T H E R E L h T I O S BETWEEN PHOTOGRAPHIC REVERSAL ASD T H E SENSITIVITY OF T H E SILVER HALIDE G R A I S BY F R A S K E. E . G E R M A N S A S D D. K . S H E S
The use of photography as an aid to research is possibly unsurpassed by any other scientific tool. Its nearest competitor, the X-ray, would be of considerably less value if it were not possible to leave a permanent record on the photographic plate. The universal appeal of photography to both scientist and layman has brought it about that many persons have tried to explain the numerous secrets of nature associated with the making of pictures. As a result there is probably no field of science in which more amateurs are working or in which the literature is more filled with contradictory theories. However, the reason for confusion is even deeper seated than this, and may be found in large part to be due to the particular character of the science. Here if ever we deal with the chemistry of the infinitely small, and as a consequence, mere traces of chemicals which easily escape the scrutiny of the most careful analyst, may completely change the character of the results. Then again failure to realize that something added for a special purpose may have far reaching results of an altogether unexpected nature, has led to much confusion. Our study of the grain sensitiveness of silver iodide in photographic emulsions illustrates how easily one may be led astray.’ The most apparent difference between silver bromide and silver iodide grains is their reducibility by means of chemical developers, a fact to which Luppo-Cramer2 attributes the apparent insensitivity of silver iodide. The silver bromide emulsion, when coated in a single grain layer, usually develops very quickly with the unbromided developer, even without having received an exposure; in other words, it fogs very readily. Work carried out by previous investigators has been confined mostly to silver bromide grains, and the developers used contained a large amount of potassium bromide to suppress the development. In order to show the development centers, the development had to be stopped at the end of about a minute. We have shown that in the case of silver iodide grains in a one grain layer emulsion the development goes on very slowly; even at the end of fifteen minutes in a developer containing no soluble halide, some grains are still incompletely developed. The fact that a grain will be either completely developed or not a t all was confirmed by developing the slides of an evenly exposed plate different lengths of time and counting the percentage number of grains developed, assuming a grain would be completely developed if it contained one or more centers. The plate and the exposure were so chosen that only a small Germann and Shen: J. Phys. Chem., 33, 864 1583 (1929). Eder’s Jahrb., 40, (1903).
* Luppc-Craner:
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fraction of the grains would be made developable, in order that fog correction, if any, could be made by having an unexposed plate developed along with it. In the short development the centers were so small that they could easily escape observation, but as the development went on, the centers increased in size until the whole grain was developed. Fig. I shows that the percentage number of developable grains made visible increases with time of development and becomes practically constant after about ten minutes. The values plotted have been corrected for fog, which, even after thirty minutes of development is less than one per cent. This is a very striking characteristic of
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FIG.I
FIG.2
silver iodide emulsions. Since prolonged developing does not increase fogging appreciably, fifteen minutes was adopted in all cases for the complehe revelation of the centers. Fig. 2 shows the relation between time of exposure and percentage number of developable grains when a one grain layer silver iodide plate is exposed, developed, and examined as detailed above. I t is to be noted that the maximum developability is almost reached at the exposure of 6 4 seconds, and that only 1 7 per cent of the total silver iodide grains are ever made developable. Further increase in exposure decreases the number of developable grains, that is, the period of reversal is entered. The relationship between sensitivity and grain size has been realized since the early days of photography. Fast emulsions usually consist of large grains, but it is not a t all true that the sensitivity of any large-grained emulsion exceeds that of any small-grained one.s Furthermore, emulsions having identical grain characteristics may differ considerably in sensitiveness. The fact that only in one and the same emulsion,' are the large grains more sensitive than the small ones, makes it apparent that grain size is not the sole factor determining high sensitivity.5 We still do not know what causes sensitivity; we know, however, that the sensitivity of an emulsion can be increased through processes which simultaneously increase the size of grains. We do not know the mechanism by which the sensitivity is increased; it seems justifiable to believe that the factors which govern the increase in a
Renwick: Phot. J., 61, 333, (1921).
' Sheppard: Colloid Symposium Monograph, 1, 346 (1921).
Svedberg: 2. wiea. Phot., 20, 36 (1920); Phot. J., 61, 325 (1921);62, 183, 186, 210 (1922); 64, 272 (1924); Renwick: 64, 360 (1924); 66, 163 (1926); Sheppard: J. Franklin Inst., 203, 829 (1927).
