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.
Fading Of Prints. It was previously stated that citric acid is present in the emulsion of printing-out papers; this will decompose hypo and liberate sulphur. If alum be present as well, we will also get sulphureted hydrogen. Most combined toning and fixing baths contain alum and hypo; therefore, if a print be placed in such a solution these two undesirable products are sure to cause a reaction with the silver in the image and form silver sulphide. The emulsion also soaks up a certain quantity of the solution, and some of the sulphur contained in the solution is likely to be precipitated with the metallic salts. This will itself eventually react with the image and cause fading deterioration. The combined baths cannot therefore be recommended when permanency is desired, and it is by all means recommended that separate toning and fixing baths be employed.
Softening Of The Film. The length of time required for the print to be in the various waters and baths tends to soften the emulsion, and it is advisable in many instances, especially where the temperature is quite warm, to harden the film in order to render it less liable to abrasion; and if it is desired to glaze the prints by placing them on glass or ferrotype plates, it is advisable to harden the emulsion to insure the prints peeling off without difficulty. A 5% solution of alum is usually employed, which when used requires that a thorough washing must be given between hardening and fixing; because if any of the alum is carried into the hypo bath the sulphur in the latter solution will be liberated in the manner already stated.
17. A better hardener would be a weak solution of formalin, as no ill effects will come from this chemical. Formalin should never be used until after prints are toned, as it is so penetrating that if used before toning, it would be difficult to tone the prints evenly.
Finishing The Print. The fixing bath being prepared with hypo, we have again a substance containing sulphur to deal with. After fixation is complete, which takes at least 15 minutes, the final washing of the prints has to be given to secure the effectual removal of the last traces of hypo.
19. The prints should be washed in at least 10 changes of water, each washing to be of 5 minutes duration. Although the great bulk of hypo is removed from the emulsion in the first three changes, yet, in order to entirely eliminate all of the hypo thorough washing is very necessary.
20. To secure the best results when enameling printing-out papers, the prints should have a bath of 5% formalin (Schering's Photo-Formalin 1 oz.; Water 19 ozs.) either before, or after, fixing and washing. As a hardening agent formalin is far preferable to an alum solution, which latter should never be employed for gelatin papers unless formalin cannot be obtained. The alum solution is quite likely to cause fading, no matter at what stage of the proceedings it is employed. If it is impossible to secure, or if you do not care to use formalin, or the alum solution, it is advisable to dry the prints without placing them on the glass or ferrotype plate. After they have dried, the gelatin becomes much harder, so by re-wetting them there is less chance of their sticking to the glass or ferrotype plate.
21. Finally, the medium employed for mounting the print must be neutral, not acid. A mountant, or paste, which will turn blue litmus paper red should never be used. The acidity acts upon the gelatin, possibly on the mount and the paper base of the print, and a yellowing of the image, or partial bleaching may result. Gold toning and fixing baths should all be neutral, or used slightly alkaline - never acid.