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.
[Illustration:
FIG. 112.—Receiving pictures from plane by parachute.]
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.
[Illustration:
FIG. 113.—Mobile photographic laboratory.]
=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.
[Illustration:
FIG. 114.—Interior of photographic trailer, developing room.]
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).
[Illustration:
FIG. 115.—Interior of photographic trailer. Enlarging camera and
printer.]
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.
[Illustration:
FIG. 116.—Tank and rack for tank development.]
=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.
[Illustration:
FIG. 117.—U. S. Air Service film developing machine for film 24
centimeters wide.]
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.