Daguerre, with his discovery in 1839 of the latent image that could be developed, became the father of photographic chemistry. Fox-Talbot is credited with the first fixing process. He used sodium chloride (salt) as the fixing agent. Upon the suggestion of Sir John Herschell, the change was made from salt to sodium thiosulfate (hypo), our present day fixing agent. Because a great number of advances made in the field of photographic chemistry, it is not practical to describe them fully. But since the greater portion of the filmmaker's work consists of processing sensitized emulsions, it is advisable that he or she understand the basic principles of photographic chemistry.

Chemistry of Development

When a photographic emulsion is exposed to light by proper means, an invisible (latent) image is formed by the silver halides in the emulsion. There are many theories as to the exact nature of the change that takes place in this reaction. It is sufficient to know that light weakens the structure of the affected halides in such a manner as to permit their reduction to metallic siver. This reduction forms the visible image. It is accomplished with a chemical, know as a reducing agent, that combines with oxygen. Oxygen enables the agent to react with the exposed silver halides and seperate or reduce the silver.
Although the reducing agent is the most important chemical in a developing solution, other ingredients are necessary to make the solution fuction properly. If used alone, most reducing agents have little or no effect on the silver halides because of their low rate of reduction. To speed up the oxidation of the reducing agent, an alkali or accelerator is generally added. This accelerator, besides energizing the reducer, also softens the gelatin of the emulsion and allows faster penetration of the solution. But a reducer and and an accelerator in solution oxidize, causing too rapid action. This reduces not only the exposed silver but also the unexposed silver halides, leaving a veil of silver deposited throughout the emulsion. This veil is called chemical fog. Excessive oxidation also causes stains in the emulsion and rapid deterioration of the solution. To prevent the reduction of unexposed silver halides, a restrainer is added, thus making the solution selective. To minimize the formation of stains and extend the life of the developing solution, a preservative, which has a chemical affinity for oxygen, is added.

Although many chemicals are capable of reducing silver halides to metallic silver, relatively few of them can be used. Many tend to reduce the unexposed as well as the exposed silver halides; that is, they are not selective in their action. The few used vary greatly in their properties.
One of the properties of a reducing agent is its reducing potential. This refers to its relative ability to develop or reduce the silver halides. A reducing agent of high potential attacks silver halides vigorously; one of low potential is slower in its action. The characteristic activity of a reducing agent is another property to consider. Some agents are more active in the highlight areas and some in the shadow areas; still others have overall activity. The temperature of the solution affects the activity of some agents more than others. The tone of the developed image is greatly affected by the type of reducing agent. Some agents produce blue-black (cold) tones, while others yield brownish (warm) tones. Metol hydroquinone, amidol, glycin, paraphenylene diamine, and pyro are some of the more widely known developing agents.

All developing agents are either neutral or slightly acid and, as such, usually have little reducing ability. To function as reducers, these agents must be made alkaline by adding an accelerator. The accelerator energizes the reducing agent and softens the emulsion, permitting more rapid penetration of the developing solution. A deficiency of alkali retards development; an excess results in an increase in activity and contrast, eventual chemical fog, and an overswelling of the gelatin that may cause frilling and blisters.
Accelerators are devided into three general types: mild, moderate, and strong. Borax, a mild alkali, is used with low contrast developers for fine grain. It is sometimes called a "buffer" alkali because in solution it slowly and constantly forms or releases its alkali, keeping the alkalinity of the solution constant. Borax is the mildest alkali in common use for the development of negatives only. Sodium metaborate, although slightly stronger, is similar in its action to borax.
Sodium carbonate (Na₂CO₃), a moderate alkali, is the accelerator most commonly used in developing solutions. It is used in many Metol-hydroquinone and pyro solutions. Potassium carbonate (K₂CO₃) can be substituted in formulas calling for sodium carbonate. It is more soluble in water buxpensive and less stable than sodium carbonate.
Sodium hydroxide (NaOH) and potassium hydroxide (KOH) are caustic alkalis used with certain developers to produce high contrast. Caustic alkalis are avoided for fine-grain developers because they soften and swell the gelatin excessively, permitting the silver grains to clump together.
Other alkalis that are in more or less common uses are ammonium carbonate, ammonia, acetone, sodium metaborate, and paraformaldehyde. A strong alkali does not give the same results as a weak one even though allowance is made for the difference in strength. Therefore substitution is not recommended.
Because the accelerator is a determining factor in the activity of a developing solution, it markedly influences the degree of graininess produced in the negative. Graininess depends on the clumping action of the silver grains during the development process. The more active the developer, the greater the clumping action; therefore the milder or less alkaline developing solutions yield finer grain.

