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.
Rocking Tray While Intensifying. The trays must be continually rocked while applying either solution in intensifying to obtain even strength. The mercury trays should never be used for any other purpose, as mercury causes no end of trouble when coming in contact with other chemicals. Therefore, in order to avoid this danger it is advisable never to use mercury trays for any other purpose than for intensifying negatives, except for soaking the negative in plain water previous to intensifying. If, for instance, a plate is developed in a tray which previously contained mercury, even after careful rinsing, should the least trace of mercury remain in the tray, black streaks would be likely to appear over the entire plate.
Plates Becoming Stained By Intensifying. All plates must be thoroughly washed and freed of hypo before attempting to intensify. Plates which are not freed of hypo and in this condition are intensified will become stained. Owing to the presence of hypo in the film the clearing bath will affect the plate unevenly, parts of it (and sometimes all of the plate) will remain white, and not return to its original color.
Strengthening Plate Without Clearing. A plate which is considerably under-timed, and has the least detail in the shadows and is not hard - dense in the highlights - can be improved somewhat by simply staining with the mercury. Such plates soak in fresh cool water the same as usual, and then immerse in the mercury intensifying bath until it has gained all the color it will. Then, without clearing in the Sulphite of Soda Solution No. 2, place the plate in clear water and wash thoroughly, and set in the rack to dry. The plate will be a cream color, and it is this color more than anything else that will aid you in obtaining detail, or printing quality. The color slows the action of the light in the printing, and your prints will be more solid, and have more depth and roundness. It is very essential, however, that you thoroughly eliminate the mercury solution from the plate by carefully washing before drying. Otherwise, the least trace of mercury will effect your prints, especially where platinum paper is used.
144. Avoid Printing From Plates Lacking Strength. Never allow a negative that is not snappy to be printed from. It is not necessary, however, that a plate be thick and dense in order to be snappy. On the contrary, it is just the opposite. A good printing plate is quite thin, with clear detail in the shadows, and the highlights crisp but not hard. There must be half-tones in the strongest highlights. Such a negative will yield good prints. The majority of thin negatives are a trifle under-developed, making highlights a little mushy, with lack of crispness and roundness. Such plates should always be intensified (strengthened), and the highlights built up a trifle.