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

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 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.