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.
278. If a 5 x 7 plate is to be reduced, five ounces solution will be sufficient. If a number of plates are to be reduced, it is advisable to make up ten ounces of solution. This bath should be discarded as soon as you are through reducing, and if more plates are to be reduced later, a fresh bath should be prepared.
279. We will now suppose that you have placed your reducing solution in your tray. Take your negative without previously wetting, and place it in the solution. As we have said before, reducing with persulphate of ammonia is best accomplished when the plate is dry. The reducer will act with greater rapidity, and will attack the highlights more freely. Rock the tray gently. Examine the plate frequently, and when it is reduced to where you want it, you can stop the action of the reducer by first rinsing the plate in clear water, and then immersing it for about five minutes in a sulphite of soda solution, hydrometer test ten degrees. Or, dissolve one ounce of sulphite of soda in ten ounces of water. After soaking in this bath for a few minutes, wash in plain running water, and then place it in the rack to dry.
Reducing Plates Hardened In Alum. The Persulphate Reducer will not act well upon plates that have been soaked, and hardened in alum. If you have plates which have been so treated, place them in plain water for ten minutes before applying the reducer. The soaking of the plates will soften the film, and open the pores, which have been closed by the action of the alum, the reducer will then act more freely.
Traces Of Hypo In Persulphate Bath. When using this reducer the negative must be freed from every trace of hypo. You are, therefore, cautioned upon the importance of using separate trays for different chemicals. If you have but one tray to use for reducing, you must see that it is thoroughly cleansed before using. For instance, should you have used the tray for reducing with red prussiate and hypo, and only rinsed out the tray with plain water, sufficient hypo would still remain in the tray to contaminate the persulphate solution. In fact, the least trace of hypo would ruin a persulphate bath. Therefore, to insure good results, cleanse your tray thoroughly, and the best solution to use for cleansing the tray is one dram of sulphuric acid added to two ounces of water. Wash the tray thoroughly with this solution, and rinse with plain water. You may be sure that the hypo and other chemicals are then removed.
282. You are also cautioned when examining the plate which is being reduced, to rinse it off under the tap with plain water, before holding it up to the light, for if any of the solution remains upon the plate it is apt to run in streaks, and the reducing will continue, and thereby ruin the plate.
Kind Of Negatives To Reduce With Persulphate. Dense negatives resulting from over-exposure should not be treated in the persulphate ammonia bath, for in such a case you must reduce and clear the shadows as well as the highlights. The red prussiate of potash reducer is the best for such negatives. Dense negatives resulting from overdevelopment of proper exposures, and from under-timed plates, that have been purposely over-developed in order to supply all the strength possible in the shadows, should be reduced in the persulphate ammonia bath. While the highlights alone are to be reduced, yet the entire plate is immersed in this bath, and the chemical action will be upon the highlights only. To apply a chemical that would act upon the shadows as well, would be ruinous ; therefore, the persulphate should be used for reducing such plates.
284. In order to illustrate more clearly the advantage of the different reducing agents we present illustration No. 9 with only one-half of the plate treated with red prussiate reducer. If you will note this illustration No. 9, which was made from a negative that was over-exposed and over-developed, and then one-half of the plate reduced with red prussiate of potash, you will note the half which was not reduced shows but little of the image, and the part that was reduced gives a good strong print with clear shadows, soft highlights, and good detail in both shadows and highlights. The reduced portion was completely printed in about fifteen minutes, while it would require hours to completely print the unreduced half and even then it would not produce as good a print as if reduced. In illustration No. 10, you will find the results produced by reducing with persulphate of ammonia. Here the plate had no more strength than was required in the shadows, but the highlights were too dense. The persulphate has acted upon these highlights alone, with the results as shown in illustration No. II
Illustration No. 9 Print from Over - Exposed and Over - Developed Negative, One-Half of which is Reduced See Paragraph No. 284.
Illustrations Nos. 10 and II Before and After Reducing See Paragraph No. 284.