All organic developing agents in an alkaline state have a strong affinity for oxygen. Therefore a preservative must be added to developing solutions to prevent excessive oxidation. The preservative prolongs the usefulness of the developing solution and prevents the formation of colored oxidation products, which cause stains. Sodium sulfite (Na₂SO₃) is the preservative most commonly used. Sodium sulfite dissolves silver bromide to some extent and therefore is useful for reducing grain size in fine-grain developers. Sodium bisulfate (Na₂₃) is also used. this is an acidified sulfate that in an alkaline developer is converted to sodium sulfite and sodium bicarbonate. Developers containing sodium bisulfite give slightly less base fog than those containg sodium sulfite. The quantity of preservative required varies greatly, according to the following factors:

1. The tendency of the developing agent or agents to oxidize.
2. The concentration of the developer. A dilute developer requires more preservative than a concentrated developer.
3. The temperature at which the developer is kept or used. The rate of oxidation increases as the temperature rises.
4. The keeping properties required and the way in which the solution is used. A solution that is to be used once and then discarded requires only a small amount of preservative. The amount of of oxidation is greater when a developing solution is used in a tray than when it is used in a tank.
5. The alkalinity of the solution. The more strongly alkaline the developer, the more rapid is the rate of oxidation.

Restrainers

Without a restrainer most developers act too rapidly and reduce unexposed halides near the surface of the emulsion, causing chemical fog, developing streaks, and producing an image lacking in contrast. When a restrainer is added, development time is prolonged and fog is minimized. Excessive amounts of restrainer greatly retard development and under some conditions cause greenish tones in the film. Potassium bromide (KBr) is the chemical most commonly used as a restrainer. All film and some paper emulsions are basically composed of silver bromide. In development the bromide is released from the silver. Although this bromide acts as a restrainer, it is usually insufficient to prevent fog. Sodium bromide or sodium chloride is sometimes substituted for potassium bromide. Another chemical, potassium iodide, is sometimes used. It gives more restraining action and tends to produce blue-black tone, but its longer fixation time limits its use.
Many factors affect the processing of an emulsion - the type of developing solution, the time, the temperature, the amount of dilution, the kind and rate of agitation during processing, the type of emulsion, and the exposure. Other factors remain constant, a rise in temperature speeds up development. This, in turn, increases contrast, fogging tendencies, and graininess. Diluting the developing solution slows development and decreases the contrast and fogging tendency, but it usually requires longer development time. Agitation also greatly influences the rate of development.
Agitation can be one of three types:

1. Constant - which means the film is constantly being moved, as in tray development or mechanical agitation.
2. Intermittent - meaning the film is agitated several times a minute, as in the usual tank development
3. Stagnant - no agitation, usually not recommended except in special cases

Rinsing

When a negative or print is removed from the developing solution, its emulsion is soft and swollen. There remains both in the emulsion and on its surface a small amount of developer that, if not removed, will continue its reaction and cause staining. To remove surplus developer, place the negative or print in a rinse bath. Rinse baths are of three general types: water, acid, and hardening. Each has its specific purpose and should be used accordingly.
A water rinse bath helps to retard development and removes excess developer from the emulsion, thus preventing contamination of the fixing bath. It is suitable for both negatives and prints and sometimes precedes the acid rinse in print processing.
An acid rinse bath stops all development by neutralizing the action of the developer, thus prolonging the life of the fixing bath. The rinse bath recommended 1s usually a weak solution of acetic acid. If an acid bath is not available emulsions should be rinsed thoroughly in water.
A hardening rinse bath (chromium alum) is used to harden the emulsion in high-temperature and tropical processing. In ordinary processing the hardening agent in the fixng bath is sufficient, thus permitting the use of water or acid for a rinse.

