Desensitizing of Photographic Emulsions
by L.P. Clerc, Photography, Theory and Practice (chapter 27, p. 222)
Use of Coloured Developers
Carey-Lea, as far back as 1877, showed that after immersing a photographic plate in ferrous oxalate developer it was possible, without risk of fog, to illuminatethe dark-room much more brightly than would have been possible during the handling of the sameplate when dry. This effect was attributed to the orange-red colouration of the developer (it has since been recognized that ferrous oxalate developer is a very efficient desensitizer), and little attention was paid to it, since the emulsions were so insensitive that in all cases a bright illumination could be used.
About 1889 various attempts were made to introduce into photographic practice the use of developers which had been coloured red by the addition of certain dyes, these dyes being subsequently destroyed in an acid fixing bath (coralline, croceine). The method, however, was not successful, since the dyes transmitted both blue and violet light. In 1900 Auguste and Louis Lumière and A. Seyewetz, with the same object in view, recommended the use of an orange-coloured compound, magnesium picrate, which allows the development to be observed at a distance of about 45 cm from a candle or at 12 cm from a 16 c.p. lamp, provided development is sufficiently rapid and the plate remains covered by a layer of developer about 1:27 cm thick. If these distances are doubled, it is possible to remove the negative from the bath and examine it very quickly by transmitted light. This method of working, although it afforded interesting demonstrations, was not used in practice.
Loss in Sensitivity of Emulsions Impregnated with Developer
After the use of organic developing agents had become general, it was frequently pointed out that after the developing bath had thoroughly impregnated the emulsion a considerable lowering of sensitivity took place, which could not be explained in these cases by any colouration of the bath. In 1901 Lüppo Cramer carried out some experiments in this direction (using the different developing agents) and discovered that this action occurred with nearly all the most common developers (with the exception of hydroquinone), both in plain and alkaline solutions, and that sulphite tended to reduce the effect. In 1920 the same worker found a very marked reduction in the sensitivity of photographic emulsions² (reduced to 1/50th or 1/100th of the original value) after they had been bathed for about 1 min in a pure solution of diaminophenol-hydrochloride of from 0.02 to 0.05 per cent strength, although no reductionin the latent image resulted from this treatment. An analogous effect may be obtained by the addition of this product to a hydroquinone developer; it was soon recognized that this desensitizing was due to traces of oxidation products, which are rapidly formed by the action of the air on dilute solutions of this developer.
R. Freund (1908) showed that the sensitivity of a sensitive plate was completely destroyed by treatment with a solution of potassium iodide, so much so that development could be carried out even in very bright white light, but at the same time a marked weakening of the latent image occurred. Since then F.F. Renwick (1920) has shown that it is possible to avoid any destruction of the latent image by the addition of certain substances to the iodide. The solution recommended contains, per 600 ml, 5.83 g of potassium iodide, 5.83 g of anhydrous sodium sulphite, and 16.85 g of potassium sulphocyanide. After several minutes’ immersion, the plate is rinsed. Development can then be carried out, even in sunlight, with any ordinary diaminophenol developer made up with sodium carbonate just before use. It should be added, however, that although these experiments may be of interest, total desensitizing does not present any advantages over the methods of partial desensitizing which are to be described.
During further experiments on this phenomenon, Lüppo Cramer discovered, several months later, the remarkable desensitizing properties of phenosafranine, a violet-red dye of considerable tinctorial power.
It was soon shown that the desensitizing properties of phenosafranine are possessed by various safranines and, in varying degree, by different substances of similar constitution. The red colour of some of these substances might lead to the supposition that the protection from fog is due to the absorption of the active radiations by the dye impregnating the emulsion. This is not the case, however, for the efficiency of these desensitizers is practically as great with panchromatic as with ordinary emulsions, and, secondly, a sensitive plate exposed behind a cell containing a solution of the dye, even in concentration greater than that used for desensitization, and under a layer of greater thickness than that of the developing bath, develops an intense fog. This is due to the fact that the red safranines absorb very little of tile violet radiations. It was discovered not long afterwards that certain violet dyes belonging to the safranine family were capable of acting as efficient desensitizers.
The first experiments were carried out with 1:200 solutions, in which the sensitive plate was bathed for 1 min. At the end of this time the dark-room may be illuminated as brightly as required, provided the blue and violet radiations are absorbed by a yellow or orange filter. A yellow light-source of feeble intensity, such as a candle or petrol flame, may be brought quite close to the dish in which the development is taking place, and it is even possible to examine the negative by transmitted light as frequently and for as long as required during the course of development.
Almost immediately it was shown that desensitizing could be brought about with even the much more dilute solutions of 1:20.000 and 1:50.000, so long as moderate intensity of illumination is observed in the dark-room, either as a bath before development or in the developer itself.
