CHAPTER XXIII

THE DEVELOPING AND DRYING OF PLATES AND FILMS

=Field Requirements.=—Developing, fixing, drying and printing in the field demand simple and convenient apparatus that may be carried about and installed with the least amount of labor. On top of these requirements military needs impose others that are more difficult. _Speed_ is, on occasion, imperative. A print may be required within a few minutes after landing, and many thousands within a few hours. Quantity production must be achieved under the most primitive conditions. Nothing, in fact, shows the calibre of the photographic officer better than his choice of workplaces as the army moves forward. Ingenuity and practical judgment are at a premium. Cellars, stables, dog kennels, or huts hastily built from packing cases, must be equipped and in working order over night. All the facilities offered by a great city are urgently needed—water, electric light, power for driving fans—but must be dispensed with if the photographic section is to be convenient to the airdrome, whose portable hangars are most apt to be pitched in the open country. Water must be carried, electricity generated, and to the photographic problem is added the military one of concealment and protection. Dugouts and bomb proofs must be built for supplies, and “funk holes” for the men. Entire underground emergency extensions have sometimes been built in stations occupied for extended periods, for airdromes are a favorite bombing target.

For getting the exposed plates to the photo section, messengers, on motorcycles if possible, are employed. In some cases, where hangars and photographic hut are forced to be widely separated, recourse has been had to parachutes (Fig. 112), a device also employed to distribute prints to infantry during an advance.

For warfare of movement, especially in sparsely settled or devastated country, where cellars are unavailable, the dark room must be taken along. _Motor trucks and trailers_ (Figs. 113, 114, 115), the former for hauling supplies and electric light generating plant, the latter fitted as a complete developing and printing laboratory, form the headquarters of each photographic section in the field. Usually altogether too small for the amount of work required, they were extended by tents and lean-to's, or ingeniously used as a nucleus for the organization of the favored stable or cellar.

=Methods of Plate Development.=—Where speed is not required the simplest and commonest mode of developing plates is in the tray, one plate at a time. Common practice is to examine the plate at intervals during development, and discontinue the operation on the basis of its appearance. This is only possible if the plates used are insensitive to some light by which the eye can see. Deep red light is suitable for ordinary and most orthochromatic plates. A faint blue-green may be used with some panchromatic plates. The best practice, however, is to develop by time in total darkness, whereby all chance of dark room fog is avoided. Development time for plates of the average exposure of the one to be developed is either known from previous experience, or is found by trial on the first one. Development by time results in negatives of densities varying with the exposures, but, as was brought out in the discussion of sensitometry, this difference can be compensated for by the choice of the paper used for printing, and by its treatment.

Where larger quantities of plates are to be handled _tank development_ is adopted. In ordinary tank development the plates are placed in grooved tanks, into which is poured first the developer, next the rinsing water, and then the hypo. It has been customary in tank development as practiced for peace-time work to use dilute developer, requiring from ten to thirty minutes, but speed requirements in war-time aerial photography dictate the use of full-strength quick-acting developer. An improvement on the simple grooved tank is provided by metal cages or racks, each holding a dozen or more plates, which may be introduced or removed from the tank as a unit (Fig. 116).

The _core rack_ system combines certain of the features of both tray and tank development. Each plate is inserted in a separate metal frame with projecting lugs to rest on the top of the tank and so suspend the plate in the solution. The process of development is the same as in the tank system, but any individual plate may be examined and removed.

=Film Developing and Fixing.=—The problem of quickly handling roll film of large size is one upon whose solution depends in large degree the feasibility of film cameras for aerial work. It presents many difficulties: a long film is unwieldy, is inherently subject to curling, and takes up much space if it is handled entire. For small scale operations roll film can be cut into short strips and developed either by drawing through a tray or, if cost of developer is no object, in a deep tank. In order to make the cutting apart of exposures easy in the dark, film cameras should make some form of punch mark in the film between the exposed parts, or the space between exposures should be uniform, so that a print trimmer set to a definite mark may be used. Racks for holding two or three feet of film, folded back on itself and clasped by spring clothes-pins, are fairly practical. One object of the use of film, however, is to greatly increase the number of possible exposures; and where hundreds instead of dozens of exposures are to be developed, this method takes up entirely too much time.

