CHAPTER XXII

PHOTOGRAPHIC CHEMICALS

=General Considerations.=—Developing, fixing and other chemicals for aerial work differ in no essential respect from those used in ordinary photography. Full discussions of these are to be found in numerous texts and articles. The aerial photographic problem is to select those most suited for the under-exposed flat negatives characteristic of photographs from the air. At the same time selection from among the chemicals of appropriate quality must be governed by considerations of the conditions surrounding work in aerial photographic laboratories. These laboratories, especially in war-time, are apt to be most primitive in their facilities.

=Characteristics of Developers for Plates and Films.=—From the standpoint of practicability, aerial negative developers should have good keeping power, be slow to exhaust, and work well over a considerable range of temperatures. From the standpoint of the photographic quality desired in the negative, the developer should bring up the maximum amount of under-exposed detail. This means that it should impart the highest possible speed to the plate, with good contrast, and low fog or general reduction of unexposed silver bromide.

There are many characteristics to study in a developer: its effect on inertia or speed, gamma infinity, fog, time of appearance, “Watkins factor,” speed of development, temperature coefficient, dilution coefficient, keeping power, exhaustion, length of rinsing, stain, color coefficient and resolving power. These are defined and described as follows:

_Effect on inertia._ The meaning of inertia has already been given under the discussion of plate speed. While this is a constant, independent of time of development, for any one developer, it is altered appreciably by change of the latter.

_Time-gamma relation._ Contrast, symbolized by γ, has likewise been discussed under plate sensitometry. Viewed from the standpoint of the developer, the point of interest is the rate at which γ varies with development, and the maximum contrast which can be reached or γ infinity. Speed of development is commonly defined by the _velocity constant_, symbolized by κ, which is arrived at mathematically from a consideration of the time of development to produce two different contrast values. High γ infinity is desired for aerial negatives, and for rapid work κ must also be high.

_Fog._ The opacity due to chemical fog is to be kept at a minimum in aerial negatives, as it is chiefly prejudicial to under exposures.

_Time of appearance and Watkins factor._ The time of appearance is measured in seconds. The Watkins factor is a practical measure of the speed of development, and is determined by the ratio of the time of development required for a definite contrast, to the time of appearance. It is useful also as a guide to development time.

_Temperature coefficient._ This is the factor by which the time of development at normal temperature (20 Cent.) must be increased or decreased in order to obtain the same quality negative, for a change of seven degrees either side of normal.

_Temperature limits_ are the temperatures between which development can be carried out with any degree of control or without serious damage to the negative. These factors are of great importance where climatic or seasonal changes have to be endured.

_Dilution coefficient._ This is the factor by which the development time is increased in order to maintain a given quality negative in different dilutions of the developer. It is useful in tank development.

_Keeping power._ The keeping power of a developer, mixed ready for use, is determined by its ability to resist aerial oxidation. A developer of poor keeping power, which must be made up immediately before use, causes delay and waste of time whenever emergency work has to be done, whereas a developer of good keeping power may be left in its tank ready for instant use.

_Exhaustion_ of a developer is the rate at which it becomes useless for developing, due both to aerial oxidation and to the using up of its reducing power by the work done in developing plates. It is conveniently measured by the area of plate surface developable before the solution must be renewed.

_Length of rinsing._ The time required for rinsing between development and fixing bath plays a not unimportant part in total development time. Dichroic fog is caused with some developers if, due to insufficient rinsing, any of the caustic alkali is carried over to the fixing bath. Stains develop also if the fixing bath is old, or if light falls on the unfixed plate while any developer remains in the film.

_Color coefficient._ The function of the sulphite, which forms a constituent of all developing solutions, is two-fold. It acts partly as a preservative, and partly to prevent the occurrence of a yellow color in the deposit. The yellow color, if present, increases the photographic contrast. This phenomenon has been purposely utilized, particularly in the British service, to give “stain” to negatives which otherwise would show insufficient printing density. The color index or coefficient of a negative (with a given printing medium) is the ratio of photographic to visual density. If we take a pyro developer containing five parts of pyro per thousand and ten parts of sodium carbonate, and then vary the amount of sulphite from none to fifty parts per thousand, the color index varies as follows:

Sulphite Color Index Parts per Thousand 50 1.16 25 1.24 15 1.30 10 1.45 5 1.80 0 2.75

The color index is somewhat different with various kinds of printing media.

