How do developers work?

Answer 1

When someone develops film (or paper, or plates but we'll call it all film for now), they put film they exposed in their camera in a bath that converts the "latent," or "hidden" image into a "visible" one. The bath's called Developer, and the developers for black & white are very different from those for color.

First, we need to talk about how the latent image got there in the first place. The film has a light-sensitive surface called the emulsion. In it there are silver halide grains--silver atoms bonded to chlorine, bromine or iodine atoms. If you expose a silver halide grain to light, the photons in the light will kinda loosen the bond between the silver and the halogen. I'm trying very hard not to make this a nuclear physics class, so just remember that unexposed silver halides are tightly bonded together, and exposed ones are loosely bonded together.

Now as to this developer stuff. Its job is to complete the loosening of the bond between the silver and the halide...and allow the water in the developer to wash the halides away, leaving the silver behind to form the image. The developer contains five basic kinds of chemicals: water, developing agents, preservative agents, accelerators and restrainers.

Water does two things for us: it carries the rest of the chemicals into the film, and it carries the halides away from the film.

The developing agents are the ones that loosen the bonds. Some of them give you a lot of density quickly, and others give real nice highlights and tonality. There are tradeoffs, of course: if you make a developer using only the first kind you'll have a real harsh, constrasty picture; a developer using only the second takes a long time to work. (There are a class of developers called "lith" developers that produce extremely contrasty negs, and they're made only with high contrast agents.) So...most developers have some of each kind. The high-contrast agent is always hydroquinone. The "soft" agent can either be Metol (Kodak's brand name for paraminophenol sulfate) or Phenidone (Ilford's brand name for 1-phenyl-3-pyrazolidone). Metol has two problems: it's a little tricky to get it into solution, and it causes contact dermatitis. Metol's big advantage is that it can be used by itself. Phenidone is essentially non-toxic, it dissolves easily, but if you don't add some hydroquinone it won't work. I think it's kinda neat that some of Kodak's own formulas--the first developer for E-6 slide film, for instance--contain Phenidone.

There are a few other developing agents out there, like Amidol, unsulfated paraminophenol and pyrocatechol, but they've largely fallen out of favor--Metol, phenidone and hydroquinone work over a very broad range of applications, so they're the agents of choice. Amidol was especially bad--it is a nice developer except that it's very toxic and it's got a working life in solution measured in hours.

Next are the preservative agents. Developers oxidize in air, so they add a chemical that scavenges oxygen to the developer. It's almost always sodium sulfite. You might see a little bit of sodium bisulfite in a developer, but sodium sulfite is very popular for this--it's cheap and it works.

It's time to talk accelerators. Developing agents work in an alkaline environment--the more alkaline, the better. Adding chemicals like borax, sodium carbonate, trisodium phosphate and sodium hydroxide to the developer makes the bath work faster, and can change the nature of the final picture. Needless to say, lith developer--made specifically to give solid blacks and no grays--contains a lot of sodium hydroxide.

And then there are restrainers. These prevent halide grains that weren't exposed from being developed. This happens, and it's called "chemical fog." There are three popular restrainers. The most common, least expensive and least powerful one is potassium iodide. There's also Benzotriazole, which works somewhat better and which Kodak calls Anti-Fog #1, and 6-nitrobenzimidazole nitrate, which works a lot better (costs more, too) and is called Kodak Anti-Fog #2.

There are also "specialty" additives you can put in a developer. My favorite class of specialty developers are tropical developers. If you develop old-school films like Kodak's Tri-X in high-temperature developer, like you'd have if you were working in the tropics, the emulsion can soften up and, potentially, separate from the film base. Normally it just wrinkles. In either case, if it happens the film's ruined. Adding a pound of sodium sulfate per gallon of developer will harden the emulsion so you can develop in 100-degree chemistry. Today, that's not a problem since most emulsions are prehardened.

Color ChemistryColor is a lot more complex than black & white because, in addition to splitting the halides away from the silver ions, you also want to form the dyes that make up the picture. There are three parts to the dye. One part is in the film already. Another part is in the developer. When you develop a silver halide grain it oxidizes, and the little cloud of oxidized developer connects the dye-part in the film to the dye-part in the developer.

This is where it gets fun: You need this oxidation to make the dye, but the developer contains a very potent antioxidation agent--sodium sulfite. If there's too much in the developer, the sulfite will compete with the dye for this oxidation...and in this contest, the sulfite always wins. They eventually discovered you can only put a little bit of sulfite in a color developer.

Developing SlidesSlides are an interesting case because you develop them twice. You know how when you take a picture you always get a negative back--lots of light gives dark areas, a little light gives light areas. In a slide you want the areas that got lots of light to be light and the areas that didn't get much to be dark, so what they do is to first develop the film in black & white developer. Then they reexpose the film and develop it in color developer. You can reexpose the film by putting it in a bowl of water and sticking it under a 500-watt lamp, but most people these days use chemical reversal baths, which are based on stannous chloride--a halide of tin, which fogs unexposed film. The color developer contains a chemical called citrazinic acid, which adjusts contrast.

Oh yeah, I always loved this: Stannous chloride reversal bath is acidic. You are supposed to drain the first developer then, without rinsing, pour this acid into your tank...where it proceeds to react with the developer, form carbon dioxide gas and pop the lid off your tank. It doesn't hurt anything--at this point you can develop the film in broad daylight, and I recommend doing it at least once because watching it is really cool--but it gets reversal bath all over the place. I usually rinse the film in stop bath to neutralize the developer. You can then go to reversal without forming gas in your tank.

A lot of new inventions are really happy accidents; such is the case with stannous chloride reversal bath. When darkrooms were first getting started, people built their own processing tanks. Some people would make them from wood, coat the wood with asphalt, and develop away. Others got ambitious: they bought tin-plated steel, bent it into the shape of a tank--for sheet film, a "tank" is more of a box--and soldered it with tin-lead solder. Then they tried developing film in these tanks, and got back a solid-black negative. They didn't realize tin would leach off the walls of the tanks and attack the bond between silver and the halide group attached to it. They did realize any film you tried to develop in a tin tank would turn black and restricted tin tanks to stop, fixer and wash. Many years later they needed a bath that would do that on purpose, and someone remembered the pictures of the Grand Canyon he ruined by developing them in a tin tank...eureka! Find a form of tin that will readily dissolve in water and we'll call it good.