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.
WETTING AGENT
After washing, water often drains from film in an
irregular manner, clinging to both emulsion and base
sides in drops, streaks, and uneven patterns. When such
partially drained or incompletely wiped films are
subjected to hot air or radiant heat, the areas under these
streaks and drops of water dry much more slowly than
the surrounding film. The swollen gelatin at these points
is subjected to stresses and shrinks unevenly, changing
the density of the silver image. When surplus water is
removed from the emulsion side and drops of water
remain on the base side, drying of the emulsion
immediately opposite the water spots is retarded and
drying marks usually result. The use of a wetting agent
helps to prevent the formation of these water spots.
Wetting agents are chemicals that "superwet" the
film to promote faster and more even drying. Wetting
agents are chemically different from soaps, but they
perform a related function. They all lower surface
tension of liquids so the film surfaces are wetted quickly
and evenly. Kodak Photo-Flo is a wetting agent used in
imaging facilities.
After washing, bathe the film in a 1- or 2percent
solution of wetting agent (prepared according to
instructions provided by the manufacturer) for about 2
minutes. Then drain the film briefly for about 30
seconds. Squeegee the film between your index and
middle finger to remove surface foam and excess
wetting agent. Be sure your hands are clean and
dampened with the wetting solution before squeegeeing
the film.
DRYING
The final step in processing is to dry the wet film.
This step should be given special attention. Film drying
has two distinct phases. The first is the removal of excess
water from the film surfaces. The second is the drying
of the film by evaporation.
When you are drying the film, the primary problems
you must guard against are uneven drying, dust,
scratches, and damage to the emulsion caused by
overheating. Dry the film in a vertical position, hanging
it from a line or beam by film clips. When you are drying
roll film, curling can be avoided by hanging another film
clip at the bottom of a strip of film. Sheet film should be
hung from one corner of the film to prevent drying
streaks. Film should not normally be dried in the hanger
or reel in which it was processed, since uneven drying
results.
Dust and water spots are the problems you
encounter most frequently when drying films. When the
dust is not embedded in the emulsion, you can remove
it with a camel-hair brush. Embedded dust in the film
requires the film to be rewashed and dried properly.
Water spots are more serious, since uneven drying can
cause not only white stains but also small craterlike
formations in the film under each spot. The white stains
can be removed with alcohol, but the craterlike spots
become a permanent defect. The best cure is prevention.
You can avoid these problems by keeping the film
surface clean and by using a wetting agent before drying.
The photographic emulsion consists of one or more
layers of gelatin with silver halides of varying sizes
distributed through the layers. After exposure and
development, the halides are changed to metallic silver
that occupies space and does not absorb water. In an
emulsion that is unexposed, the undeveloped silver salts
are made soluble and are removed from the emulsion
during the fixing and washing stages. Only the gelatin
and the space occupied by the halides remain, and these
do absorb water. Because of these conditions, dense
10-8

Basic Photography Course












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