REDUCTION 457. Choice of a Reducer. The usual reducers act by gradually dissolving the silver which forms the photographic image, but widely different results are obtained according to the substance (or mixture of substances) which is used for this purpose.
Uniformity of action of a reducer is obviously only possible if it can penetrate to all parts of the image at the same speed; in the case of a dry negative, however, and particularly if fixation has been done in a bath containing alum, the permeability of the gelatine is often very different in the various parts of the image, and thus there is a great risk that the process of reduction will produce markings which cannot be If, therefore, reduction has not been carried out before drying the negative, the latter should be soaked in water until the gelatine has swollen uniformly, which will some times require three or four hours.
The characteristics of a reducer depend largely on the dimensions of the grains forming the image, and on the distribution of these grains in the depth of the layer, hence there are considerable variations in effect according to the type of emulsion, and even with nega tives of a given emulsion developed under very different The various reducers may be schematically classified into three groups— r. Surface reducers.
2. Proportional reducers.
3. Superproportional reducers.
Superficial reducers arc very active solvents of silver. They attack the silver almost as fast as they penetrate the film of gelatine, so that the different densities in the image are decreased by approximately the same amount (Fig. 176, I), and if the action is allowed to go on too long some parts of the image may disappear alto gether. These reducers are specially suited to the clearing of fogged negatives. They are generally chosen for local reducers in retouching so as not to tire the patience of the operator.
A proportional reducer decreases every density in the image in the same proportion. This may be brought about by converting the silver of the image into a less absorbing substance or mixture of substances (Fig. 176, II), e.g. by blue
toning (L. P. Clerc, 1899) or by iodizing the image. The removal of the same fraction of the quantity of silver at every point of the image is never realized except, approximately, by the use of solvents whose action is so slow as to be negligible during the time taken to penetrate the film of gelatine. These re ducers are most suitable for diminishing the contrasts of negatives which have been over developed.
A superproportional reducer removes a greater proportion of the silver from the dense parts of a negative than from the lighter parts, as if its action started from the support of the film and moved outwards towards the surface of the gelatine (Fig. 176, III). The only reducer known to belong to this group is ammonium persulphate ; it dissolves silver slowly, and its activity is increased by the silver sulphate produced in the course of the reaction (auto catalytic reaction).
Lastly, in certain cases, it is required to reduce only the dense parts of the negative. This may be done indirectly by allowing the solvent of silver to act only after all the silver contained in the surface layers of the film has been con verted into an insoluble compound.
In using any of the reducers of the first three groups, the negative should always be withdrawn from the bath a short time before the desired effect has been produced, because the solution of the silver continues during the first moments of washing.
458. Surface Reducers. Whichever of the reducers of this class may be used, anything which tends to in crease the rate of solution of the silver (increase in concentration of the active substance) or to diminish the velocity of penetration into the gelatine (excessive swelling of the film, thickening of the liquid, etc.) exaggerates the peculiar character of this class of reducer. The con verse tends to make these reducers behave in a proportional manner.