This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
257. There are times when, because of faulty exposure, one finds it necessary to over-develop the plate, thereby producing, in case of over-exposure, a negative which is too dense. While by the over-developing the required values of the highlights and shadows have been retained, yet the plate will be so dense that the printing from it would be very slow and unsatisfactory. In the case of under-exposure, the shadows having been insufficiently timed to supply detail, the development is prolonged in order to obtain all the detail possible in the shadows; consequently the highlights are over-developed, and are too dense. The result is too much contrast, the negative having only highlights and shadows, but no half-tones, and no gradation from the highlights to the shadows. Necessarily, negatives of either the above classes are slow printers.
258. In order to improve the printing quality of such plates, it is necessary to reduce them. Many negatives that would be considered worthless, and are often discarded, may be remodeled and doctored up to produce excellent prints. In fact, in many cases the very finest prints are produced from negatives that have been doctored; not only reduced, but intensified, in order to produce the proper gradation from the highlights to the shadows.
Reducing Over-Developed Plates. As all negatives that have been over-developed should be reduced in order to produce the best printing quality, we will, therefore, consider the action of the reducing agent, of which there are a number of different kinds, although the action of all are practically the same. If a plate was placed under a microscope of high power so that the action of the reducer on the silver granules could be observed, you would find that the reducer was dissolving, or removing these small silver granules of which the image on the plate is composed. The longer the solution remained upon the plate the more it would reduce, and if the reducing agents were allowed to remain on the negative long enough the entire image would disappear. As this reduction, however, can be checked at any time by immersing or washing the plate in water, one has but to judge when the plate is reduced sufficiently.
Red Prussiate Of Potash Reducer. There are quite a number of formula for reducing. There are also prepared preparations on the market. We will, however, consider the two which are most generally used, and which we recommend. The first is ferricyanide (red prussiate of potash ). The reducer acts evenly on the entire plate, and therefore the thinner portions (the shadows), are liable to disappear before there is much change on the dense parts (the highlights). For this reason we recommend red prussiate reducer for over-exposed and over-developed plates, which are not only slow printers, but lack in contrast.