To read a brief out-line by Ilford (written 1984), on the basic steps involved in home developing a black & white film, click here to download the article as a pdf file.

The following is an excellent article on the fundamentals of developer chemistry.
It is taken from Practical Photography magazine, September 1962. Chemical prices quoted in the text apply to that date.

1. A DEVELOPING AGENT, which chemically 'reduces' the light-struck silver halides to black metallic silver.
2. An ALKALI, which accelerates the activity of the developing agent.
3. A PRESERVATIVE, which keeps the developer from premature oxidation and reduces the risk of stains.
4. A RESTRAINER, which inhibits the formation of 'fog', i.e. the development of unexposed silver halides.
5. A SOLVENT, which is invariably water.

Understanding a developer's action and its constituents is not essential to the production of a reasonable negative of a normal subject. All you have to do is follow explicitly a few simple instructions included with a bought packet or bottle of developer. BUT, the more you know of developer types, their constituents and capabilities the greater the accuracy with which you can obtain exactly a required effect in abnormal as well as in run-of-the-mill conditions.

What is a developer and why is it so important ?
A photographic emulsion comprises a suspension of light-sensitive crystals of silver halide in gelatin. If you expose this emulsion to image forming light for long enough the light struck crystals will darken naturally to form a negative representation of the image. However, you would need to expose your films for hours in bright sunshine before you obtained a satisfactory image.

This is not the case, because silver halides, when immersed in a suitable (developer) solution, increase their sensitivity. As a matter of fact, a developer produces a gain in emulsion sensitivity of about a thousand million times. In other words, a developer permits an exposure that is so brief that the image cannot be seen before development and is therefore called a "latent" image and which only becomes visible after development.

Here's what happens during development
On exposure to image-forming light the crystals of silver halide do not alter visually—but they do chemically. On each crystal is formed a latent image speck which varies in size in proportion to the intensity of the light that hit it. During development the crystals are reduced to black metallic silver, the action starting at the latent image specks, until you see the image in full density in its familiar negative form.

It is a fact that only a few chemicals can distinguish between light-struck crystals and those that were unexposed and which therefore earn the label "developing agents".

Developing agents
A developing agent is a chemical which will blacken the parts of the emulsion which have been exposed to light. Only a few are able to distinguish the difference between light-struck crystals and those that were unexposed. Here are a few common developing agents and, since different developing agents are useful for different purposes, their main characteristics.

Metol
Metol is an abbeviation of the true chemical name monomethyl para-aminophenol sulphate. Metol is a white crystalline powder and costs 4s.9d (24p) for 25 grams. Metol produces images characterised by fine grain, very low contrast and high emulsion speed. Because they are so soft working, metol solutions are particularly useful when maximum shadow detail is required at the expense of bright highlights. However, metol is generally used in conjunction with another developing agent, usually hydroquinone.

Hydroquinone
The full name of hydroquinone is para-dihydroxybenzene, though it is often abbreviated still further to quinol. Hydroquinone is a fine white crystalline chemical which discolours if stored in an open jar. The main characteristic of hydroquinone as a developing agent is high contrast when used in conjunction with a strong alkali, such as caustic soda. Hydroquinone costs 3s.3d (16p) for 25grams and 7s.3d (36p) for 100 gram bottle.
When metol and hydroquinone are used in conjunction with one another in a single developing solution it is known as an MQ developer. MQ solutions combine the advantages of both individual developing agents, i.e. the high film speed and fine tonal gradation of the metol with the high contrast and good density of the hydroquinone. In normal developers the metol : hydroquinone ratio is about 1 :4, although this ratio may be decreased for a softer working solution.

Amidol
Amidol solution is characterised by rapid development but poor keeping properties. Amidol solution is said to display the Sabattier effect markedly and at one time was used as the developer in the bromoil process. Amidol solutions must be stored in an airtight bottle away from light. Amidol costs 4s.3d (21p) for 25grams and 11s.6d (58p) for a 100gram bottle.

