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.
Parts Not Improving By Intensifying. If the negative has been fixed in an acid hypo bath, or the plate was placed in a hardener, it will be almost impossible to intensify successfully. A plate of this kind should be thoroughly soaked in water for hours, and often you will find it necessary to strengthen the intensifying solution.
Intensifier Spreading. To overcome this difficulty be very careful when applying the intensifier. As soon as there are signs of intensifier spreading, rinse and drain for a few minutes, and then apply again, being careful to have only a little of the intensifying solution in the brush, and applying it exactly in the center of the part you wish to intensify. If it again spreads rinse at once, and repeat the operation. If this is not successful use the glycerine formula.
Judging When Sufficiently Intensified. Only practice and close observation can teach you this. It is always safe to stop the intensifying a little before you think it is strong enough, and repeat the operation if you find the parts have not gained enough in strength.
Intensified Portions Stained After Going Through Clearing Solution. If there is any hypo present in the plate it will always stain. If the clearing solution is not strong enough you will obtain a stain. If the plate originally was discolored a trifle, this stain will appear considerably stronger. Be sure and use fresh clearing solution, and see that your plates are thoroughly freed of hypo before attempting to intensify.
Parts Not Improved By Intensifying. A very badly underexposed plate can never be locally intensified successfully, as the shadows have nothing in them. The highlights should always take on a certain amount of density. If any part you wish to strengthen turns to a white or cream color, you can be sure that it is somewhat strengthened. In some cases you will find it best not to clear the plate after intensifying; allow it to remain this cream color, as the stain color will slow the printing, and assist in supplying strength and detail.
Plate Refusing To Clear In Clearing Bath. See Difficulty, Intensifying, Part I, paragraphs 159 - 160.
Applying The Glycerine Intensifier To Dry Negative Without Effect. This is probably due to the plate being fixed in a hardening bath, or an acid hypo. Hold the plate flat, and apply the solution freely, and allow it to remain on the plate for a little while. It may take some time for the solution to take effect, and make any change.
Glycerine Intensifier Spreading. To overcome this difficulty add a drop or two more of the glycerine, and be careful not to apply too much of the solution to the part of the plate which you intend to intensify.
Any other difficulties met with are very likely covered in Part I. -Intensifying.
AT PEACE Study No. 4, See Page 343 Dr. A. R. Benedict, Montclair, N. J.