This staining effect is a variable one, depending upon length of development, dilution of the developer, length of rinsing, temperature, the fixing bath used (plain hypo being necessary for a maximum effect), the length of washing after fixation and the properties of the water used. Standardization of these conditions in the field is difficult; hence any developer which will give the same effective contrast without resorting to stain is to be preferred.

_Resolving power._ Some developing processes and conditions will introduce bad grain into the negative. Hence the resolving power which a developer brings up must be investigated among its other characteristics.

=Practical Developers for Aerial Negatives.=—In the English service a pyro metol developer was generally used, producing stained negatives. The French, American and Italian practice was to use metol-hydrochinon, without staining. A special chlor-hydrochinon developer, worked out by the Eastman Research Laboratory for the United States Air Service, has probably the greatest merit of any yet tried. A comparison, given below, between it and a pyro metol formula used on a representative plate, illustrates the use of the various bases of study given above.

PYRO FORMULA

Solution A Solution B Pyro, 3.75 grams Sodium carbonate, 53 g Potassium metabisulphite, 3.75 g Metol, 3.05 g Potassium bromide, 1.5 g Water, 500 c.c. Water, 500 c.c.

Use 1 part of A to 1 of B

CHLORHYDROCHINON FORMULA

Solution A Solution B Chlorhydrochinon, 25 g Sodium carbonate, 30 g Metol, 6 g Sodium hydrate, 10 g Sodium bisulphite, 2.5 g Potassium bromide, 3 g Sodium sulphite, 25 g Water to 670 c.c. Water to 330 c.c.

Use 2 parts of A to 1 of B

Pyro Chlorhydrochinon H & D speed 150 180 Gamma infinity 1.45 2.12 Fog (at maximum gamma) .32 .60 Time of appearance 5 seconds 5 seconds Watkins factor 25 10 Velocity factor “κ” .320 .400 Temperature coefficient 1.40 2.0 Temperature limits 4° to 32° C 4° to 32° C Keeping power 45 minutes 8 days Exhaustion (100 c.c.) 30 sq. in. 300 sq. inches Dilution coefficient 2 2 Color coefficient 1.50 1.00 Resolving power 47 53

Owing to the difficulty of securing pure chlor-hydrochinon a metol hydrochinon of very similar properties has been worked out. Its composition is

Metol 16 grams Hydrochinon 16 grams Sodium sulphite 60 grams Sodium hydroxide 10 grams Potassium bromide 10 grams Water to 1 litre

To keep the ingredients in solution in cold weather, 50 c.c. of alcohol should be included in every litre of solution. All things considered this is probably the most practical and satisfactory developer for aerial negatives.

=Developers for Papers.=—The following formula has been found very satisfactory for papers:

Metol .9 gram Hydrochinon 3.6 gram Sodium carbonate 20.0 gram Sodium sulphite 14.0 gram Potassium bromide .5 to 1.0 gram Water to 1 litre

=Fixing Baths.=—For plates the following fixing and hardening bath is recommended:

Sodium thiosulphate (hypo) 350 grams Potassium chrome alum 6 grams Sodium bisulphite 10 grams Water to 1000 c.c.

During hot weather, the above quantities of chrome alum and bisulphite are doubled.

For papers the following:

Hypo, 35 per cent. 100 volumes Acid hardener 5 volumes

The acid hardener is constituted as follows:

Alum 50 grams Acid acetic 28° 400 c.c. Sodium sulphite 100 grams Water to 1 litre

=Intensification and Reduction.=—These processes have been little employed in air work. Reduction is rarely necessary, for obvious reasons. Intensification would often be of value, but the common practice, which saves some time, is to use printing paper of strong contrast for those negatives which are deficient in density and contrast. When intensification is desirable or permissible, either the ordinary mercury or uranium intensifier may be used.

=Water.=—In the field it is found necessary in many cases to purify the water that is to be used in mixing up chemicals. Water may contain suspended matter or dirt, dissolved salts, and slime. It is important to remove the suspended matter, as it may cause spots on the plates and papers, while any slime would coagulate, forming a sludge in the developer which would also tend to settle on the plates and cause marks during development. The dissolved salts may or may not cause trouble. Two methods of purification are possible:

(_a_) Filter the water through a cloth into a barrel, add about one gram of alum for every four litres of water, and allow to settle over night. Draw off the clear liquid from a plug in the side as required.

(_b_) Boil the water and allow it to cool over night. If the water contains dissolved lime, boiling will often cause this to come out of solution.

V METHODS OF HANDLING PLATES, FILMS AND PAPERS