Phenidone
Phenidone produces images characterised by high emulsion speed, low contrast and high fog level. However phenidone is usually used in conjunction with hydroquinone (P.Q. developer) with which it makes a good general purpose developer. We have said that the ratio of hydroquinone to metol is generally 4 : 1—but the ratio of hydroquinone to phenidone is 40 : 1, which makes for economy, higher concentrations of stock solutions, and better keeping properties. Phenidone costs 7s.3d for 10grams.

Pyro
Note well:—Pyro is very poisonous. Used as a developer, pyro has peculiar properties. The image formed by it consists not only of black metallic silver but also of a brown stain formed by developer oxidation products. However, as well as staining the photographic image pyro also discolours hands, fingers and dishes. Several years ago pyro was a very common developer but is now unpopular because photographers want a non-staining solution. Pyro developed negatives tended to be thin in density but crisp in definition.

Glycin
Glycin, unlike pyro, does not stain the image and has good keeping properties. One disadvantage of this developing agent is that it is very slow working and in extreme cases, takes several hours to produce a good contrast image of satisfactory density. The main use of glycin developers nowadays is for warm tones on printing papers. Glycin costs 5s (25p) for 25grams.

Para-aminophenol
This is a chemical relation of metol and has similar developing characteristics. Its main use nowadays is for highly concentrated developers which are diluted from 10 to 30 times to form a working solution. Para-aminophenol costs 2s.6d (12.5p) for 25grams.

Paraphenylene diamine
This is a fine-grain developing agent which, if used alone, demands long development times. A combination of paraphenylene diamine and pyrocatechin produces the developing agent commonly known as meritol, one of the developing agents in the well known solution meritol-metol. Paraphenylene diamine costs about 3s.6d (16p) for 25grams.

Pyrocatechin
Pyrocatechin is a tanning developer like pyro and when used in conjunction with a caustic alkali gives high contrast in short development times. Its main use nowadays is in warm tone developers for printing papers. Pyrocatechin costs 4s.6d (22.5p)for 25grams.

Developing agents are never used alone. A developer contains other constituents which are essential for a satisfactory working solution. The other constituents generally found in developers are:—

1. An alkali. To accelerate the activity of the developing agent.
2. A preservative. To keep the developer from premature aerial oxidation and to reduce the risk of stains.
3. A restrainer. To check fog—(fog is the density that results from the development of unexposed grains).
4. A solvent. Invariably water.

Let's have a close look at each of these extra constituents in turn:—

An alkali
The most common alkali in developing solutions is sodium carbonate—but that does not mean ordinary washing soda for this is impure. Also note well that the "carbonate of soda" sold by stores is not sodium carbonate at all but sodium bicarbonate. Many fine-grain developing solutions use borax as the alkali instead of sodium carbonate.

The Preservative
The two most common preservatives are sodium sulphite and potassium metabisulphite. When making up your solutions it is important to remember that sodium sulphite is sold in two forms:—crystalline and anhydrous. One part of anhydrous = two parts of crystalline.

The Restrainer
The most widely used restrainer is potassium bromide and even if the developing solution begins life without the bromide as a basic constituent, it certainly contains it as soon as the developing action starts for it is liberated in the process.
The whole purpose of a restrainer is to stop the unexposed crystals from developing or, in other words, to prevent fog. However, restrainers also tend to check the development of exposed crystals to a small extent and they therefore slow down the speed of the emulsion. Some organic restrainers are claimed to check fog without affecting film speed. These are known as "anti-fogging" agents. Paper developers always include a large amount of bromide for any trace of fog on a print is immediately noticeable and therefore MUST be avoided.

From the different types of developing agents and from the other constituents of a developing solution it must be obvious that there are innumerable combinations of developers with different characteristics for specialised purposes. For example, there are fine grain, high contrast, compensating,high definition, special paper developers, those that will cope both equally well with both films and
papers, and developer/fixer combinations.

