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.
Reducing Plates After They Have Dried.
When the plate is dry, and you wish to locally reduce it, it is necessary to first place the plate in plain cool water for ten minutes. This softens the film, and opens the pores. Then transfer to the regular hypo bath, and allow to remain for ten minutes more, so that the pores of the film being open allow the hypo to penetrate quickly, thereby avoiding stain and spots, as would be the case had all the film not been evenly saturated with hypo.
302. After the plate is sufficiently soaked in hypo, it is advisable to immerse the entire plate in a very weak solution of reducer for only a moment ( just enough to slightly stain the film). Then rinse with plain water, and apply the solution locally, as you require. Finally, when you have doctored the plate to your entire satisfaction, again immerse the entire plate in the reducing solution for a moment, after which rinse off quickly. Then place it again in the hypo bath for ten minutes, and finally wash for fifteen minutes in running water. If the negative requires much doctoring, the reducer may slightly discolor the reduced portions. If this should occur after the plate is thoroughly washed immerse it in the clearing bath. ( See paragraph 270 of Part I.)
Reducing With Persulphate. While most local reducing can be accomplished with red prussiate bath, yet, some workers prefer the persulphate for certain class of plates. In treating with the persulphate reducer, Part I, of this Instruction, you were told that the persulphate has a tendency to reduce the highlights without effecting the shadows ; therefore, this reducer should be used only on plates where the shadows are already thin enough. Prepare your persulphate of ammonia reducer according to instructions in paragraph 277, Part I. Remember that it is necessary when using this reducer that the negative be absolutely free from every trace of hypo. The application of the solution is exactly the same as that of the red prussiate. After you have reduced portions of the plate as much as you desire, rinse the entire plate in water. Then place it in sulphite of soda bath ( see paragraph 279, Part I), after which wash in plain running water, and place in rack to dry.
Applying The Reducer To Films. The application of the local reducing solution applies to films as well as plates. Both are reduced exactly alike. When applying the red prussiate locally to film, in order to hold the film perfectly flat, lay it on a piece of plain glass which has previously been wet in cold water. The surplus water on the glass and film will hold the film perfectly flat, while you are applying the reducer. If the film is of the curling kind, it will not lay flat by the above method. With ordinary tacks fasten the four corners to either a card board, or a small pine board, which has previously been thoroughly soaked in clear water, and while wet tack on the film.
305. If the persulphate reducer is used, it should be applied to the film while dry. In event of the film curling, the board to which it is attached must be dry and not wet. As any results produced by persulphate can be obtained with the red prussiate and many more effects that cannot be produced by persulphate can be obtained with the red prussiate, which is easily prepared, and much simpler to manipulate than the persulphate, it is advisable to use red prussiate for all local reducing.
306. While the best time to reduce or doctor a negative with red prussiate is while it is wet, or as soon as possible after the plate is developed and fixed, yet it is advisable, for experimental purposes, to use old or discarded plates or films. These being dry must be treated according to instructions before reducing. After one has become experienced in the manipulating of the reducer locally, it should be applied on all negatives requiring local reduction immediately after the plate is developed, and thoroughly fixed.