Safranine is precipitated by developers containing hydroquinone or pyrogallol. It cannot be added to concentrated solutions of other developers, but only to the diluted bath ready for use, and even then it can only be introduced when ina very diluted form itself (at most 1:2.000).
With either of the two methods of desensitizing, safranine prevents or reduces considerably the development fog, so that it is possible, when necessary, to extend development beyond the limits usually fixed by the growth of the fog, and also to use a much more alkaline developer. Finally, hydroquinone developer, which works somewhat slowly in the normal state, acquires almost the properties of a rapid developer, such as metol.
Desensitizers, and particularly Basic Scarlet N, although they may not reduce the so-called chemical fog, at least suppress the aerial oxidation fog which occurs when emulsion, impregnated with developer, is left in contact with the air for any length of time, as, for example, during the examination of the negative while development is in progress. This fog is formed particularly with developers containing hydroquinone (E. Fuchs, 1924).
The only defect of safranine is its strong staining action, which causes a considerable colouration of the fingers (more particularly the nails) and of the gelatine, especially if it has been used in the concentrations which were first recommended. The colouration of the gelatine is removed for the most part during the subsequent manipulations and the washing, any defects in this last process being very clearly shown by the unequal colouration of the different parts of the plate. This colouration, however, even when it is fairly intense or irregular, does not affect printing adversely, and maytherefore be ignored.
The most useful employment of desensitizing is emphatically with panchromatic plates, which in the past have had to be developed in practically complete darkness, but which now, after desensitizingin a preliminary bath, or in the developer itself, may be developed in a much brighter light than that previously used for ordinarv emulsions. Avoid the use of a green light with a red desensitizing dye, or vice versa, since the negative appears uniformly black during the course of development.
All desensitizers are not, however, suited to all emulsions which are panchromatic, or sensitive to infra-red, probably owing to reactions between the colour sensitizers and the desensitizer. The effect of a given dye on a given emulsion varies greatly from one spectral region to another.
Many contradictions may be noted in publications on the properties of desentizers owing to the use in experiments of impure or wrongly labelled products. A large number of substances which are energetic desensitizers in plain aqueous solutions are unusable in practice, the desensitization being annulled or decreased during development by the action of one or other of the components of thedeveloper; tests of a desensitizer must therefore include a practical development test in an abundance of yellow light (Miss F.M. Hamer, 1931). Failures have been obtainted on many occasions while using an acid diaminophenol developer for desensitized plates and films. It is necessary in this case to use only dilute solutions of the diaminophenol-hydrochloride for the desensitizing.
The systematic study of a large number of dyes undertaken by various workers has shown that the number of basic dyes which exercise this narcosis or numbing effect on silver bromide emulsions is very considerable. Although certain of them desensitize the emulsion, others cause fog. Others either desensitize or fog the emulsion, according to the concentration of the solution employed. Methylene blue, for example, at a concentration of 1:1.000.000.000 (1 g in 1.000 m³ of water), exerts an appreciable desensitizing action, although admittedly far from complete (at such a degree of dilution the solution is absolutely colourless), but at a concentration about a thousand times greater produces a marked amount of fog. Others weaken the latent image to a greater or less extent. Prolonged action of red light causes in all cases a marked regression of the latent image. Also, the latent image is very much weakened if, after desensitizing, the plate is left for a long while before development.
Among the desensitizers which do not belong to the safranine group we may mention the following: rhoduline red, auramine, chrysoidine, fuchsine (Lüppo Cramer), tolulyl red, aurantia (Auguste and Louis Lumière and A. Seyewetz, 1921), a green non-tinctorial mixture of secret composition, supplied commercially under the name of pinacryptol green (E. Koenig, 1922), which possesses desensitizing properties equal to those of the safranines, but which, at least with certain emulsions,increases the chemical fog instead of diminishing it; finally, there is a red dye, which has all the properties of safranine and, above all, stains the fingers, nails, and gelatine only very feebly, namely, basic scarlet N (Cie. Nat. des Matières Colorantes), first recommended in 1924 by the Pathé-Cinéma Laboratories (basic scarlet N is a mixture of approximately equal weights of a safranine and chrysoidine. This latter, when used by itself, stains the skin and the nails very strongly. The Pathé-Cinéma Laboratories have also shown that the fog caused by certain desensitizers may be avoided by adding to their solution in suitable amounts other dyes which do not affect the desensitizing but exercise a strong retarding action on the fog; acridine yellow, for example, prevents the fog caused by methylene blue). In many cases skin eruptions, similar to burns, with eczematous eruptions have been caused through handling solutions of aurantia; it is, therefore, wise, when using this desensitizer, to prevent the solution from coming into contact withthe fingers (by using developing frames or rubber finger-stalls).
