Ignoring, for simplicity, all wash steps and steps that do not have a major effect on the film layers, then a reversal process consists of a first developer, reversal exposure, color developer, bleach and fix. The first developer is a specially formulated black and white developer which, like all black and white developers, converts all exposed silver halides to black metallic silver. At the end of this step the film resembles a black and white negative, but recorded in three separate layers.
The film is now given a reversal exposure to light which exposes all the remaining silver halides. Some processes dispense with this step by including a chemical fogging agent in the color developer, or in a separate step prior to color development.
The color developer reacts with the now exposed (fogged) silver halide to give black metallic silver and dye. The top layer of the film develops to give silver and yellow dye, the middle layer silver and magenta dye and the bottom layer silver and cyan dye. The film now contains black metallic silver throughout and dyes in the appropriate layers.
The purpose of the bleach step is to convert all the metallic silver formed in both developer steps back to its original form of silver halide. The film now has a "milky" appearance.
In a similar manner to black and white processing, the fix removes the silver halide. The film now consists only of dye images, which will reproduce the original scene in terms of its blue green and red content.
Having looked at the process in outline, it is now necessary to examine each processing step in detail and to include the washes and other steps that have been so far ignored. It is important to understand the reason for each step.

When you expose a color film in your camera, a chemical change takes place in each of the three emulsion layers where light has reached it. The change is in direct proportion to the intensity of the light; the brighter the light the greater the change. In this state the film is said to have a latent image registered on it. As mentioned, the first developer converts this latent image into a visible black silver image in each of the three emulsion layers.
The first developer is basically similar to a conventional black and white developer but containing a few additional ingredients which are necessary to control the quality of the final transparency. Most first developers have a combination of two developing agents--metol and hydroquinone or Phenidone and hydroquinone. These need an alkaline environment in which to work and this is provided by a chemical such as sodium carbonate, borax, or sodium metaborate, sometimes in conjunction with a stronger alkali such as sodium hydroxide. Since the activity of the first developer depends to a great extent on the alkali content, the actual concentrations of these chemicals are very important. The alkalinity of a developer is indicated by the pH of the solution.
To prevent the developer from oxidizing rapidly, a preservative is used--invariably sodium sulphite. Potassium bromide is usually added to prevent unexposed parts of the film from developing (fog); organic anti-foggants are often used in place of or as well as the bromide for a similar purpose. The most important of the special additive chemicals used in the first developer is a silver solvent in the form, usually, of sodium or potassium thiocyanate. The purpose of this chemical is to dissolve residual silver salts in the emulsion during development to increase sensitivity and to clear highlights in the finished film.
A very small quantity of potassium iodide is often included in the formulation to act as a restrainer, having greater effect on the top blue sensitive layer than on the other two layers. This means that the yellow dye content of the film can be controlled quite accurately by varying the iodide content (always assuming, of course, that you are mixing your own solutions from raw chemicals).
First development is the most critical stage of processing a reversal color film because it determines the characteristics of the finished movie. Any deviation from the correct time, temperature and agitation will lead to a change not only in the overall density of the film, but also to a change in the color balance. The effects vary from film to film, but in general terms, reducing the first development time produces a yellow or red cast while increasing it gives a cyan cast.
Similar effects are produced if the alkalinity of the developer of the developer is too low or too high. Increased first developer time also gives thin movies which have the appearance of being overexposed while reduced development time has the opposite effect.
The Kodak E4 process starts with a pre-hardener and nuetralizer before pouring in the first developer.

When the first development is complete, you must remove excess developer left on the surface of the film and in the film spirals by giving the film a short, vigorous wash, usually of fifteen to sixty seconds in length. The main purpose of this wash is to prevent carry-over of developer into the next solution--the stop bath--where it would tend to neutralize the acidity of the stop bath.
The thing to remember is that the developer absorbed by the emulsion carries on working during the wash, so the length of the wash is important. if you wash too long you could end up with the film being too thin.
In soft water areas you may have trouble with the emulsion on the film frilling (a ripple effect on the emulsion surface). This is caused by the gelatine in which the silver salts are in suspension swelling when wet. The swelling is dependent to some extent on the degree of alkalinity of the solution; the higher the alkalinity, the softer the gelatine becomes, and the greater the degree of swelling. When you transfer a film into an acidic solution from an alkaline one, such as when you immerse it in a stop bath after being developed, the swollen gelatine contracts. This can, in extreme cases, cause the emulsion to tear. Another controlling factor on the degree of swelling is the concentration of metallic salts in the water you use for the brief wash between development and the stop bath. In hard water areas metallic salts are present in fairly high concentrations, but in soft water areas they are not. So you can use an intermediate bath to introduce these salts artificially. This bath consists of a solution of magnesium sulphate (Epsom Salts) or sodium sulphate (Glauber's Salt). You immerse the film in this solution for one minute before you give it the short wash.
The intermediate bath has a secondary beneficial effect: it halts the action of the developer more efficiently than does water. This means that the life of the stop bath is extended slightly. But you may find that if you use an intermediate bath of this type you will need to increase the first development time by half a minute.

