During fixing the frame should be kept rocked so that every trace of undeveloped silver salts may be removed from the film. Then it is transferred to the washing tray and submitted to a thorough washing in frequent changes of water for some twenty minutes. The film is now ready for its final treatment. This is immersion in the soaking solution:—
| Avoirdupois. | Metric. | |
| Water | 32 oz. | 1,000 c.c. |
| Glycerine | 1 " | 30 " |
This final bath is not always used, but it is desirable if the negative film is to be kept for any length of time. Immersion in this soaking solution prevents the gelatine coating of the film from becoming hard and horny. After remaining in this bath for five minutes the frame is lifted out, and returned to its stand, where the excess of glycerine and water is removed by wiping with a soft cloth.
It will be seen that once the film is wound upon the frame it is not removed during the whole process of development, and may be left in the same position during the drying period. But if the drying is to be quickly performed the frame must be of what is known as the spring type, so that it does not keep the same area of film constantly pressing upon the curved end-bars. Otherwise the film would retain this shape when it has been dried and the kink would be irremovable. To avoid this defect it is just as well to transfer the film from the frame to a drum (see [illustration facing p. 72]). This is an easy matter. The drum is mounted upon a stand so as to be free to revolve easily. Detach one end of the film from the developing frame, and attach it to the drum by means of a drawing pin. The emulsion side, of course, must face outwards. Then by unwinding the frame and rotating the drum simultaneously the film becomes wound spirally upon the drum. Another pin will secure the second end of the film. The drum is an inexpensive and very handy accessory to the moving-picture photographer, especially in the drying operation, when forceful methods have to be adopted.
Unfortunately the drying of the film cannot be accelerated to any great extent. The hardening of the gelatine emulsion cannot be hastened, as in glass plate work, by immersion in a bath of methylated spirits or some other evaporative agent, since the alcohol contained therein would dissolve the celluloid base. The only available means is a current of warm, dry, clean air. While the well-equipped factory is fitted with a special drying room, such a facility is beyond the resources of the average independent worker, who must therefore be content to revolve his frame or drum continuously, until the gelatine has hardened sufficiently. The process can be accelerated to a certain extent by revolving the drum or frame over a steam radiator, or some other form of heating which emits no smoke or flame, but the temperature of the air must not be raised too high or the gelatine coating will be injured. If the weather is fine and calm, the drying may be done upon a lawn in the open air, but in any event extreme care must be observed to prevent dust settling upon the gelatine while it is wet and soft, or irreparable injury will be inflicted. So it behoves the worker to keep his dark room and drying room free from dust. Drying should not be done in the dark room because there are small particles of chemical dust always floating about in such surroundings. If these should settle upon the emulsion they would play sad havoc with it.
When the gelatine has hardened the film may be transferred direct from the drum to a spool by means of a winder. When the beginner has become expert he will be able to do this by hand, but it is never a wise practice since the coating is liable to become scratched. The spool-winder is inexpensive and does the work much more quickly, while the risk of damaging the film is eliminated.
As has been mentioned, it is well to develop the film as soon as possible after exposure. Although the exposed film is kept in a dark box, the chemical action set up by exposure before the lens, continues, as in dry-plate and snap-shot photography, and in a more rapid manner. Consequently the film should not be left undeveloped for more than a few days at the utmost. While prompt development is usual in topical work, there are other sorts of work in which the operator may feel tempted to put the film on one side for development at a later and more convenient time. Perhaps several weeks may elapse, and then complete amazement is expressed at the result. In the unexposed condition, however, Eastman stock will last many months so long as it is not removed from the case in which it is packed at the works. Equal care must be used in storing the developed negative films. They must be kept in a cool dry place, protected from severe fluctuations in temperature and climatic effects.