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. 10-7 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 60°F to 75°F (15.6°C to 23.9°C). 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  75°F  should  not  be exceeded.  Water  at  temperatures  above  75°F  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