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.
Clearing The Plate. After this immersion, rinse the plate carefully for a minute and then immerse in the sulphite of soda clearing bath, until the entire plate has resumed its original color. Use the clearing bath given in instruction on Intensifying, Part I. (See paragraph 136.) There is absolutely no excuse for a flat printing negative, for if by improper treatment in the developer or improper exposure the developed plate is flat and lacks snap and contrast, one can strengthen the highlights, lighten the shadows, supply catch lights to drapery, lighten the hair where it is too dark and build in detail in the dense shadows, etc. For landscape work the improvements are still more apparent, in fact, with proper care one can produce almost any result. You must work cautiously, however, as great care must be exercised when applying the mercury locally, or you may ruin the negative.
174. Caution: - Bear in mind that the plate must have been thoroughly washed and freed of the fixing salt (hyposulphite of soda) before you attempt any intensification. If, after the negative is dry, you find it is not strong enough, repeat the operation, and in this case you need not place the whole plate in the intensifier, but soak in clear water for ten minutes; after which apply the intensifying solution with the brush to the parts you desire strengthened, exactly as in your first operation. When you have the proper strength, clear the plate in the sulphite solution, and then wash thoroughly and dry as usual.
Locally Intensifying Negatives While Dry. Another method which is very practical, and which one should experiment with, is intensifying the negative while it is dry. To do this place on a piece of glass a few drops of the weakened mercury solution. To this add two or three drops of glycerine, and mix thoroughly. The glycerine will prevent the solution from spreading on the plate. Dip your brush in this solution, and apply carefully to the parts to be strengthened, always touching the center of the part first, and being careful not to have too much of the solution in your brush. Allow this to remain on the parts until the desired strength has been gained. Then rinse and place in the sulphite of soda clearing solution, after which thoroughly wash and set up to dry.
Plates To Use For Experimenting. For your experiments we would advise that you use discarded plates. After you have full control of the solution and can apply the liquid as you want it, without spreading, take up a good plate which you think can be improved by this treatment, Then proceed to carefully apply the solution where it is needed, working from the center, and blending gradually from the stronger highlights.
When To Apply The Intensifier. As in reducing, the best time to apply the intensifier is immediately after the plate has been thoroughly fixed and washed. The gelatine film being swollen is most receptive and the intensifying is quickly done. Always be sure that the plate is perfectly washed after developing and fixing before intensifying, for if any trace of hypo is left on the plate it will cause a stain. The plate must also be thoroughly washed after intensifying.
178. In preparing results of this instruction, the same methods must be pursued as with the other instruction. Make good proof prints from each experiment, numbering them in regular order, and noting all data pertaining to the manipulation on the back of each proof, thus supplying valuable data for your future guidance.