Photographic film and paper developers generally consist of four components which work together to develop your images. Here’s a quick overview of the components and their purposes.
Please note: I’ll mention some chemicals in this article. Some of them are pretty harmless, but some can be downright dangerous. Please educate yourself about safe handling of chemistry before you start messing around with any of them.
The active part of a developer is the developing agent. It converts silver halide crystals into stable metallic silver that’s captured in the emulsion. Metallic silver is awesome because it blocks light. And as a noble metal, it’s also very stable.
There are a bunch of developing agents. metol, hydroquinone, and phenidone are some common ones. Each developing agent acts differently on silver halides, which is why we have so many different developer formulas. Some formulas combine multiple developing agents for the way they work together. One common combination is metol with hydroquinone (we call these MQ developers.) These two work together more strongly than each one does individually. This is called superadditivity.
Developing agents are easily oxidized during development or even just while they’re stored in a bottle before use. To prevent this, we add a preservative to keep them fresh. It’s the same idea as adding a preservative to processed food so it keeps longer. In fact, it’s often the same preservative. Go through your pantry and see how many packaged foods contain sodium sulfite. That’s the same preservative used in most developer formulas.
As naturally reactive as they are, most developing agents don’t really get going until they’re in an alkaline environment. The accelerator raises the PH to provide that environment.
Many of the chemicals we use as accelerators also have uses in the home as cleaners. Borax is used in many cleaners, as is sodium carbonate (a. k. a. washing soda). Sodium hydroxide, a strong base, is commonly used to open clogged drains. These three are frequently used in photo developers (but not simultaneously!)
All of the components so far keep our developer converting silver halides to metallic silver. This one slows down the developing action.
If the developing agent is overly activated (too strong an alkaline environment) it will reduce all of the silver halides, not just the exposed ones that make up our image. To avoid this fog of uncontrolled development, we use a restrainer in our developer. This pulls back the action of the developing agent and encourages it to only work on the exposed silver halide crystals.
The most common restrainers are potassium bromide and benzotriazole. Some developers, such as D-76, are formulated so carefully that they don’t need a restrainer.
Go read Troop and Anchell
Now that you understand what the four basic developer components are doing, you may be thinking about your preferred developer and wondering how its components work together. I highly recommend that you get your hands on one or both of the following books.