Fixing

When a negative is removed from the rinse bath, there remains in the emulsion a considerable amount of silver halide that has not been affected by the developing solution. these undeveloped silver halides are sensitive to light and, if allowed to remain in the emulsion, eventually darken, making the negative unusable. These halides are removed by changing them to a soluble state in a fixing bath. This bath usually contains more than one chemical agent. The chemical agents most commonly used are a silver halide solvent, an acid or neutralizer, a preservative, and a hardener.
Silver halide solvent All fixing baths must contain a chemical that is a silver halide solvent, also known as a fixer or fixing agent. The chemical most commonly used for this purpose is sodium thiosulfate, known as hypo. The sodium thiosulfate changes the silver halides to a compound that is soluble in water. Another chemical sometimes used as a fixing agent ls ammonium chloride. To remove the unused silver halides, a fixing bath composed of sodium thiosulfate and water may be used. However, other factors to be taken into consideration require the addition of other agents.
Acid or neutralizer After development the pores of the thickened emulsion retain a considerable amount of the developer. If allowed to remain, it will continue its developing activity. Even though an emulsion is thoroughly rinsed in a non-acid rinse before it is placed ln the fixing bath, a sufficient amount of the developer remains ln the emulsion to continue this activity, causing the emulsion to become stained and unfit for use. To stop development and prevent staining, acetic acid is added to the fixing bath. This neutralizes the alkalinity of the developer.
Preservative When a sufficient quantity of acid is added to the fixing bath to neutralize the alkalinity of any remaining developer, the sodium thiosulfate is decomposed into free sulfur and sulfurous acid, making the bath unusable. To prevent the decomposition of the fixing bath, a preservative, sodium sulfite, is added. It combines with the sulfur and forms new sodium thiosulfate. Sodium sulfite not only prevents the acid from decomposing the sodium thiosulfate, but ls also prevents discoloration of the solution and aids in eliminating stains.
Hardener It was previously stated that during development the gelatin softens and swells. If processing is continued without hardening the emulsion, frilling, scratching, or other undesirable effects may result. In the discussion on rinse baths, it was pointed out that the softened gelatin of the emulsion is sometimes hardened by treating it in a hardening solution before fixation. However, the most common practice is to include the hardening agent in the fixing bath. This allows the emulsion to be fixed and hardened at the same time. The most common hardening agent used in a fixing bath is potassium alum.

Plain fixing bath The standard plain flxing bath is a 25 percent solution of hypo. It can be mixed by using 2 lbs. of hypo to each gallon of water. Plain fixing baths are seldom used for fixing solutions except for special purposes.
Acid fixing bath A satisfactory fixing bath may be made by adding sodium blsulfate (acidified sodium sulfite) to a hypo solution. The acid of the bisulfite stops development, and the sulfite preserves the solution and prevents its discoloration. Hypo combined with the proper proportions of acetic acid and sodium sulfite also makes a suitable acid flxlng bath. This type of bath is unsatisfactory for negatives because it has no hardening quallties. It is primarily intended for fixing prints.
Acid hardening-fixing bath An acid hardening-flxing bath contains a hardening agent, usually potassium alum. ln addition to a silver halide solvent, a neutralizing agent, and a preservative.
Boric acid hardening-fixing bath The hardening agent in an acid hardening-fixing bath causes the precipitation of aluminum sulfite when the acid becomes neutralized. Because of their much lower sludging tendencies and excellent hardening characteristics, fixing baths containing boric acid are recommended for films.
Chromium alum fixing bath This bath is especially suitable for hot weather fixing because chromium alum hardens gelatin better than does potassium alum. It has one important disadvantage - its hardening properties rapidly deteriorate, so that the bath must be replaced frequently. It is recommended that chromium alum be used ln a separate rinse bath, followed by a regular fixing bath. This results in a saving in chemicals.
Double fixing bath The use of two fixing baths is recommended especially for mass production of negatives or prints. Such a procedure results in a more uniform and thorough fixation, conserves chemicals, and speeds the production of a great number of prints. The method usually practiced for double fixing baths is to have two trays of equal amounts of the solution. The prints or negatives are fixed for half the time in the first tray and then moved to the second tray. Since the products of development are usually elminated in the first bath, this bath deteriorates more rapidly than the second. When the first fixing bath shows signs of exhaustion, the second bath is moved into its place and a fresh bath is placed ln the second position.
Exhausted fixing bath Some of the characteristics that signal exhaustion of the fixing bath are milky appearance, sulfurous odor, slippery feel, and bubbles that do not disappear. A fresh fixing bath will have the distinct pungent odor of acetic acid and will have a grippy feeling to the fingers. The large bubbles that form ln an exhausted fixing bath during agitation do not disappear readily; those in a fresh bath do. The temperature and dilution of the fixing bath and the amount of agitation the print or negative receives during this process have a considerable effect on the rate of fixation and quality of the results obtained. All solutions in photographic processing should be maintained at approximately the correct working temperatures.