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sensitivity coincidently favor the growth of grains. In one and the same emulsion, the conditions under which the large grains are produced are decidedly different from those in the formation of the small-grain fraction; a difference in sensitiveness between these two classes of grains should not be surprising. If the sensitivity is due to the presence of some impurity in the gelatin, allyl thiocarbamide, according t o Sheppard,Band the degree of sensitivity depends upon the size of the sensitive specks situated on or in the silver halide grains,7 the large grains would be more sensitive because the sensitive specks on the large grains are likely to be larger. If we assume that the amount of the sensitive material is limited in the gelatin, the amount adsorbed on the silver halide grain would be greater in the earlier stages of precipitation when the concentration of the sensitive material is the greatest. When an emulsion is made by pouring a silver nitrate solution into the soluble halide solution to which gelatin is added, it is obvious that at the earliest stage of precipitation, the concentration of the soluble halide is also the greatest, which favors the so-called Ostwald ripening, L e . , the grains produced will attain the largest size. From these considerations, therefore, the large grains which are more sensitive are produced during the earlier stages of precipitation, during which time the concentration of both soluble halide and sensitizing material is greatest. The insensitiveness of the silver iodide emulsion cannot be explained to be merely due to the incapability of the normal developers to reduce the silver iodide grains, as suggested by Luppo-Cramer, because such explanation cannot satisfy the fact that seventeen per cent of the total grains are developable, unless the assumption is made that the rest of the grains are materially different and entirely lacking in sensitivity. If sensitivity is due to the presence in the grain of some foreign material derived from gelatin, then only this seventeen per cent of the total grains is supplied with such material, and the lack of sensitivity of the rest of the grains would be due to deficiency of the sensitivity material on account of the fact that they are formed during the latter stage of precipitation in which the sensitivity material is practically exhausted. If this were the case, the developable grains would have been those formed at the earlier stage of precipitation, L e . , those of the largest size. In order to test the above conclusions, two extreme fractions containing the largest and smallest grains of an emulsion were obtained by repeated centrifuging, and coated on separate glass plates. Both plates were exposed and developed in exactly the same manner; it was found that the developabilityexposure curves for the large- and small-grained fractions coincided with that of the uncentrifuged emulsion. This is shown in Fig. 2 , plates I, 11, and I11 representing the uncentrifuged, large- and small-grained emulsions respectively, all of which coincide. This would indicate that the undevelopability Sheppard: Phot. J., 65,380 (1925). Sheppard: J. Franklin Inst., 200, 51 (1925); Colloid Symposium Monograph, 3, 86 (1925).
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of eighty-three per cent of the grains cannot be due to the depletion of the sensitivity promoting material in the solution. It would, furthermore, indicate that there is no appreciable difference in sensitivity between the large and small grains of the same emulsion. When one of these plates consisting of grains which were only 17 per cent developable, was bathed in a one per cent solution of either hydroquinone or pyrogallol, prior to exposure, it was found that the number of developable grains increased with time of bathing until roo per cent was reached. Pro-
longed bathing in the sensitizer produced no further effect. The sensitized plate, after being thoroughly washed in running water, does not resume its initial state of low percentage developability, a fact confirming LiippoCramer's early observations.s Metol, glycin, and amidol mere also tried as sensitizers, and were found to act the same as pyrogallol and hydroquinone. Fig. 3 is a composite of many curves and shows their interrelationship better than would be possible with single ones. I n cases where the same curve has various numbers, not all points are given, as there would be too much overlapping. With the scale used, practically all of the points fall on the curves, and the various curves represented by one are identical. Thus Plates I, 11, and I11 all yield identical curves, giving a maximum developability of about 1 7 per cent. Plates IV and V were sensitized for ten minutes with I per cent hydroquinone and pyrogallol respectively, and dried without washing. The points a t 4096, and 8192 seconds showing reversal, belong only to I\', the maximum time of 2048 seconds having been adopted for all other experiments. Plates VI and VI1 were sensitized for ten minutes in I per cent pyrogallol, the one being dried and then washed in running water for two hours and again dried, the other having been washed for two hours immediately after sensitizing and then dried. 8Liippo-Cramer: Phot. Korr., 38, 158 (1901);40,25 (1903).
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In order to find the relative effect of sensitizers on large and small grains, Plates T’III and I S were prepared. Plate T’III consisted of the large-grain fraction prepared by centrifuging the emulsion as previously described, and Plate I S the small-grained fraction. Both were sensitized with a I per cent pyrogallol solution for ten minutes, and dried without washing. Thus Plate I1 before sensitizing corresponds to T’III after Sensitizing, and similarly Plate I11 corresponds to Plate IX. The power of sensitizing silver iodide plates is not limited to the usual developers, as is shown by Plates X to XT’inclusive. Plates S and X I were sensitized for ten minutes with 0.2 and 0.j per cent solutions of acetone semicarbazone respectively, and dried without washing. Plates XI1 and S I 1 1 were sensitized for ten minutes with I and j per cent solutions of sodium nitrite respectively, and dried without washing. Plate SIT’ was sensitized for ten minutes in a I per cent solution of sodium sulphite and dried without washing. Sodium nit,rite and sodium sulphite, although much stronger halogen absorbers than the usual developers are not such good sensitizers. Plate XT’ u-as sensitized for ten minutes with a I per cent solution of sodium bisulphite and dried without washing. I t is seen to be as effective a sensitizer as are the usual developers.