The desensitizing power (ratio of the sensitivities before and after treatment) varies with the concentration. In concentrations of about 1:5.000 it is approximately 100 for safranine, pinacryptol green, and basic scarlet N, and for the other dyes mentioned it is considerably lower. In very dilute solutions, the desensitizing power is markedly reduced; it is only 10 for safranine in concentrationsof about 1:1.000.000. The values of the desensitizing power given above refer to illumination by white light. The chromatic sensitivity of orthochromatic and panchromatic emulsions is lowered proportionally to a much greater extent than their sensitivity to the blue.
Lüppo Cramer attributes desensitizing to the weak oxidizing properties of all substances acting as desensitizers (they all can be reduced by hydrosulphites), these properties permitting them to retard considerably the formation of nascent silver, although they are unable to attack any silver which has been formed previously. Some facts appear to confirm this hypothesis: in slightly acid solution in the presence of a bromide, the latent image is destroyed. Some investigations of Sihvonen and Baur on the mechanism of photochemical reactions enable the rôle of a desensitizer to be interpreted as follows: the energy received by the silver bromide is transferred to the desensitizer, this substance alone being decomposed; this would explain why with very long exposures desensitization no longer protects the silver halide, the adsorbed dye having been completely destroyed. Desensitizers are ineffective in an atmosphere without oxygen or in a vacuum (Frln. M. Blau and H. Wambacher, 1934). It is an interesting fact that a uniformly fogged plate, if treated with a dilute solution of safranine (for example) containing potassium bromide, then put to dry without intermediate rinsing, will give, by exposure to light followed by development, a positive image. The first latent image is destroyed by the second exposure, the effect being more marked with increase in the second exposure.
At the usual solution strengths desensitization increases with the duration of the treatment, first very rapidly and then more and more slowly. Prolonged washing of a desensitized photographic emulsion diminishes the desensitization without annulling it.
Practical Methods of Desensitizing
In order to obtain the maximum advantages from the use of desensitizing methods, there should be two systems of illumination in the darkroom, with arrangements for changing rapidly from one to the other (different lamps or different current supply), or an arrangement by means of which an additional safe light may be removed from the front of a permanent yellow screen. It should be pointed out that, even in a dark-room different parts of which are being used for different operations (many operators working separately, or the treatment of cinematograph films in continuous machines), the elimination of oxidation fog is a sufficient advantage in itself to make the adoption of desensitizing, either before or during development, worthwhile. It is for this reason that desensitizing is carried out on the commercial scale with positive films, which in ordinary circumstances could be handled in light of sufficient intensity for the superintendence of the operations.
When choosing a desensitizer, it is necessary to remember that coloured desensitizers, even those which are most easily removed, are generally held very strongly by paper fibres. It is therefore necessary, for the treatment of paper negatives, to choose either a colourless desensitizer, or one which has already been shown to be readily eliminated. If the desensitizer is miscible with the particular developer in use, less efficient desensitization is obtained, for equal concentrations, by the use of a combined bath than by preliminary treatment before development (Miss F.M. Hamer, 1929), althoughthe reverse has frequently been stated. In all those cases, however, in which the sensitive layer is wetted previous to development (cinematograph films are treated in this manner, to prevent the adherence of air bubbles) the desensitizing may be combined with this preliminary treatment.
The desensitizing baths may be used a considerable number of times, but any baths which have been left unused in the dishes for any length of time generally contain a certain amount of suspended matter (residues from evaporation, dust particles, products formed by slow reaction with the calcium salts in the water used for dilution), which may cause spots on the sensitive film. After use, solutions of pinacryptol green often throw down a gelatinous precipitate resulting from the coagulation of the dye, owing to the accumulation in the bath of potassium bromide yielded up by the emulsions that have been treated. It is, therefore, necessary to make sure that the bath is quite clear; if it is not satisfactory it must be filtered.
The desensitizer may be employed in solution of increasing dilution according as the intensity of illumination in the dark-room is cut down. Whatever method is used for the use of the desensitizer, one minute must be allowed for its penetration through the layer of an ordinary plate or film, and a little longer in the case of multiple-coated emulsions and very thick layers.
Basic scarlet N 1:5.000 gives excellent results, the solution requiring to be frequently renewed. It is made up by diluting a stock 1:500 solution, which latter keeps well if a little formaline (about 1.2 ml per 600 ml) be added.
It is necessary to make allowance for the fact that desensitizing usually modifies the time of development required to reach a given degree of contrast, and affects the time of appearance of the image and the time required for its complete development to a different extent (sometimes in an inverse manner). The method of estimating the time of development from a measurement of the time of appearance of the first details of the image (Watkins method), is only applicable if the desensitizer is added to the developer. The “factors” of the different developers require to be modified in the following manner—the “factor” for hydroquinone developers should be multiplied by 4, for metol-hydroquinone developers it remains the same, while for pyrogallol or diaminophenol developers it is reduced by a third (R.E. Crowther, 1921).