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.
Discoloring Of Hypo Bath. A strong fixing bath should always be used for fixing plates. To more fully understand the reasons for this, let us study the peculiar properties of this salt (hypo-sulphite of soda). Suppose we were to dissolve sixteen grains of nitrate of silver in one-half ounce of water and twenty-four grains of hypo-sulphite of soda in one-half ounce of water, and then add one solution to the other. A dense deposit of hypo-sulphite of silver would immediately be formed and rapid changes would take place in this deposit; first, white and curdy, next canary color, then a rich orange-yellow, afterwards a liver color, and finally almost black. This change is due to the sulphuric acid formed by the mixing of the silver and hypo-sulphite of soda. The black deposit is sulphide of silver, the yellow and orange being in the early stages of decomposition. This will explain why a plate fixed in a weak hypo bath is sometimes covered with a brown deposit of sulphide of silver and the hypo bath discolors so rapidly, whereas if a full strength hypo bath has been used the plate will be free of the deposit and the bath will remain clear much longer.
29. When you place your developed plate in the fixing bath you are carrying a certain amount of silver into the hypo.
OLD CEDARS Study No. I J. S. Neary, Trenton, N. J.
If, therefore, your hypo bath is weak the silver in the plate overpowers the hypo-sulphite of soda and sulphide of silver is formed and the same chemical action takes place as when you mix the silver with hypo-sulphite of soda. A strong hypo bath is, therefore, recommended. The hypo is also more easily eliminated when a strong bath is used.
30. Developing". - A plate you believe to be properly exposed should be started in a normal developer. A correctly exposed plate can be completely developed with the solution of normal developer given in this instruction without any alteration whatsoever. It is advisable to always save the last developer used, pouring it into a large mouthed bottle, placing a cover over the bottle to protect it from dust and also from the air. The developer will become discolored, but this need not alarm you, as it will make no difference for the purpose it is intended, - that is, the developing of over-exposed plates. However, it should only be kept from day to day. The developer which you use once as your fresh normal developer, should be used the next time as old developer.
31. It is well to use two trays for developing. Into one tray decant the clear solution of old developer. By clear solution we mean that which is free of particles of film or dirt, but not clear in color. Add to this one-third fresh normal developer. The other tray use for fresh normal developer.
Restraining; Development. If a plate flashes up quickly, indicating over-exposure, place it in the tray of old developer at once, rocking the tray quickly so that the bromide in the old developer, which was liberated from the emulsion of plates previously developed, will penetrate the plate at once, and check development. The bromide acts as a re-strainer, keeps the shadows clear, and allows the highlights to build up. The plate will develop up much slower in this bath, as the bromide acting on the shadows permits the highlights to build up first. Unless plates are very much overtimed, we advise using no other means of restraining than those described above.
33. If you have no old developer on hand, and upon placing the plate to be developed in normal developer it shows indication of over-exposure, then remove the plate immediately from this bath and if you have a tap of water, run the fresh water over the plate quickly and at once drop four to six drops of a ten per cent, solution of bromide of potassium into the normal developer. Then return the plate to this tray, and conclude the development.