• If you want to emphasize minute detail — use a high-definition developer;
• To cope with extremes of contrast — use a compensating developer;
• To boost the contrast of a flat subject — use an energetic developer;
• For maximum economy — use a universal developer;
• If you want maximum quality in broad areas of even grey tone — use a fine grain developer;
• For convenience — use a monobath.

In a normal developer the density difference between shadow and highlight area is a simple step, illustrated in the left-hand diagram as X.

In a high-definition developer, however, the density difference between shadow and highlight area is greatly increased due to the enhanced edge effect, indicated by Y.

High-definition developers
A high-definition developer works in this way:—
Theoretically, the difference in density between an adjacent highlight and shadow is a simple step. However, a high definition developer achieves its aim by increasing the contrast of this boundary by making the step steeper. It does this by forming a light line just within the edge of the shadow area and a dense line at the edge of the highlight. This is called an "enhanced edge effect" and produces a bitingly crisp image.
How does it work?
Bromide liberated in the developing process in the heavily exposed area (highlight) diffuses into the lightly exposed area (shadow) stopping development at this edge. This produces the light line. On the other hand, still fresh developer that has not been used in the lightly exposed area of shadow seeps into the highlight and builds up a line of high density at its edge. This produces the dark line. When using high-definition developers it is important that you give minimum development time and as little agitation as you can get away with. A good general guide is about 20 sec. agitation initially and then two or three seconds every minute. Over develop or over agitate and you will nullify the effect of using a high definition developer.
Excellent examples of high definition developers which you can obtain from your local photographic dealer are:—

• Ilford's Hyfin which costs 4s. for a carton containing 5 sachets each of which will make 600cc of working solution.
• Kodak's High-Definition Developer which costs 3s.3d. for 50cc of concentrated solution that will make 600cc of working strength.
• Johnson's Definol which costs 3s. for 100cc size of concentrated solution, 10s. for 500cc and 27s.6d. for 2¼litres.

Fine-grain developers
Fine-grain developers work in two main ways:—

1. Soft working action. If the silver grains in a negative were in an absolutely even pattern—in other words, if no two grains overlapped or were closer to one grain than another—there would be no problem of print graininess. However, such an emulsion which would produce a perfect negative has not yet been manufactured and the granularity of a negative is often evident after only 10 times enlargement. Why? Because the individual grains do not remain separate but clump together during development. If the developing solution contains less alkali than usual it is less energetic inaction and minimises the clumping of the grains and so reduces negative granularity. In fine grain solutions, borax is used as the alkali instead of sodium carbonate. Also, developers specially compounded for fine grain contain a relatively large amount of sodium sulphite.
2. Restricted development. When you first immerse an exposed emulsion in a developing solution the silver halide crystals begin to be converted into black metallic silver. Cut development time and the negative granularity will remain fine because the silver halide crystals are only partially developed. However, it must be obvious that if each individual crystal is developed to give only a small grain of silver more crystals must be exposed to build up negative density to a satisfactory level. In other words, all true fine-grain developers that work on this principle require an increase in exposure.

Compensating developers
A compensating developer has the ability to cope with high subject contrast. In other words, it boosts shadow detail yet prevents the highlights from blocking up by holding back their development. How is this effect achieved? By reducing the amount of alkali in the bath or using a softer working alkali such as borax. The effect is also produced by increasingthe amount of potassium bromide, i.e. the restrainer, in the solution. Compensating developers attack the surface of the emulsion first and only gradually penetrate it which means that you can leave the negative in the solution until the shadows are of full density without the fear that you will burn out the highlights. From this description it is noticeable that there is similarity in composition between a fine-grain and compensating developer. Because of their low alkalinity most fine-grain developers are also compensating in their action.

High-contrast developers
As a general rule, the higher the alkalinity of a developer the higher the resultant image contrast. Following this guide, as hydroquinone is characterised by high contrast this is generally used as the developing agent and to increase contrast still further, sodium hydroxide instead of sodium carbonate is used as the accelerator. If you make up your own MQ developer, increase the proportion of hydroquinone over the metol to 10 : 1 and use the solution five times stronger than normal. Proprietary high-contrast developers usually contain a high proportion also of potassium bromide.
High-speed developers—those giving development times of about 30 seconds to 1 minute—also work on the principle of high alkali content. However, because these solutions are so strong that even potassium bromide will not eliminate fog, phenosafranine is also a major constituent.

