3-27. FIXING PROCESS
a. When the film is placed in the fixing solution, it is milky in appearance
because of the residual silver bromide crystals; but as it is moderately agitated and both
film surfaces are completely bathed by the solution, the milkiness gradually disappears.
The action of the acidifier immediately neutralizes the residual alkaline developer and
any continuing development action ceases. Since the gelatin is still swollen and porous,
the clearing agent dissolves out the unexposed and undeveloped silver bromide
crystals, leaving untouched the developed silver image. This is the clearing action.
b. As clearing abates, the hardening action begins, causing shrinking and
hardening of the gelatin emulsion containing the silver image. This hardening action is
most important, since it prevents swelling of the emulsion to any marked degree in the
later washing operation. Do not turn on the white light in the processing room until the
film is entirely clear, as white light will streak and fog the film.
c. The duration of the fixing process is dependent upon several factors. These
include the strength and nature of the fixer; the temperature of the solution, the amount
of film agitation, the volume of fixing agitation is related to the number and surface area
of films being fixed, and the emulsion thickness. The minimum fixing time is that
needed to clear and harden the film. For best results, the film should remain in the
fixing bath three times as long as necessary to clear the film.
d. Although radiographs may be left in the fixer for a slightly longer period, it is
not wise to do so, for it will only take longer to wash them free of residual silver and
fixing bath salts. The hardness of the emulsion in a given fixing solution decreases with
solution exhaustion and the initial fixing time must be extended as the solution is used
to maintain normal hardening.
e. Also, the higher the temperature, the greater the speed of fixation; however,
the optimum range of temperatures is from 60 to 75F. With higher temperatures, a
fixing solution tends to sulfurize, thereby shortening its life. Therefore, it must be
specifically treated when high temperatures prevail.
3-28. SINGLE-TANK FIXING SOLUTION
a. Most processing tank systems only provide for a single tank of fixing solution.
The use of a single bath is not the most efficient nor economical method for fixing. The
gradual accumulation of silver salts in the bath makes longer washing times necessary
because of the difficulty in replacing these salts with pure water.
b. Normally, the usefulness of a single tank fixer ends when the bath loses its
acidity or when more than twice the normal times is required to remove the unexposed
silver from the film. The bath should then be discarded, especially if it has become
turbid. An exhausted fixing bath permits abnormal swelling of the emulsion due to
deficient hardening action. Consequently, drying is prolonged and reticulation or