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.
Life Of Hypo Bath. The question which arises in the mind of both amateur and professional is, "How long, and for how many prints, can the hypo bath be safely used?" In paragraph No. 715, Printing and Developing, was given definite directions in regard to the number of prints a given quantity of hypo bath would properly fix. The following advice, combined with the photographer's past experience will be a splendid guide to judging when the fixing bath is practically exhausted and, therefore, unsafe for further use. If an acid hypo bath is being used it will, when exhausted, be found to turn milky. There will be a white sediment which does not readily precipitate, but continually floats. The solution will appear soapy, forming suds and bubbles, not unlike soap suds. When the hypo bath acts thus, it should be discarded and a new bath made up at once. Hypo baths should be prepared fresh at least once a week, even if only slightly used.
811. Scum on Surface of Prints - Scum is caused by sulphurization. If the hypo bath is too strongly acid, the acid will release the sulphur from the hypo, causing the extremely milky appearance. This can generally be charged to the acetic acid and alum, the latter being strongly acid. Exact proportions of alum, hypo and acid are essential to avoid this sulphurization, as the relative strengths of these iv - 15 chemicals vary from time to time. It is almost impossible to judge when the proportions are right, and an excess of either will cause the trouble. With the ordinary means at hand it is impossible to decide which is in excess; so if you are troubled to any extent, it is wise to pour a portion of the fixing bath into a small tray and add a trifle more hypo. Try a print in it and if the results are not better, add twice as much alum as you did hypo, and try again. The effect of sulphurization will then very likely be overcome.
812. Scum is also caused by allowing prints to lie for some time without being separated. These developing papers, having a gelatin surface, are apt when wet to sink to the bottom of the tray and mat (stick together). It is, therefore, necessary to handle them over and over during fixing. If careful to use the acid clearing bath (formula given in paragraphs 738-740) there will be less danger of forming this scum.
Removing Yellow Stains From Prints. At times when vignetting or printing - producing a white border on the print - yellow stains are apt to appear. These can be readily removed with the following bleaching solution: In a 10-oz., wide mouthed bottle place 1 ounce of red prus-siate of potash (ferricyanide of potassium). To this add 6 ounces of water. Shake well. The red prussiate will dissolve slowly. Wrap this bottle with black opaque paper, label the bottle "Red Prussiate of Potash Solution," and keep in the dark room away from strong light.
814. To remove the stains from the print, take one pint of a clear solution of hyposulphite of soda, 10° hydrometer test, add two - not more than three - drops of "Red Prussiate Solution." Too much prussiate would quicken the action and if it acts too rapidly it is apt to bleach the entire print, possibly staining it; so be sure not to have the bath too strong. With a tuft of cotton swab the parts of the print that have stained yellow. If the print is mounted swab both the print and the mount with the solution until the yellow disappears. Then rinse off with clear water. If the mount was not swabbed, the solution would leave streaks. It is well to use absorbent cotton or a very soft sponge to remove the water, as the chemicals will thus be more quickly and thoroughly eliminated.
815. After the prints are washed clean, lay them out to dry. The bath must be used only while fresh, not after it becomes discolored. Usually a bath of this kind is good for only half an hour. Should it be found necessary to employ the bath for a longer period, make a fresh solution.