First stop bath

To stop the action of the first developer and to prepare the film for it's reversal exposure, you must treat it in a stop bath. This is basically a weak acid solution which neutralizes the developer retained in the three emulsion layers. It pays to keep a careful eye on the state of the stop bath, testing it regularly with litmus paper, or even replacing it regularly to ensure that it remains acidic. If the stop bath loses its acidity, usually by the first wash being inefficient and allowing excess developer into the stop bath, the developer will continue to work and give overdevelopment.
Stop baths often include a chemical such as chrome alum or potassium alum to harden the emulsion before reversal exposure. This reduces the risk of damage to the film, especially if you remove it from the container for the re-exposure.

Second wash

This wash is most important, its job being to remove all traces of the first developer and stop bath from the film. If you do not remove it, it can affect the color development stage, producing color shifts and other "undesirable" effects.
To be really efficient, this wash--and all others--must be very vigorous, with plenty of water flowing through the tank and turbulence to increase its scouring action. It is important that the water is fed to the bottom of the tank by a tube or by pressure forcing it through all the lengths of film, so that all residual chemicals are carried off of the film
The temperature of the wash water is important, too. If it is too cold the chemicals are not removed efficiently and extra washing time becomes necessary. There is also the danger of reticulation (cracking of the gelatine) if the temperature of the wash water is more than two or three degrees C lower than that of the stop bath. On the other hand, of the wash water is too hot, the emulsion can swell excessively and cause frilling, even though the film has been previously hardened.
Many workers prefer to wash their films by giving several changes of fresh water rather than by using running water. There are advantages and disadvantages with both systems. In general, which procedure you use will depend on the facilities at your disposal.

At this point in the processing procedure, the film has as appearance something like a black and white negative film. The parts of each layer which have not been blackened by the first developer comprise unexposed silver salts and it is in these which help form the final positive image of the color movie. But first you must expose these silver salts to light.
The problem is that, during the first development, the film has lost a large proportion of its original sensitivity and this loss of speed is further hampered by the negative image which now exists. These two factors mean that the reversal exposure must be very intense. The most usual way to carry out the reversal exposure is by using a Photoflood bulb.
The length of the reversal exposure depends on the particular make of film you are processing. Some need only thirty seconds while others need as much as three minutes. But generally, it is all too easy to underexpose at this stage but virtually impossible to overexpose, so if in doubt, err on the side of overexposure.
One other point, Place a sheet of glass between the film and the lamp to avoid splashes of water on the lamp. Remember that the lamp gets extremely hot and a drop of cold water can easily shatter it.
Kodak has eliminated the need for reversal exposure in their E6 process by incorporating a separate fogging bath in the process at this point. While this is, in many ways, a great convenience, the chemical foggant has the disadvantage of being extremely toxic and can cause unpleasant sickness if absorbed through the skin. So when using the E6 process (or any process --Ed.), always wear rubber gloves. In fact, whatever color process you are using, always wear rubber gloves.

The color developer contains a special developing agent which, after reaction with the silver halide to form metallic silver, reacts with the color couplers in the three dye layers of emulsion to form dyes. So, in each layer of emulsion, the areas which were not blackened during the first development now develop to form a combined silver and dye image, the color of the dye being yellow in the top layer, magenta in the middle layer, and cyan in the bottom layer. The amounts of dye and silver formed are in direct proportion to the amounts of silver halide left in the emulsion after the first development.
In essence, color developers are similar irrespective of the particular process, the main difference between individual solutions being the particular color developing agent used. Other differences are usually in the additives which are used to modify contrast, assist the penetration of the developing agent, reduce graininess, and so on.
Like the first developer, the color developer is an alkaline solution, usually of higher alkalinity than the first developer. Sodium carbonate or tri-sodium phosphate, generally in combination with sodium hydroxide (caustic soda), forms the alkali in the color developer.
As with the first developer, the alkaline strength is important.
Sodium sulphite and potassium bromide act as preservative and restrainer again, but the quantity of the sulphite is very small compared with that in the first developer, because a large amount can cause a reduction in the mount of dye produced. Instead, hydroxylamine hydrochloride or sulphate is often used in combination with the sulphite to retard oxidation of the color developing agent.
Color development is not quite as critical of temperature as first development. Whereas the temperature of the first developer must be controlled to within half a degree C, the color developer often has a tolerance of two degrees either way. But this is by no means universal so check the instruction sheet for the process you are using before you start. You must control the timing carefully, though, and standardize it to give you the results you want. In general, too long in the color developer will give you film that has a magenta bias and too short a time will give greenish results.
At this stage of the process the film is opaque; it contains, in each of its emulsion layers, a silver negative image, a silver positive image, and a positive dye image. It is usually just possible to discern an image if you look at the emulsion side of the film, but it is very faint.

