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