Following the practice in moving picture development, _film developing machines_ of various designs have been devised. Among these may be described the G. E. M. machine; the Ansco machine; the Eastman apron machine; the Brock frame and tank apparatus; the Eastman reel machine; and a modification of the latter by the United States Air Service.

The G. E. M. film developing apparatus, similar in idea to the Eastman “apron” method of film developing, as exemplified in the familiar amateur film developing machines, has the film wound in a spiral on a long linked metal frame or chain. After being wound it is placed in a tub of developer, from that to a tub of water, thence to a tub of hypo, and finally to a tub of water, where it is washed in several changes. The objections to the method are that it takes up much floor space for the various tubs, and that it requires such large quantities of solution. To develop a thirty-five foot length of 18 × 24 centimeter exposures requires approximately 28 gallons of developer; for the rinsing, 28 gallons of water, and the same for hypo, and at least three times that for washing. In all 168 gallons of water must be brought to the developing hut or lorry.

The Ansco machine makes use of an idea frequently applied in the moving picture industry. The film is carried spirally, upon two cross-arms which bisect each other at right angles, and which contain vertical pins around which the film is looped, beginning at the center and working out. After it is wound it is placed in a tub of developer, as in the G. E. M. machine. It has an advantage over this apparatus in that the shape of the tubs or tanks is square instead of round. But it is equally extravagant of space and water.

This same criticism may be made of the Eastman apron apparatus for film developing. This is similar to the G. E. M. machine, but differs from it in using a perforated celluloid apron to support the film during the various operations, instead of a metal chain.

The Brock developing outfit consists of a rectangular wooden frame and a three-compartment tank. The frame, which is approximately 3 by 4 feet in size, is used as a support for the 4 inch wide film, which is wound spirally around it, between guiding pins. A special support is provided, on which the frame may be rotated as the film is fed off the camera spool. The frame, with the film on it, is lowered successively into the three narrow but deep compartments of the developing tank. The first compartment holds developer, the next water, the next hypo. The amount of developing solution required is rather large (96 gallons of water in all for a strip of 100 4 × 5 inch exposures), but because of the small surface exposed to the air, it keeps for a considerable period. The chief demand for floor space with this apparatus is for feeding the film on to the frame.

In the Eastman twin reel machine the film is wound on a wooden drum or reel of large diameter, to form a helix. The drum is suspended so that the bottom edge touches the developing solution, and, upon revolving the drum, every portion of the helix of film is brought into contact with the developer. By shaping the developing tank so that it closely conforms to the shape of the reels, a high economy in quantity of developing agent can be achieved. When developing action is finished, the developer is emptied out, rinse water put in; hypo follows, and then comes the final washing with water. With this apparatus the whole cycle is completed, for the 35 feet length of film above considered, with seven gallons of water.

The Air Service apparatus differs from the above only in the drying method, which will be described below.

Heavy cut film, such as is marketed under the name of Portrait Film, has not thus far been used in aerial work, except for printing transparencies. It is conceivable, however, that film in the cut form may be used in some future design of camera. This may be developed expeditiously in a tray, six or eight films being handled at a time, in a pile, pulling out the lower one frequently and placing it on top. The core rack system is also available for film in this form, special racks with clips to hold the film being necessary.

=Plate Drying.=—The drying of negatives on glass is a comparatively simple matter, owing to the rigid nature of the emulsion support. A large number of plates may be placed in a compact mass in the ordinary plate racks of commerce with the wet sides accessible to a draft of air. Two dozen plates separated from each other by a quarter of an inch and left to dry spontaneously in a room of ordinary humidity and living temperature will dry in two hours and a half. If the surface be wiped with soft cheese-cloth or chamois, so as to absorb all the surface moisture before the plates are placed on the rack, this time may be appreciably reduced. By placing the plates in a forced draft of air, from an electric fan, this time may be reduced to an hour.

