Film Chemistry

This is intended a a brief overview of the chemistry involved in processing film. Perhaps the best in-depth resource is Steve Anchell's The Darkroom Cookbook, 3rd edition, (Focal Press, 2008, 978-0-240-81055-3). This is an exhaustive resource for photographic chemistry, and is highly recommended.


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.


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.


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.


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.

Stop Bath

What Stop Bath Does

As mentioned above, stop bath, a weak acid, chemically neutralizes developer, which is alkaline, or a base.

What Stop Bath(s) Is (Are)

Most stop bath is acetic (around a 1% working solution) or citric acid (around a 5% solution). Stop bath may contain an indicator, or a chemical which changes color as it is exposed to developer. Indicator stop will go from a bright yellow when mixed to a working solution from concentrate, to clear, then purple-blue when exhausted.

The Case for Water

The following information is paraphrased from Steve Anchell's The Darkroom Cookbook, 3rd edition, (Focal Press, 2008, 978-0-240-81055-3), pages 103-104.

A one-minute running water bath may be used instead of chemical stop bath. Alternately, fill the tank with water, agitate for 20-30 seconds, and repeat three times. There is a risk of producing pinholes, as a common ingredient in developers, sodium carbonate, will react with acids to form dioxide gas, which in turn can cause blistering of the emulsion. The water bath alleviates the chances of this happening. It does not stop development as fast, but is generally considered fast enough.

Another problem with (particularly) acetic acid stop bath is the fumes. These are an irritant, and over time can damage the sinuses. Using citric acid stop, or water baths will solve this.


What Fixer Does

As stated above, fixer removes under- and unexposed silver halides from the emulsion, thereby making it insensitive to light. Fixer first converts the unused halides into "an insoluble but not very stable compound". Further fixing breaks these compounds down into a soluble form, which can be washed away.

What Fixer Is

Although there are some exotic fixers, for the most part there are two types, regular, which is mainly sodium thiosulfate, and rapid, which uses ammonium thiosulfate. Additionally, regular fixer is acidic, while rapid is alkaline. Rapid fixers have the advantage of making hypo-clearing unnecessary, and require shorter times, but they are more expensive, and have a shorter shelf life than acidic fixers.

Fixers may or may not have a hardener added, such as sodium acetate. This is designed to enhance the stability of the gelatin emulsion. Hardeners may not be necessary, as modern films incorporate a hardener.

Hypo-Clear (Wash Aid)

What Hypo-Clear Does

Hypo-clear helps convert residual fixer to a more water-soluble form. This decreases the amount of wash time needed. It is used for film, and for fiber papers. RC papers to don require hypo-clearing.

What Hypo-Clear Is

Sodium sulphite, 200g/liter. Sodium bisuphite can be added for film, 50g/liter.

Drying Aid (Wetting Agents)

What a Wetting Agent Is

Wetting agents are generally a mild detergent, greatly diluted in the working solution. Woolite™ can actually be used.

What a Wetting Agent Does

Wetting agents break the suface tension of water, causing it to sheet off of film, rather than bead up. This helps eliminate water spots. Use care when diluting from the stock solution, as not enough dilution can leave 'soap scum' on the film.