negatives, or negatives containing many heavily
exposed areas, contain less water and dry faster than thin
negatives. When you think about this for a moment, you
can see that since the heavily exposed and lightly
exposed areas are distributed randomly throughout the
average negative, drying occurs rapidly in the dense
areas, more slowly in the intermediate areas, and most
slowly in the thin or unexposed areas. Obviously, then,
film does not dry uniformly.
When gelatin dries, the water it contains must first
migrate to the surface and then evaporate into the air.
Ideally, these processes should take place
simultaneously and at the same rate. However, when the
surface moisture evaporates too rapidly, the surface
becomes hard, and the internal moisture is unable to
escape it. In addition, when drying is too rapid, the outer
surface shrinks while the rest of the gelatin layer is in an
expanded state. This causes strains that can have a
harmful effect upon the emulsion.
For a negative to dry, it must be surrounded by dry
air; that is, air that contains a lower relative percentage
of moisture than the gelatin. When the heated air
circulates, the damp air moves away from the surface of
the wet film and replaces it with dry air that permits the
drying process to continue. This is the principle behind
the air impingement dryers currently in use. Heated air
accepts more moisture than cool air. When the air does
not move, air can become heated and rapidly reach a
state of equilibrium with the moist film, and drying
stops.
In an air impingement drying system, air is warmed
and blown against (impinges upon) the surface of the
wet film. The warm, dry air picks up moisture and moves
on. It is immediately replaced by more warm, dry air,
and the process continues until the film is dry. The rate
of drying is controlled by adjusting the velocity,
temperature, and humidity of the air in the drying
chamber. In hot and humid climates where the air is
saturated with moisture, the air must be passed through
a dehumidifier before it enters the drying chamber.
When this is not done, the film does not dry. In dry
climates, you must reduce both the heat and the air
velocity to prevent overdrying.
Photographic films begin drying at the corners and
edges as well as in the areas of heaviest density. This
introduces strains in the direction of the dry areas. As a
film continues to dry, the strains gradually begin to
equalize, and the film, when dried properly, ultimately
lies flat. The surface is not moist to the touch, but it is
firm and soft enough that flexing does not damage it. If
10-9
overdried, the film curls toward the emulsion and can
become brittle.
The rate of drying and the amount of curl also
depend upon how thick the emulsion layer is and
whether or not the film has a gelatin backing. Naturally,
the thicker the layer, the longer the drying time. A gelatin
backing takes time to dry, but it introduces an opposing
curl and causes the dried negative to lie quite flat.
FILM PROCESSING EQUIPMENT
Hand processing of photographic film is best carried
out in a darkroom that is properly equipped. Whether
the darkroom is large or small, certain essentials are
necessary for good quality processing.
The darkroom must be clean and well ventilated.
Shelves, bottles, racks, processing equipment, walls,
and floors that are spotted with dried chemicals are
harmful to photographic images. Navy photo lab
equipment, therefore, must always be spotlessly clean.
The arrangement of a darkroom should be
convenient, “a place for everything and everything in its
place.” There should be adequate and correct safelights
placed at recommended working distances. Only
necessary sensitized material should be in the darkroom.
Temperatures in the lab should be maintained as closely
as possible to the normal processing temperature-about
70°F to 75°F. The well-equipped darkroom should
contain the following items: sink, graduates, required
chemicals, waterproof aprons to protect clothing, clean
towels, accurate thermometer and timer, and the
necessary film hangers, trays, reels, and tanks. All
darkrooms should be well stocked with prepared
chemicals in containers that are labeled properly. In
general, good photographic quality demands that all
work must be conducted in a clean, orderly, and
systematic manner.
DARKROOM SINKS
Sinks in the photographic darkroom should be sized
adequately and constructed properly. Most sinks in
Navy labs are factory-made and meet all the
requirements for photographic work Sinks should be
big enough and built so they drain thoroughly. The sink
should have duckbords to keep trays and tanks off the
bottom and to permit water to circulate under and around
the solution tanks to maintain correct and constant
temperatures. Sinks, also, should have a mixing valve
to control the temperature of the water in the sink and a