CHEMISTRY OF WASHING

Negatives and prints are washed to remove the chemical byproducts of the fixing bath. If these byproducts remain in the emulsion because of insufficient washing, they will in time cause it to change color, stain, or fade. The rate of washing depends largely on the dlffusion of the hypo from the sensitized material. The rate of dlffusion depends on the amount of fresh water coming into contact with the emulsion. The temperature of the water has little effect on the rate of washing as long as it is within 50 to 75 ¡f. Although most salts diffuse more rapidly in warm water than in cold, when washing photographic material the warmer the water, the more the gelatin swells. Swelling retards the diffusion of the chemicals from the emulsion in about the same proportion as the rise in temperature accelerates it. Hardening the gelatin in the fixing bath does not influence the rate of washing unless the emulsion has been dried after fixing. If the emulsion has not been dried first, its shrinkage or contraction is negligible and has little or no effect on the rate of washing. If the gelatin has been dried, it will not swell as much when it is soaked again, and consequently the chemicals deeper in the emulsion will not be washed out as quickly. An idea of the actual rate of washing may be obtained if you realize that the hypo remaining in the sensitized material is continually halved in equal periods of time as the washing proceeds. An average negative, for instance, will give up approximately half of it hypo in 15 sec of direct contact with running water. After 30 sec one-fourth of the hypo remains, and so on, until eventually the amount of hypo remaining becomes negligible. The rate of washing then depends on the degree of agitation and the amount of fresh water with which the emulsion is brought into contact.

Washing in Trays
There are three methods of washing negatives and prints in trays. The simplest is to place the negatives or prints in a tray full of water and then change the water frequently. The second is to run a continuous stream of water into the tray for at least 20 min. The most efficient method uses a device attached to the edge of the tray that siphons the water from the bottom of the tray while fresh water is being run in at the top. In any method take care to separate the prints or negatives to ensure that sufficient fresh water reaches all areas.

Washing ln Tanks
A very satisfactory method of developing negatives is the tank method. Metal or plastic frames hold the negatives and suspend them in the tank. Negatives developed ln such a manner are washed in the same or a similar tank with fresh water flowing into it.

Mechanical Washers
Mechanical washers are convenient for washing large numbers of small and medium-size prints. These washers spray fresh water onto the prints while siphoning off the contaminated water from the bottom. This type of washer sometimes contains a large tray that is revolved either by the force of the water spray or by motor power. The rotation, together with the spray of water, constantly agitates the prints. In such washers the water is completely changed every few minutes.

Testing Solutions

It is possible to determine accurately whether prints and negatives have been sufficiently washed by using chemical tests. One method of testing is to remove several prints or negatives from the wash water and allow them to drain into a violet, permanganate test solution. If the test solution becomes colorless, a large concentration of hypo is present; if it changes to an orange tone in about 30 sec., only a slight amount of hypo is indicated. In either case the prints should be returned to the wash water until further tests show no change in the color of the test solution.