The pH range of the fixer is limited. It must be low enough to neutralize the activity of the developer and also be high enough to prevent sulfurization. The reduced acidity of the bath is gradually neutralized by the alkali of the developer carried into the fixer by the film. When the active acidity is neutralized too far, a sludge of aluminum sulfite forms that can make the fixer useless. An antisludge agent, such as boric acid, is added to the fixer. This agent is capable of absorbing a large quantity of the developer before sludge occurs, thus lengthening the useful life of the fixer.

Time Required for Fixing

The time required for film to fix depends on several factors:

The type of emulsion and its thickness. All else being equal. fine-grain emulsions fix faster than coarse-grained ones. Thin emulsions require less time to fix than thick emulsions.

The type of fixing bath and degree of exhaustion. When sodium thiosulfate is the fixing agent, a concentration of about 75 percent gives the fastest rate of fixation. However, because of the tendency of hypo to bleach out the image, most fixers for negatives have a concentration between 20 and 40 percent.

The fixing bath temperature. An increase in the temperature increases the rate of fixation. (Do not interpret this to mean that you can raise the temperature of the fixer above the temperature called for by the particular process being used.) The temperature of the fixer is not as critical as the temperature of the developer. However, you should keep all processing solution temperatures constant to avoid an increase of graininess.

The amount of agitation. The rate of fixation is affected by diffusion of the chemicals, so agitation reduces fixation time.

The amount of exposure. The more exposure the film has to light, the less unused silver halide to be removed by the fixer, and hence the faster the rate of fixation.

As a general rule, a film is considered completely fixed after twice the time it takes to clear it. Clearing or fixation occurs when all visible traces of the silver halides (a milky appearance) have disappeared. The clearing time can be determined by taking an undeveloped piece of the same type of film and agitating the film in the fixer until it clears. This procedure can be performed under normal room lights. The tongue cut from the beginning of the 35mm film may be used for this purpose.

Life of a Fixing Bath

The useful life of a fixing bath depends on several factors. One of which is the amount of material treated in the fixing bath. You cannot state accurately the exact amount of film or paper that can be safely fixed in a given amount of fixer. It is common practice to consider the fixer exhausted when the clearing time for the film is double the time it was originally. For a fixer used solely for prints, this is not easy to determine; therefore, the life of the fixer is considered ended after a given amount of paper has passed through it. This is usually about 200 8x10 prints (or equivalent) per gallon of fixer.

Using an exhausted or near exhausted fixing bath may cause the staining of films and paper. To avoid such staining, use two fixing baths in succession is the best practice. Initially, two fresh fixing baths are used. The materials are treated in the first bath until they are just cleared; then they are transferred to the second fixing bath for an equal period of time. In time, clearing time in the first bath (which is doing most of the fixation) is doubled from the original time required when the bath was fresh. When this occurs, the first bath is removed from use and replaced by the second bath. The second fixing bath is replaced by a completely fresh solution. This process is repeated as required, so the second bath is always relatively fresh. Using this procedure ensures that all film (and photographic paper) leaves the second fixer in stable condition and does not fade with time. This method is also economical, because all fixer is used to a point beyond that at which a single bath could be used.

WASHING

An unwashed or improperly washed emulsion will stain, crystallize, and fade. Therefore, the washing of the photographic emulsion is as important as any other part of processing. Removing as much of the salt and fixer

from the emulsion is essential. Only by good washing techniques can image permanence be assured.

The purpose of washing is to remove the soluble salts from the emulsion. Fixing converts silver salts into soluble salts that must be removed. If the fixing process is incomplete, even prolonged washing cannot make the image permanent. This is because the compounds of silver sodium thiosulfate remaining in the emulsion discolor in time and produce stains. Thorough washing is necessary to remove the fixing agent that, if allowed to remain, slowly combines with the silver image to produce brownish yellow stains of silver sulfide and causes the image to fade.

Water containing iron should not be used for washing. However, impurities, such as rust, dirt, or silt, can be removed by installing a 5 micron water filter in the supply line.

Seawater may be used to wash negatives if it is followed with a freshwater rinse. Salt water removes the hypo from film in about two thirds of the time required for a freshwater wash. However, a short rinse with fresh water is required to remove the salt from the film.

Temperature, chemical contamination, and rate of water change all affect the time required to wash film correctly.

Temperature

The wash should be kept within a range of 60F to 75F (15.6C to 23.9C). Within this range of wash temperatures, the warmer the water, the shorter the washing time required. However, for black-and-white film, a wash temperature of 75F should not be exceeded. Water at temperatures above 75F swells the gelatin and tends to inhibit diffusion. It also can damage the emulsion. Therefore, you should keep the temperature of the wash water constant with the processing solutions.

Chemical Contamination

Adding negatives fresh from the fixer into a tank of partially washed negatives nullifies the effects of previous washing, and you must start the washing procedure again. The reason for this is that the negatives with the higher concentration of fixer add enough chemicals to the washed negatives to contaminate the partially washed film. This situation can also occur if your hands are contaminated by chemicals when you place them in the wash tank.

Rate of Water Change

The length of washing time also depends on the diffusion of the hypo from the emulsion of the material. The rate of diffusion depends on the amount of fresh water coming into contact with the emulsion. Hypo remaining in the emulsion is continually halved in equal periods of time as the washing proceeds; for example, the average negative gives up about one half of the hypo it contains in 1 minute when in direct contact with running water. After 2 minutes, one fourth of the hypo remains, and so on, until the amount of hypo remaining eventually becomes negligible. Thus the rate of washing depends upon the degree of agitation and the amount of fresh water that comes in contact with the emulsion. The minimum washing time for negatives in running water is 20 minutes when a complete change of water occurs every 3 minutes.

Rapid film washers are designed to provide a constant freshwater exchange across the film emulsion. When rapid film washers are used, such as a Hurricane type of film washer, the film can be washed satisfactorily in 5 minutes.