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.
Yellow Negatives. Yellow negatives are generally caused from long development due to under or over-exposure. The developing of an under-exposed plate is always slow owing to the fact that a weak developer has been used, the emulsion on the plate is apt to become soft and this gives the pyro an opportunity to stain. The developing solution by the long development becomes discolored, therefore, acts as a stain upon the film. This yellow stain, however, can be eliminated by immersing the plate or film in an alum solution. ( See paragraph 270, Chapter X (Varying Water Conditions. Their Effects Upon The Manipulation Of Sensitized Papers), Negative Reducing, Part I.)
Using Old Developer. When using old developer be sure and decant and filter the solution free of dirt or particles of film before use. The normal developer used for one developing should be your old developer for the next developing.
235. Determining When to Check Development of a Plate Started in Normal Developer - A plate should be transferred to the restraining bath - or in other words, the tray containing old developer - just as soon as you see the slightest signs of the shadows growing weak, fogging or veiling over. As stated before, the first few moments a plate is in the developer, it should be watched more carefully than at any other time. If you are developing several plates at a time, and one or more of them show signs of fogging in the shadows, transfer them at once to the old developer. Watch your plates closely and act quickly. Do not hesitate to transfer the plate to the tray of old developer if it shows the least sign of fog or flatness, for even should you be mistaken you cannot injure the plate by so doing.
Clearing Stained Negatives. To remove the stain from negatives. (See Chapter X (Varying Water Conditions. Their Effects Upon The Manipulation Of Sensitized Papers), Negative Reducing.)
Uneven Development. Uneven development - or plates with streaks in them - is generally caused by insufficient developing solution. It is also caused by not rocking the tray constantly during the development, or by allowing the plate to remain in the bromide solution, or in any restraining solution without agitating. A plate should never remain in any solution, no matter what it is, without being agitated and the solution kept constantly in motion. Too harsh rocking will give harsh, grainy effects. Rock gently sufficient to keep any sediment from settling on the plate.
Mottled Negatives. This you can overcome by carefully rocking the tray during development. Sometimes this mottled appearance is visible on plates that have been reduced. This is caused by too strong a reducing solution, and not rocking the tray while the plate is reducing. Avoid too strong solutions of any kind.