Sensitirzty and Speed. The curves in Fig. 3 are similar to the characteristic curves for the various emulsions, since the percentage of developable grains is proportional to the density of the plate. Khen the middle, or straight line, portions of these curves, which represent the range of correct exposure, are projected until they cut the exposure axis, it is ,seen that they intersect a t a common point marked (i). This point has been called the inertia of the emulsion by Hurter and Driffield, and may be taken as a measure of the speed. Obviously, the greater the inertia the less the speed of the emulsion. Since all these curves intersect at one point, it is obvious t,hat the sensitizers have had no effect on the speed, but have merely increased the developability of the grains. Sensitivity is measured in ternis of the amount of light that will make the grain developable. A change in sensitivity involves, therefore, a vertical displacement of the curve without changing the value of the inertia (i), while a change in speed involves a horizontal shift in (i). By sensitivity of a grain we mean that the grain is developable if a sufficient exposure is given. Speed represents the degree of sensitiveness. Obviously we cannot state that all the grains of a given emulsion have the same speed, but the average of all speeds is proportional to the reciprocal of the inertia. The terms sensitizers and desensitizers should, therefore, be assigned t o t,hose substances which increase or decrease the developability of the grains. Substances which increase or decrease the speed should be called accelerators and retarders respectively. The R61e 0.f Sensitizers. Assuming that in a given emulsion we have a wide range of speeds of the various grains, it is obvious that the grains possessing the highest speeds would be developable after a very short exposure. If the
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exposure is prolonged, then we enter the period of reversal for the fast grains, while at the same time making some of the slower grains developable. A wide range of speeds would accordingly yield a plate of low maximum percentage developable grains. If something can be added to such an emulsion which will prevent the reversal of those grains which have become developable, without interfering with those which have not become developable, then it should be theoretically possible to make all the grains in an emulsion developable if given a long enough exposure. This apparently is actually what happens when a sensitizer is added. When the sensitizer is not washed off, but allowed to dry in excess on the plate, the perwntage number of developable grains increases to I O O per cent in some cases and remains a t that value for a considerable overexposure. Finally, when the exposure is very long, the sensitizer seems to be exhausted, and the period of reversal is entered. When excess sensitizer is washed off, reversal comes quickly The r6Ze of a sensitzwr thus a p p r a r s to be that of preuenting or delaying rewrwal. If, therefore, sensitivity is merely a case of inhibited reversal, it is very possible that many substances which are at present regarded as insensitive to light may be made sensitive when suitable substances are found to prevent cheir rapid reversal. The insensitivity of pure silver iodide eniulsions is probably d u - to thc existence of a wide range of speeds among the grains, combined with the phcnomena of quick reversal. There might not be any inherent difference i n sensitivity between grains, but we must admit that grains in a given emulsion possess different speeds. The constancy of the percentage number of dr.velopable grains in the normal unsensitized silver iodide emulsion throughout a wide range of exposures may be explained by assuming that by a given exposure those which have higher speeds may be reversed, while others are just made developable. Equilibrium may finally be reached when the number reversed per unit of exposure equals the number made developable and the horizontal portion of the curve results. If a certain critical amount of light energy is required to make a grain developable, and a definite small increment of that amount would make it reverse, the curve of the reversal should exactly repeat the curve of developability, but in a reverse direction. This is most probably the case as indicated by Curves VI and Y I I , Fig. 3 . This simply means that the course of reversal follows exactly the rule that governs the developability of the grains. Since the phenomenon of reversal actually takes part even in a normal exposure of a modern fast plate: it becomes evident that such a phenomenon cannot be neglected in the study of photographic processes. The failure of the reciprocity law seems very likely to be due, among other factors, to the intervention of reversal. SoZarizaCion or Reversal. Numerous theories have been offered to explain the phenomenon of reversal; the question still remains unsettled. If the action of light is to affect the grains in such a way as to initiate development, it is 8
Svedberg: Phot. J., 64,272 (1924).
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rather difficult to see how excessive exposure could destroy the effect already produced. The solarization cannot be the reversed reaction of that producing the latent image, because reversible reactions would come to an equilibrium as their ultimate stage. It is thermodynamically impossible that a reaction goes to completion in one direction and then completely reverses to its origin:il state under the same external conditions. The view that the chemical composition of the solarized image is different from that of the normal latent image and that the silver halide grains having received a solarizing exposure are chemically different from the normal unexposed grains, is a very debatablc assumption. Arguments of this nature will ultimately lead to the ever-present controversy over the chemical or physical nature of the latent image. In fact, we have proven experimentally that solarized grains are practically identical to the normal unexposed grains in the case of silver iodide emulsions so far as their behavior towards sensitizers is concerned. When a silver iodide one-grain layer plate which has been exposed to complete solarization is sensitized with a I per cent pyrogallol solution, the exposure-developability curve falls exactly on that of the similarly sensitized normal plate. This is shown by Plate ST-I of Fig. 3 which was a one-layer silver iodide emulsion exposed twenty minutes to a 150-watt incandescent lamp a t a distance of 0.j meter, then sensitized with a I per cent pyrogallol solution for ten minutes, and dried without washing. Photo-Rei,oyressz'o~~.Another interesting characteristic of the silver iodide plate is its rapid photo-retrogression. On one occasion, a silver iodide plate was exposed and left undeveloped for twelve hours. On developing, the maximum percentage developable grains was reduced to half of what it would have been had the plate been developed immediately after exposure. The nature of photo-retrogression, though very little understood, is probably due to the same causes as reversal. I'nit.ersity of Colorado, Boulder, Colorado.
