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360 XI. PRESERVATION AND STORAGE
Before it is actually printed, it should be cleaned. Cleaning consists of wiping away the surface dust and dirt with very good facial tissue (similar in quality to lens tissue) soaked in carbon tetrachloride. The tissue is usually folded in a piece of silk velvet to prevent the hand of the cleaner from touching the film. When the film has been satisfactorily cleaned in this manner, it is rubbed once more with the silk velvet to remove traces of lint that may have been left behind by the tissue.
Each such handling adds additional tiny scratches and abrasions to the film. Some laboratories do not bother to clean the film before it is printed, and the dirt accumulated by the original is copied onto all subsequent prints. Other laboratories try to use machine cleaning, but, unfortunately, there are very few cleaning machines that do not add scratches and abrasions far more quickly than the laborious and more expensive hand cleaning. In any event, every handling adds deterioration ; the amount of deterioration varies with the cleanliness of the place in which the film is handled and the skill and care of its personnel.
The operations of the laboratory — particularly in the printing room — add their important share to the scratches and abrasions added to the film. If the film path is not kept scrupulously clean and the hardened stainless steel gate or chromium-plated film path with which the film is in contact is not kept mirror-smooth, still more scratches and abrasions are added. A particle of dirt lodged in the film gate of a printer does just as much damage as a like particle of dirt lodged in the gate of a camera; it scratches the film as a diamond cuts a piece of glass. If the machine starts with a jerk or has been improperly threaded, a large amount of damage is done in a single trip through the printer.
The printing room can add still more mars by its manner of handling the precious film. In some laboratories the printers run in only one direction. When the end of a reel is reached, the film must be rewound — usually in the dark — in order to make the next print. Rewinding is done by hand or by machine. Hand rewinding is slow and tedious if it is well done, and is 'costly in terms of labor. Rewinding by machine is usually faster and less costly, but is usually considerably more damaging than the printing operation itself. Although printer operators are quite careful, the amount of scratching and abrasion added to the film by a rewinding, however accomplished, and by the threading of the machine is ordinarily appreciably greater than that added when the film is run through the printer several times. If a machine runs at high speed, it is far