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RACK MARKS AND AIRBELL MARKINGS ON MOTION PICTURE FILM*
J. I. Crabtree and C. E. Ives
WHEN developing motion picture film by the rack and tank system it is very difficult to secure uniform development throughout the entire length of the film. Unless special precautions are taken, more development occurs at the top and bottom of the rack where the film passes over the end slats or bars than along the sides, so that bands of greater density occur at intervals corresponding with the height of the rack, which cause an objectionable flicker when the film is projected. These dark markings are termed "rack marks."
Another difficulty arises from the clinging of airbells to the film as the rack is immersed in the developer. These airbells prevent the access of developer to the film locally thus causing white spots.
Both the above defects can be overcome by correct manipulation, but their presence on much of the film shown in the present day theatre indicates a need for a better knowledge of the subject on the part of many laboratory workers.
It is the purpose of this article to explain the nature and cause of rack marks and airbell markings on motion picture film and to indicate methods for their prevention.
Rack Marks
When film is developed on the usual rack in a vertical tank, more development invariably occurs where the film passes over the top and bottom of the rack than along the sides, causing the film to appear as shown in Fig. 1. The marking where the film passes over the top of the rack is usually mottled and consists of a double line, while at the bottom only a single dark line is produced.
Cause of Rack Marks. — At various times rack marks have been wrongly attributed to causes such as a difference in temperature between the rack slats and the developer, which might cause an ac
* Communication Xo. 250 from the Research Laboratory of the Eastman Kodak Company.
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