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 With Persulphate Of Ammonia. The persulphate of ammonia does not keep well when made in solution, and, therefore, it should be made up in small quantities, mixed for immediate use. This chemical, which is a most valuable addition to the stock of photographic materials, is less known and made use of less at the present time than its virtue would warrant. This is probably due, in part, to the want of knowledge of its properties, and the conflicting results that have been obtained with the first experiments. This, too, explains the reason for the various recommendations as to the strength the persulphate should be used at, which has varied in many instances from on2 to ten per cent. With a uniform preparation, however, the variations of results will be little or none, even when taking into consideration the different requirements of the amateur and professional. The amateur with fewer negatives to attend to, thinks nothing of spending a half-hour on a negative that he values, but on the other hand, the professional, whose time is money, would scarcely like to spend more than five or ten minutes on any plate. If longer time were required he might be tempted to leave the negative for a few minutes to attend to some other work, and probably forget it, and the nega-11-7 tive in consequence would be spoiled by reducing too far. As there is nothing gained by the use of a very weak reducing agent, we recommend a method that will, on an average, require ten minutes to complete the necessary doctoring.
Persulphate Reducer Grows Stronger With Age. The persulphate of ammonia from the moment that it is made into solution decomposes, and gradually becomes more and more acid. A good commercial sample of persulphate has a slightly acid action to start with, and this acid action rapidly increases when the persulphate is made into solution. A freshly made ten-per-cent. solution from C. P. persulphate has but a slight acid action, and can safely be used. This same solution, however, at the end of a week has a very strong acid action. A one-per-cent. solution at the lapse of a week acts more quickly than a fresh ten-per-cent. solution. Hence, our reason for using a fresh ten-per-cent. solution, and for making up only sufficient quantity for the plate, or number of plates, to be reduced. If the solution is made up of ordinary tap water, which usually contains chloride of carbonate, the action of the chemicals is quickly seen by the gathering of a milkiness on the surface of a negative. This is not the case if distilled water were used. It is, therefore, advisable to use ordinary tap water, as it acts as a guide. If old solution is used the reduced negative will assume a sickly sepia tint, and while the strong parts apparently are reduced, yet the color of the plate is such that the negative has not been improved for printing quality by the reduction,
276. From the above one will readily see that the persulphate is of uncertain action unless used fresh, and it is for this reason that we wish to impress on the worker's mind the necessity of using a fresh solution for each lot of plates to be reduced.
Persulphate of Ammonia Crystals..........
If a larger bulk of solution is required, add ten grains persulphate of ammonia for every ounce of water.