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.
White Spots. White spots are generally caused by air-bells gathering on the print when first placed in the water and allowed to remain while the developer is poured over the print. Wherever an air-bell appears it prevents the developer from acting on the sensitive emulsion of the paper and consequently it leaves a white spot. Particles of dirt in the developer or water settling on the print will cause white spots. Drops or splashes of water striking the print before being immersed in the water and before developing will cause white spots.
Small Black Or Brown Spots. Generally caused by some foreign metallic matter, such as iron rust, in the water or developer.
Print Develops Weak In Shadows With Gray High-Lights. This is an almost certain sign that you did not use enough Bromide in your developer.
Prints Gray In The High-Lights, Having Mottled Shadows. Caused by over-exposing and then not carrying the print far enough in the developing. If you did carry the print far enough the print would be too black.
Prints Weak And Too Light. A certain sign that you under-exposed or possibly under-developed.
Prints Very Light And Yellow. This is caused by underexposure and prolonged development.
Prints With Large Yellow Stains Of Different Sizes. This is almost a certain sign that the print was not thoroughly immersed in the Fixing Bath. If any air-bells are allowed to gather on the print when in the Hypo they prevent the Hypo from attacking the film and this will result in yellow spots.
Dark Prints. Caused by over-printing.
Blisters. Caused by either too warm a developer, too strong Hypo, or uneven temperatures of baths and wash waters. Give your prints a strong salt bath immediately after fixing. This will generally overcome this difficulty.
Prints Developing Gray Even After Sufficient Bromide Has Been Added. This is a sure sign that your paper has become fogged either by your dark-room light not being perfectly safe or because the package was opened in daylight.