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.
Restraining The Sensitive Emulsion. To illustrate: The sensitive emulsion on the plate is composed of layer upon layer of these minute sensitive silver particles. If you exposed the plate and gave two seconds' exposure and the correct exposure required but one second, it becomes necessary to cut off or make a number of these layers of minute silver particles unchangeable. Or, in other words, restrain them from developing so that when the developer comes in contact with them it will have no effect. This is generally done either by immersing the plate before developing, in a bromide of potassium solution, or, developing in old developer (developer that has previously been used), and which, of course, contains bromide; the bromide having been liberated from plates previously developed. For description of the action of bromide, see paragraph 35 - Dry Plate Developing.
Why Plates Are Over-Timed. The best photographers are apt to misjudge the strength of the light employed, sometimes due to the variance of the light at different times of day, or conditions of weather (cloudy or dark days), but more especially when they are in a hurry. While no two photographers will work alike, although they are aiming for the same effects, each has his one peculiar way of controlling and measuring the light employed in producing the desired result. Yet, there is one point upon which all agree, that is, that an over-timed plate, or film, is preferred to an under-timed one. The reason for this is that sufficient exposure is always required to supply detail to the shadows, and in an over-exposure we are always sure of the necessary detail even in the deepest shadows, while in an under-exposure this detail is generally lost. It is much easier to rectify the exposure so as to retain the detail in an over-exposed plate than it is to obtain detail in a plate, or film, which has been under-exposed. The reason is that with an under - timed plate the exposure has been insufficient to supply the necessary detail, and even with the most careful handling in the developer sufficient detail cannot be produced, especially if the plate is very much under-timed. In some instances, however, if not badly under-exposed, fair results can be obtained in an under-timed plate, but the results are uncertain. With an over-exposed plate one can always regulate the developer so as to produce negatives with good printing qualities. It is for this reason that many plates are over-timed.
Use Of Old Or Once Used Developer. In order that you may at all times be supplied with a restraining bath that may be applied to plates that are over-exposed, it is advisable to save the last normal developer used. It is a good practice also, after each developing in normal developer, to pour the solution used into a wide-mouthed bottle. The reason for using a wide-mouthed bottle is that it is much easier to decant the clear liquid from such a bottle. It is also much easier to pour the used developer into a large-mouthed bottle without the use of a funnel. In order to keep the developer free from dust, cover this bottle with a piece of glass, or cork it. This old developer having become charged with bromide liberated from the plates previously developed, will make a good restraining bath. This used developer will become somewhat discolored, but the very fact of its being discolored makes it all the better. This discoloration will prevent the light from the ruby lamp - even though slightly actinic - from affecting the plate while in the developer.