Monobaths—combined developer/fixer
As their name implies, monobaths carry out the two processing stages of development and fixing in one solution. This makes for convenience, speed and simplicity.
A combined developer/fixer sounds paradoxical — why doesn't the fixer cancel out the action of the developing agent and so render the bath unusable ?
A monobath works like this:—The developing agent is rapid working and converts the exposed silver halide to black metallic silver within a few minutes. The fixer, on the other hand, is slower working and does not attack the unexposed silver halide crystals before the developer's action is complete. The temperature of a monobath is not critical for it affects the rate of both development and fixing proportionally. Also, the time the film is in the solution is unimportant after a basic minimum.
A monobath is alkaline which leaves the gelatin soft and in an "open" state (a normal fixer is acid and often contains a hardener) which means that the byproducts of fixing are washed out easier and quicker.
The few monobaths available in this country are:—May & Baker's Simprol; Tetanel's Monotenal; Ilford's Monophen; and Johnson's Unibath.

Paper developers
The main difference between developers for film negatives and prints is that the former are generally used quite dilute whereas papers are developed in a much more concentrated solution. The image colour is particularly important in prints which means that no staining developer, such as pyro, can be employed. Nearly all positive developers are based on the standard MQ formula and the stock solution will develop bromide, chlorobromide and contact papers simply by altering the solutions' concentration.
It is a fact that a basic MQ developer is useful for almost any emulsion—including negatives. However, for optimum results use an MQ solution which is specially compounded for use with papers for two reasons:—

1. The potassium bromide level is usually low in order to achieve rich maximum black.
2. Print MQ developers are usually more alkaline than their negative brothers, since paper emulsions are harder than that of negatives.

How to make up your own MQ developer
Without a doubt you achieve greater economy in your darkroom if you make up your own developing solution. However, there are several disadvantages in homemade "brews" which you would do well to bear in mind before you start:—

1. If you allow chemical dust to escape into your darkroom during weighing out the chemicals and allow this dust to settle on shelves, there is a risk that you will contaminate other solutions and ruin films and prints that are subsequently placed on these surfaces.
2. If you keep a large stock of different chemicals in your darkroom there is a danger that you will not use them all quickly and that some will deteriorate in time.
3. Everyone is fallible and therefore likely to make an error in making up these solutions which means that unless you develop a test film in the solution first, unrepeatable negatives may be ruined.
4. The weighing out of chemicals accurately is troublesome and time wasting — although if you are one of the rare types who get a kick out of dabbling in chemistry you can ignore this factor.

If after considering these points you still think that they are outweighed by the economy factor, here's how to make up your own developing solution.
Make up the developer in three quarters of the final volume of warm water—and warm is the operative word—for developers containing metol must not be mixed in water above about 120 degF (48C). After all the chemicals have dissolved, make up the exact volume with cold water.
The order of dissolving the chemicals is important and as a general rule is:—
FIRST-the preservative. SECOND-the developing agent(s). THIRDLY-the alkali. FOURTHLY-other constituents, such as the restrainer.

It is important that the preservative is added first in order to prevent the developer from oxidising while making up the solution. It is also important that the alkali is added after the developing agent.

There are two basic exceptions to this general rule:—

1. Metol—because metol is insoluble in strong solutions of sulphite you must first add a pinch (only) of the sulphite, to prevent oxidation, and add the rest after dissolving the metol.
2. Phenidone and glycin—are not very soluble in plain water but they are in alkali solutions. Therefore, dissolve the developing agents after the alkali.

It is imperative that you make sure that all the chemicals are fully dissolved before you use the solution as solid particles of chemicals will form irremovable spots and blemishes on the negatives.
That's all there is to it !