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