developer and the accelerator from working so quickly as to cause chemical fogging, or reduction of un- or underexposed silver halides. Simultaneously the restrainer helps control excessive contrast. Potassium bromide is a common restrainer.
- Preservative
- One final additive is sodium sulfide, which slows the oxidation of the developer, increasing its shelf life as a stock solution.
Developers are normally treated as one-shot, or used once at working strength. This has the distinct advantage of ensuring fresh developer. Most can be used again, but great care must be taken in determining the area of film that has been processed in order to calculate the extended development for subsequent film. This is extremely difficult to do in a classroom lab situation. Another method of extending developer life is to use replenishers, which replace components of the developer used in processing. This again is very difficult in a classroom lab, but may work fine in the more tightly controlled conditions in a processing lab.
