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.
168. The object of Local Intensifying is to strengthen only parts of the plate which are weak. There are cases where, if we strengthen only a portion of a negative and allow the rest to remain as it is, the printing quality of the negative will be greatly improved. There may be portions of a negative which are a trifle soft, which, if strengthened, will produce stronger highlights. Often in a landscape the sky and distant objects are strong, but the foreground, especially if there is much green foliage, is thin and prints black. This foreground should be strengthened by local intensification. As there is danger of the solution spreading beyond the parts you desire to strengthen, great care must be exercised in doing the work. The greatest difficulty lies in applying the intensifier only to the parts that you desire to improve, avoiding its spreading to the parts that are already strong enough.
How To Proceed. Place the negative which you desire to intensify locally, in a tray of water, and allow to soak for about ten minutes, or until the film has become thoroughly saturated with water. While the plate is soaking prepare the intensifying solution : -
Intensifying Stock Solution No. 1.........
the negative for a moment. Do not wait until the plate has turned white. The object of this is to slightly stain the entire plate, so that when you are applying the intensifier locally there will be less danger of staining should it happen to spread. Should it spread slightly beyond the parts you wish to strengthen it will spread gradually, and do no harm. Still had you not previously immersed the entire negative, giving it only a slight stain, it would be very noticeable. After immersing the entire plate for a moment, and rinsing as already stated, set it up to drain for a few minutes. While the plate is draining, pour about one ounce of the intensifying solution into a small dish, or saucer, and add to this two ounces of water. Next dip the fine point of a small camel's-hair brush in the solution which you have poured into the saucer, or dish, and apply to the parts that you wish to intensify. Be careful to use only a little intensifier, and apply only on the parts to be strengthened. If it spreads, immediately rinse the plate under the tap, and try it again. Always touch the brush in the exact center of the part which you intend to intensify.
Why A Weak Intensifying Solution Is Used.
You will note the intensifying solution for local intensifying is much weaker than the solution used when intensifying the entire plate. This is because with a weaker solution you have better control of the intensification, and there is, therefore, less danger of over-doing or intensifying parts of the negative which should not be strengthened. Too strong a solution will have a tendency to spread. A weak solution is easily controlled during the application, with little danger of injuring other parts of the plate.
172. After you have obtained the desired strength in the weak portion, rinse in clear water, and then re-immerse the entire plate in this weak intensifying solution. This final immersion is given for the same reason that you immerse the plate before the local application. However, at this time your plate has been materially strengthened locally, and if any of the solution has spread beyond the parts intended, it is apt to be noticeable unless the plate is again immersed for a moment or two, just enough to nicely blend the local work to the remainder of the plate.