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.
Judging Proper Strength. It is impossible to give any method by which one could judge absolutely the proper strength of development under all conditions. Only practice and close observation can teach you this. The difference in exposure, the lighting, etc., all have some bearing on the strength to which one should carry the development. One rule can be followed, however, and that is when the distinction between the highlights and shadows is as it appeared on the ground-glass, making due allowance for the fixing, then the plate is fully developed. There are many ways of judging density. A very convenient one is to hold the plate before the ruby light and place one finger close to the film and near the strongest highlight, and when you find the highest light is as dense as the finger, you can consider the plate of the proper strength. It is a good practice in developing, when in your judgment the plate is developed far enough, to try and impress upon your memory the density of the plate developed, and, after fixing, examine the plate in daylight by looking through it. If it is too dense or too thin, you must govern yourself accordingly in developing the next plate. With this practice you will soon be able to judge the proper strength of development under all conditions.
Judging When Plate Is Fixed. A plate is generally considered fixed when all the white or creamy effect has disappeared from the back of the plate, but even then, it is safe to allow your plate to remain in the fixing bath ten minutes longer. A plate may appear fixed and yet not be thoroughly fixed. If your fixing bath is cool and not too old, you could allow your plate to remain in it for hours and the hypo would do no harm. There is practically no danger of over-fixing; however, if the hypo bath is old and warm it would be apt to reduce the plate and soften up the emulsion so badly that the image on the plate would.
be destroyed, or the emulsion would slide off entirely; therefore, it is advisable to renew your hypo bath often.
Discolored Hypo Bath. This need not alarm you, as the hypo bath is good as long as it will fix the plate in a reasonable length of time, say twenty minutes, and not stain. Always rinse your plate (both sides) before placing in the hypo; otherwise you will be carrying the developer into the hypo, and this, with the silver in the plate, will cause the discoloration of both plate and hypo.
Mottled Negatives. Generally caused by allowing the plate to remain in the developer without agitating. This is more frequently caused in extremely slow development; as for instance, when you are using old developer. Remedy: Obvious.
Finger Marks. Caused by carelessness in handling plate be-fore developing. Remedy: Never allow the fingers to come in contact with the film side of the plate.