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.
Circular Marks On The Plate. This is usually caused from drops of silver on the back of the glass plate. To overcome this, either wipe off the back before placing in the holder, or it will be good practice to back up the plate with another plate stained a dark color. This will usually overcome such difficulty.
Streaks In The Developed Plate. These may be caused by uneven flowing of the developer, or not flowing the plate with one sweep. If the plate is allowed to stand in the holder for some time before use, this will cause the upper portion to slightly dry out, and, therefore, the dry part will not develop as freely as the wet portion. Streaks may also be caused if, by accident, water was first poured over the plate in place of developer, and afterward the plate was flowed with the developer. The water coming in contact with the plate first will give the surface an oily appearance, which the developer cannot overcome; therefore, care must be exercised when working near the tap. that even a drop of water does not fall upon the plate before the developer has been applied.
Fogged Plates. This you will find may come from any of the following causes: By over-exposure; by light entering the camera; by lack of sufficient acetic acid in the developer; or by the silver bath becoming alkaline. When such trouble presents itself it is advisable to first test your exposure. Prepare another plate and give less exposure than given the former. If the plate still appears fogged, examine the camera and plate-holder. See that there is no trace of light entering. Should even slight cracks in the bellows, or a small hole in the plate-holder appear, these are sufficient to fog the plate, and you should at once repair them. Should the camera prove safe and light-tight, then test your silver bath with both red and blue litmus paper. Should the bath test neutral - if neither litmus papers turn from their original color - then you will know that the bath is at fault, as it should be worked acid. This may be accomplished by adding a few drops at a time of nitric acid C. P., stirring with a glass rod and testing with blue litmus paper. When the bath turns blue litmus paper red it is in proper shape and this difficulty will be overcome. If the bath is not at fault, then look to your developer. You may have omitted the acetic acid, or you may have added an insufficient amount. To test this, expose another plate and pour the regular portion of developer in a tumbler or graduate, adding to it a few drops of acetic acid; then develop the plate with this developer, when you will very likely find all traces of fog disappearing. Exercise care that there are no vapors, fumes from ammonia, or gas in the room, as these will affect the manipulation of the plates.