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.
Bromide Solution. To make a ten per cent, solution of bromide, dissolve one ounce of bromide of potassium in ten ounces of water. Place this stock solution in a twelve-ounce bottle with cork stopper, and split the cork to the center. If it fits too tightly you may cut a little notch in the side and again insert the cork tightly, and you will find when the bottle is turned bottom side up, the solution will drop from the cork very nicely and will be a very convenient way for measuring the required number of drops quickly. The bromide of potassium when used in the developer prevents oxydization of silver in those parts of the sensitive plate on which the light has not acted. For example, the shadows. It also retards the oxydization on the parts on which the light has acted. For instance, the highlights, white draperies, etc., etc. If one is quite sure that a plate is over-exposed, a drop (or no more than two drops) of bromide can be added to the normal developer before placing the plate into it to be developed. This will slightly restrain the shadows and a more brilliant negative will be the result. A plate placed in a fresh developer, which from the start contains two drops of bromide, will restrain the plate more than six drops of bromide if added to the developer after the plate has been once started in normal developer.
Action Of Old Developer. When a gelatine-bromide plate (dry plate) is exposed to the action of light, the sensitive film undergoes a change, the elements of which it is composed (silver and bromide) lose their affinity for each other and a state of incipient decomposition is set up. If the exposed plate is then subjected to the action of a developer, for instance, pyro, and all developers have a reducing power over modified silver bromide, the action of the light is continued and intensified by further decomposition of the molecules of the film impressed by light. This continuing of action constitutes development, and by it the image impressed on the film is made visible, a dark deposit of silver resulting from the application of the developing agent in those parts of the silver affected by light in proportion to the intensity of the light action. When the developer is too strong, we have a general reduction of silver over the entire sensitive surface, resulting in what is commonly called chemical fog. The bromide in the sensitive dry plate is really bromo - iodide of silver; originally it was bromide of potassium and iodide of potassium, but when they became a part of the nitrate of silver, they, by decomposition, became bromo - iodide of silver.
36. The principal difficulty, therefore, to overcome in developing an over-exposed plate is chemical fog. While bromide added to the normal developer will prevent this to a certain extent, yet the development is prolonged considerably and the action of even the ruby light upon the plate has a tendency to fog, while if old developer was used (one which has become thoroughly ripened and which contains bromide liberated from previous plates developed) the color of this developer over the plate protects it from the continuous action of the light during prolonged development and with less likelihood of fog. Therefore, the use of old developer is preferable to normal developer with fresh bromide added.