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.
Persulphate Ammonia Reducer. The second reducer which is very successfully used is the persulphate of ammonia. This reducing agent will first attack the denser portions of the negative, as they are more susceptible to the action of the reducer. The thinner portions of the negative (the shadows), are only slightly affected, therefore, for a plate which is under - exposed; or a contrasty lighting; or an over-developed, under - timed plate; we recommend the use of persulphate of ammonia. It will act upon these strong highlights first, and when the highlights are reduced to their proper stage there will be proper gradation from the highlight to the shadows.
Action Of Reducers. The persulphate of ammonia not only reduces the size of the silver granules, but also alters the character of the deposit. The granules become opalescent, reflecting a milky or pearly light, resulting in the required soft tones in the negative. With the red prussiate (ferricyanide), however, the silver granules after becoming reduced remain opaque. The action of the red prussiate is also much quicker than that of the persulphate.
Use Discarded Plates, Or Films, For Experimenting. As the successful performance of this work depends on the proper mixing of the chemicals in the required proportions, we advise for the first experiments the use of old discarded negatives. When able to produce the desired results on these discarded plates, make use of these same rules and formulas in your regular work, and apply one of the above reducers whenever you feel that you can improve the negative, or negatives. The expert has these reducing chemicals always ready at hand in the dark room, and when his practiced eye sees the occasion for their use, on account of under or over-timed exposures, he immediately applies them and, by correcting their faults, obtains good negatives.
The Best Time To Reduce Plates Or Film. The best time to doctor negatives that are to be reduced with red prussiate, is immediately after fixing, while the negative is wet and most receptive. In case you are to reduce plates that have already been dried, or for the benefit of practice you are to reduce old or discarded plates, it will be necessary for you to place the plate first in the regular hypo bath for at least thirty minutes, so as to give the gelatine an opportunity to become thoroughly saturated with the hypo. If the plate is not thoroughly saturated with the hypo before the red prussiate touches it, it will produce stain, and the reducing will also be slow and uneven. When reducing with persulphate of ammonia, however, it is different. The reducing with this chemical is best done when the plate is dry. It will act more quickly, and the reducer will attack the highlights more readily.
Red Prussiate of Potash (Ferricyanide)
You will find a little difficulty in dissolving all the crystals, but by constant shaking most of them will dissolve.
This Stock Solution must be kept in a well-stoppered bottle. A glass stoppered bottle is recommended. Wrap the bottle with black or yellow paper, and keep in a dark place, as strong daylight will affect this solution, and cause it to rapidly deteriorate. It is necessary that you observe the caution in regard to bottles for preserving your solution, and the manner in which it should be preserved. By doing this you will always be able to produce the best of results.
266. For use prepare as follows: To one ounce saturated solution hypo, add one - half-pint of water (eight ounces). Add to this about one dram of Stock Solution No. 1. To make a saturated solution of hypo, dissolve in a certain amount of water all of this chemical that the water can hold. When the chemical is dissolved in the water, the first portion added may disappear quickly enough, but as more is added, the dissolving proceeds more and more slowly until finally it ceases altogether, and some of the chemical failing to dissolve will sink to the bottom and remain there. The solution is then what is called saturated.
267. If upon applying the reducer to a plate you find that it does not reduce rapidly enough, add a little more of No. 1, and a proportionate amount of hypo, remembering that too little hypo will cause yellow negatives. Always bear in mind that the hypo is your color controlling chemical, and prevents the red prussiate from staining.