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.
Negatives Appearing Oily. If the negative has not been sufficiently washed and there is hypo present in the negative, or if it was previously dried and then not soaked sufficiently in the water to soften up the film thoroughly, the negative will appear greasy. Remedy: If there is hypo present, intensify with the formula which contains hypo-sulphite of soda. If the plate has been dried a long time, soak longer in water before intensifying.
Judging When Plate Is Carried Far Enough In Mercury. If the negative needs only a slight intensification, allow it to remain in the intensifier just long enough to whiten all over. If, after clearing, it is not strong enough repeat the operation. In time you will be able to judge by the appearance of the image on the surface, and by looking through the negative. The negative, as it grows white in the intensifying solution, apparently becomes a positive and, if a black rubber tray is used, it acts as a black backing, and with very little practice you will readily see the image gain in strength on the surface. By close observation and practice you will soon be able to judge very accurately.
Negative Intensifying Unevenly. If there are spots which do not turn white, this is a certain sign that the plate was not thoroughly fixed, was not allowed to remain in the hypo long enough. The portions not fixed will not be affected by the mercury. In such cases, intensify with the formula which contains hypo - sulphite of soda, and is intended for intensifying plates which are not thoroughly washed.
Negative Stained After Going Through Clearing Bath Sulphite Of Soda Solution. This is a certain sign that the negative was not thoroughly washed after fixing. If the negative is stained evenly it will do no harm; on the contrary it will add strength to the printing quality of the negative.
Negative Not Clearing In Clearing Solution. If the sulphite clearing bath has been properly prepared, and if good sulphite was used, the negative will usually return to its original color. Should it fail to do so, first test the bath and see that the sulphite of soda is full strength. Use a fresh clearing bath. An over-worked clearing bath will work slowly. Often a weak solution of ammonia will act when the sulphite has failed.
When To Use Ammonia Clearing Bath. On under - timed plates the increase in intensity is generally not very great, and as the plate loses some of its density in the sulphite of soda clearing bath, and as ammonia clearing does not reduce as much as sulphite, it is advisable to use ammonia for clearing on all plates that are under-timed, and need strengthening. Ten to twelve drops stronger ammonia in half-pint of water will clear the plate nicely, and may be used in the above cases.