Extra rapid drying of plates may be accomplished by placing them in a bath of alcohol before putting them in the racks. The alcohol displaces all the water in the film, and is itself very quickly dissipated into the atmosphere when the plate is taken from the tray. The plate must be left in the alcohol tray long enough for the substitution of the alcohol for the water in the film to take place. Five minutes is long enough. The alcohol before use must be as nearly free from water as possible. The best way to make sure of this is to place in the bottle of alcohol some lumps of calcium oxide, which will take up the water and form calcium hydroxide, which settles at the bottom of the bottle.

Another method of quick plate drying takes advantage of the extraordinary greediness of potassium carbonate for water. The wet plates are placed in a saturated solution of potassium carbonate and left for a minute. If a plate be now taken from the solution and its surface wiped with a soft cloth, it will be found that the film has a greasy, slippery feeling, but that it contains no water and can be printed from at once. Plates so treated should be washed, however, at some time in the succeeding four months, or the traces of potassium carbonate left in the film cause deterioration.

=Film Drying.=—Unlike the drying of plates, drying of film negatives is a very puzzling problem, and may be considered as the crux of the successful use of film in aerial cameras.

Apron and similar machines have very poor drying efficiency if the film is left in place, for not only the film but the apron or chain must be freed of water. This may be hastened, as in the G. E. M. machine, by blowing air through with fans, but even with their help drying a 35 foot film is a matter of two hours or more. Passing the film through wringers or a squeegee to remove excess water is a considerable aid; the film may either be re-wound on a dry reel, to be put in a forced draft of air, or may be hung up in short lengths or festooned, either method taking up a great deal of space. The use of alcohol is not advisable as it may abstract camphor from the celluloid and cause the film to become distorted.

The Eastman twin reel machine had an upper reel joined to the lower or developing reel, with a chain and sprockets, so that the upper reel revolved at the same time and rate of revolution as the lower, when the lower was being revolved at the gentle speed appropriate to the developing process. Fans blew a draft of air over the upper reel. This method necessitated over an hour for drying.

The Air Service model of film developing and drying machine (Fig. 117) introduces an essential modification in the drying scheme of the Eastman apparatus. The upper reel is quite independent of the lower reel and is revolved at a high rate of speed, so that a whirling action is introduced into the drying. Large rotating fans at the same time drive a considerable volume of air across the film surface, and the combination of the two agencies makes it possible to dry 35 feet of 18 × 24 centimeter film in 20 to 30 minutes. This for large numbers of pictures makes the use of film even quicker than that of plates. The only practical drawback to the apparatus is its bulk, which calls for a separate room or trailer. This, however, seems to be inevitable in the use of large roll film.

Cut film can be dried with speed only if placed in a draft of warm air. Drying boxes, with a chute or chimney and with fans to drive the air through from an alcohol stove, will dry several dozen films in an hour. The films must not be closer together than about one inch, which makes the drying boxes rather cumbersome.

=Marking Negatives.=—After development and drying, and before filing or printing, each plate should be marked with data for purposes of future identification. This is most easily done with pen and ink on the film side (in reversed lettering) either along an edge in the unexposed portion covered by the sheath or in a corner, so as to lose as little of the photograph as possible. Just what data shall be inscribed is dictated by the purpose for which the negative was made. The date, altitude, time of day, true north (from known permanent features or from shadow direction and time of day), number of the camera used, the focal length of the lens. Other records, such as the plane and squadron numbers, or even the pilot's and observer's initials, may be called for (Fig. 75). For mapping work the scale of each of a set of negatives, once found, may be marked, either in figures or by means of a line of length corresponding to a fixed distance on the ground. Rectifying data can similarly be inscribed, so that the negative can be printed in the enlarging and rectifying camera with the minimum of delay.