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.
Shadows Too Black Before Sufficient Detail Has Been Produced In The Highlights. This difficulty generally occurs when a negative is contrasty. In a landscape picture there may be extremely heavy foliage in the foreground, with the highlights probably rather dense. In the ordinary way, when printing dark enough to produce detail in the highlights, the shadows are overprinted, and, therefore, develop too black. Generally, they are too black before you have enough detail in the highlights. To overcome this difficulty interpose a piece of cardboard between the light and that part of the negative which prints too quickly, exposing the hard portions for a few seconds only. Keep the cardboard moving, backward and forward, with rotary motion, to produce a light vignette effect. If the cardboard is allowed to remain motionless, the result will be a sharp line between the highlights and shadows. After holding back the shadows for a few seconds in this way, expose the entire plate evenly. By carefully following the instructions in developing, these dense, deep shadows will most always be overcome.
Stains On Edges Of Vignette. If the vignette is produced in the developing, the stain sometimes obtained on the edge of the vignette is of yellow or brownish color. This is usually caused by developer becoming discolored or too warm, and printing too dark. Follow the instructions in regard to removing this stain.
White, Milky Deposit On Prints. This generally occurs if the hypo bath is not properly balanced, having either too much acid or alum, or vice versa. The milky deposit will do no harm if removed from the prints after they get into the wash water. Allow the hypo bath to stand without agitation, and the milky deposit will settle. Then decant the clear solution, and use it for fixing the prints.
Prints Yellow Before Placing In Hypo. If the print was under-printed and you attempt to force it by prolonged developing, it will generally turn yellow. To avoid this difficulty print to the proper depth.
Prints Becoming Yellow In Spots While In The Hypo. Yellow spots are caused by air-bells gathering between the print and the hypo solution. Wherever there is an air-bell there is sure to be a yellow spot. To overcome this difficulty, see that prints are thoroughly immersed and air-bells removed when first placed in the hypo. The prints should also be picked over and over, to separate them, while fixing.
Prints Becoming Yellow All Over In Hypo. If your hypo is warm, or has become exhausted, too many prints having been fixed for the amount of hypo, the prints will yellow. Remedy: Do not fix too many prints in the hypo bath, and see that the bath is cold - never warmer than 60° Fahr.