As with the first wash, the purpose is to remove excess developer from the film before it goes into the second stop bath or clearing bath as it is sometimes called. It is even more important at this stage to avoid the frilling effects of soft water because the high alkalinity of the color developer makes the film even more suscepetable than after the first development stage. For this reason, I think it is wise to use an intermediate bath in all cases where this wash is specified, irrespective of the hardness of the water supply. As before, the intermediate bath is a solution of magnesium or sodium sulphate.
Not all processes call for the third wash. The Kodak E4 and E6 processes, for example, specify that you should place the film straight into the second stop from the color developer. On the other hand, some processes such as the Agfachrome 41 do not use a second stop bath. Instead, they prolong the third wash from its usual minute or so to about twenty minutes.

Second stop bath

After you have removed surplus color developer from the surface of the film, that absorbed by the three emulsion layers must be neutralized and this job is carried out by the second stop bath.
Like the first stop bath, this is acidic in nature, being usually compounded from acetic acid or an acid sulphite such as sodium or potassium metabisulphite.

Fourth wash

This must be a particularly vigorous and efficient wash to remove all traces of the color developer and stop bath from the three emulsion layers. If you do not do this, staining of the film can occur in the bleach bath with the result that the film will be "ruined". This staining is usually in the form of magenta or blue streaking and it is impossible to remove.

Before you can see your film in its full colors, you must remove the silver produced by the first and color development stages in a bleach and subsequent fixer.
The bleach is generally made up from potassium ferricyanide and potassium bromide with the addition of other chemicals to adjust the acidity or alkalinity of the solution to suit the requirements of the process. Although this acidity or alkalinity is only slight, it has a big influence on the final color balance of the film and for this reason it is important to use only the bleach specified for the particular process you are using.
The bleach bath does not itself remove the silver from the emulsion layers of the film; it merely converts it to a form which can be dissolved by the fixer.
During the bleaching phase the appearance of the film changes quite drastically. The overall opacity is replaced by--viewing the emulsion side of the film--a yellowish-brown appearance, and from the back of the film the color is predominantly bluish-green. This is because, at this stage, the dyes are still partly concealed by the white or creamy silver halide to which the silver has been converted by the bleach.

To prolong the life of the fixing bath a wash is often included at this point to remove as much of the bleach solution from the emulsion layers as possible. This wash is not strictly necessary, in fact some processes such as the Kodak E4 do not call for it, but it is as well to use one if only in the interests of economy.

The purpose of the fixing bath is to dissolve the silver halide produced at the bleach stage to leave the final film comprised of just a dye image. As with the bleach bath, the precise composition of the fixer depends on the specific process. Many processes call for a simple neutral fixer consisting of a 15 or 20% solution of sodium thiosulphate (plain hypo), but others need the addition of small amounts of sodium bisulphite or metabisulphite to acidify it. Still others need a small amount of borax or a phosphate to make them slightly alkaline. This alkalinity is often necessary to stabilize the cyan dye.
With some films, you can now see the full colors of the film, as they only change slightly on drying. Others though, notably those of the Ektachrome type, have oily droplets in the gelatine which have been used to suspend the color couplers. These droplets have a refractive index different from that of the gelatine when wet and give the film an opalescent appearance: the color balance also looks wrong when the film is wet.

To complete the process you must now remove all residual processing chemicals, together with any remaining silver halide--now converted into soluble silver salts by the fixing bath. The final wash must, therefore, be very thorough, because any chemicals left in the film can effect the stability of the dye image and the deterioration this produces can form stains in the emulsion.
In most cases a wash of about ten minutes is called for and I usually empty the tank two or three times during this period to increase the efficiency of the wash.

Stabilization

Some processes include a stabilizing bath after the final wash to help prevent discoloration of the dyes over time. Other films do not require this step, but personally I always use one and I suggest that you do the same. If the particular kit you use does not include a stabilizer you can make one up by mixing about 5 ml of 40% formalin with a liter of water and adding a few drops--about 2 ml--of a wetting agent. Or get your local chemist to mix it for you.
After stabilization, do not wash or rinse the film again or you will remove the formalin which stabilizes the dye.

Drying

Because color reversal films have three layers of emulsion instead of just one, they take a lot longer to dry than black and white films, but do not be tempted to try to speed up the drying by using excessive heat; the film is very delicate and you can easily damage it. If you use a drying cabinet for your film make sure it is thermostatically controlled and set the temperature no higher than 33°C (100°F).