American cinematographer (Jan-Dec 1926)

January, 1926 AMERICAN CINEM ATO GR APHER Nine Rack and Airbell Markings on Cinema Film Concluding Part of Exhaustive Treatise Begun in December Issue of American Cinematographer. Big J. I. Crabtree and C. E. Ives Findings of Authorities Reported in Research Laboratory Communication from Eastman Co. (Continued from last month, in which complete illustrations appeared) If the airbell forms on the film along the sides of the rack, owing to the tendency of the air to rise to the surface, the airbell frequently becomes elongated so that the area of contact is not circular but oval. The tendency for distortion is greater with the larger airbells, which explains why the larger airbell markings are rarely circular, while the small markings are invariably circular. A typical group of circular and irregular airbell markings is shown in Fig. 7. Unless the surface of the emulsion is locally greasy or burnished, the points of attachment of the airbells are determined merely by chance. However, there is usually a greater propensity for the airbells to become attached where the film passes over the ends of the rack so that rack marks are usually accompanied by airbell markings. See Fig. 8. Classification of Airbell Markings Airbell markings may be of the following types : /. Clear white spots. These may be either circular or irregular in shape as explained above. See Fig. 7. The clearcut edge of the spots indicate that the area of contact of the airbells did not materially alter during the course of development. 2. Grey spots. These are' similar in shape to those illustrated in Fig. 5 but are not perfectly clear and contain more or less silver grains. They are caused by the airbell breaking or becoming dislodged during development so that the spot was protected for only a part of the total time of development. 3. Clear spots surrounded by a dark ring. See Fig. 9. The dark ring is probably a result of developer oxidation fog caused by local oxidation of the developer by the airbell. This type of marking occurs only rarely and with freshly mixed developers which are susceptible to aerial oxidation fog. in such a case if the film remains stationary during development the oxidation products of the developer flow down the film and frequently produce a fog streak or tail as shown in Fig. 10. 4. Clear spots surrounded by a grey ring. See Fig. 11. The grey ring is probably caused by a diminution in the area of contact of the airbell with the film due to a change in shape during development as explained above. 5. Clear spots with a dark central ring. See Fig. 12. Examination of the dark nuclear ring showed that this consisted largely of silver. The exact method of formation of such markings is not known though they could be formed by bursting of the airbell just before the film was removed from the developer so that the whole airbell area became saturated with developer, and the reforming of a smaller central bubble when the film was immersed in the fixing bath. This second bubble would prevent the access of the fixing bath and permit of development of the image underneath by the developer absorbed by the film after the bursting of the first bubble. Such a marking could also result from the printing of a positive image from a negative containing airbell markings similar to those described under "3" above, namely "clear spots surrounded by a dark ring." 6. Clear spots with a nucleus of silver halide. The appearance of these spots by transmitted light is essentially the same as those shown in Fig. 12, although the dark central ring consists largely of silver halide instead of metallic silver. The method of formation of such spots is probably as follows: During development the airbell prevents access of the developer to the emulsion and persists until the film is removed from the developer. On reimmersion in the fixing bath a small airbell forms where the larger bell previously existed, thus protecting the emulsion from fixation. The difference between the spots indicated under 5 and 6 is, therefore, merely a result of slight wetting of the previously protected airbell area with developer immediately before fixing. A nucleus of silver halide is produced in one case and a mixture of silver halide in the other. Factors Affecting the Number of Airbells Formed The quantity of airbells which may ac i continued on Page