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Rack Marks and A irhell Markings — Crahtree and Ives 109
is then lifted completely out of the developer and resubmerged, a much larger quantity of airbells will be formed than when the film was originally immersed.
Short immersion of the film in the developer followed by exposure to the air leaves the film in a partially swollen state, and in this condition it has a much greater propensity to carry along airbells with it on subsequent immersion than the dry or completely swollen film. It is usually necessary to allow the film to soak for at least twenty to thirty seconds after the first immersion in order to remove this tendency.
c. The degree of agitation of the rack. In many cases airbells can be dislodged after the film has been thoroughly soaked by rapid agitation of the rack or by slapping the end slat against the surface of the developer, though when developing by time it is necessary to duplicate the rack agitation precisely and too much rack manipulation is not practical. It is preferable to remove the airbells manually as described below.
2. The quantity of grease on the film.— K very slight trace of grease or oil on the film will so affect the surface of the emulsion that it has a greatly increased tendency to attract airbells. Any appreciable quantity of oil or grease will also act as a resist and prevent the access of the developer. Preliminary soaking of the film in a solution of sodium carbonate will often overcome this tendency (see below) .
3. The condition of the developer. — Experiments have shown that old developer which frequently tends to foam badly has a greater tendency to give airbells than new developer. This foaming is the result of the presence of decomposed gelatin produced by the action of the alkali in the developer on the small particles of emulsion removed from the film by abrasion. The effect of the addition of ethyl alcohol to such a foaming developer was tried, but no beneficial effect was observed by the addition of increasing quantities of the alcohol up to 10%.
Methods of Preventing the Formation of Airhells. — The formation of airbells may be prevented as follows:
1. By soaking the film in water or a solution of sodium carbonate {about 2%) before development. This has the effect of thoroughly soaking the gelatin, in which condition the propensity for airbells to form is a minimum, while the carbonate solution tends to remove traces of grease which would otherwise cause airbells and prevent