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5 minutes, washing is much more thorough than if the rack remains in one large tank for, say 15 minutes. Attempts have been made to wash racks of film by spraying with water, but this is entirely unsatisfactory because the spray does not wash the hypo out of the wooden rack, especially at those points where the film touches the slats.
The completeness of removal of the hypo from the film can be roughly determined by tasting the film, when it will taste sweet if any dissolved silver salts remain. A more precise method is to soak strips of the film in a small quantity of water and titrate this with a standard solution of iodine, taking care to make a blank test with the same volume of fresh water.
The Water Supply
Water may contain as impurities either dissolved salts, suspended matter, or colloidal vegetable matter. The extent of these impurities should be determined by placing a few drops of the water on the back of the film, allow to dry, and observe the quantity of residue. Excessive quantities of calcium salts are apt to leave a white deposit on the film surface, though if the film is thoroughly wiped the dissolved impurities must be present in excessive amounts before any visible deposit is perceptible on the gelatine side after drying. Water marks on the back of the film are easily removed by subsequent polishing. The matter of water purification has been fully dealt with in the article "How to Prepare Photographic Solutions."5
The water should always be filtered with a suitable sand filter to insure the absence of grit and suspended matter.
Troubles in Washing
1. Grit. The main trouble in washing is caused by grit in the water which deposits on the film. If it is not possible to filter the entire water supply, particles of dirt and grit may often be removed from the surface of the film by passing the rack under an elongated spray which throws a jet of well filtered water on either side of the film before entering the drying room.
2. Reticulation is characterized by a leather like graininess of the surface of the film. It is usually caused by subjecting the film to a sudden change in temperature, such as transferring from a warm fixing bath to cold wash water, especially when the film is not hardened sufficiently in the fixing bath.
DRYING
Motion picture positive film which has been fixed and washed at a temperature of 65° F. and the excess moisture removed contains about one pound of water per 1000 feet of film, while negative film contains two and one-half times this amount. The problem of dryingis to evaporate this moisture from the film efficiently without distorting the film, and without producing drying marks, or dust spots.
Apparatus
After washing, the film is generally removed from the film rack to a drying reel, although some laboratories dry the film while still
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