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