186. The second trial alluded to is known as "weakness" of the films, and is caused most generally from over - exposure. A bath too strong for the collodion used, or a bath that is weak; collodion insufficiently salted, development) we have a wonderful power if we observe closely. Then, again, in intensifying, we possess another power of modifying the resulting picture; so that, with care and judgment, by the secondary exposure, a negative that has had insufficient time in the camera may be made to give a fairly-good print instead of being rejected. There is not a photographer, I suppose, who is not fully alive to the importance of keeping the chemicals at a suitable temperature during cold weather, and there is no way - so good as having the dark-room continually warm both night and day. - Alfred Hughes.
This method of reducing the intensity of the negative has reference only to that class of negatives which is produced by under-exposure, too strong contrast of light and shadow, collodion too thick, or bath too acid, where the high-lights are so dense that the negative yields only white and black, hard, chalky prints. In such cases pour over the negative, after the fixing solution of hypo, or cyanide has been washed away, a very dilute solution of perchloride of iron. Allow this to act on the deposit of the high-lights until a slight change of color takes place, then well wash and pour over again the usual hypo, or cyanide fixing solution. The dense deposit on the high-lights will be found to be reduced in intensity proportional to the action of the perchloride of iron solution. The best advice I can give is to bear in mind the principle of the chemical change that takes place. The original dense deposit forming the high-lights of the negative is composed of metallic silver; this is not soluble in either hypo, or cyanide. By pouring on solution of perchloride of iron, the outer layer of the deposit is changed into chloride of silver, and chloride of silver is very soluble in the fixing solution. If the perchloride solution be allowed to remain on too long, all the silver forming the deposit will be changed into chloride of silver, and will all be dissolved away. This is clearly doing more than is required; the real point to be gained is to dissolve away only that excess which renders the negative too dense. The whole operation is a delicate one, and requires quite as much judgment to be exercised as in developing or intensifying a negative. It is really a process exactly the reverse of intensifying; but there is this difference - that intensifying is usually done at one operation, and the process can be observed as it proceeds. In this, however, the real action is done by the perchloride, and the exact amount of the action cannot be told until the fixing solution is employed to show the change that has taken place. The greatest care is, therefore, necessary in using the perchloride, otherwise a negative may be injured instead of being improved - Jabez Hughes.
186. A correspondent of the Philadelphia Photographer"says: "As the tendency of all intensifies is to flatten and destroy the beauty of the photograph, it is desirable to omit them as much as possible. I often avoid their use by the following ' dodge.' If, after the negative is developed, fixed, and washed, it needs a little reinforcing, I dry it rapidly by the stove or otherwise, which usually brings it right for printing. Any one who will try the experiment and insufficient immersion of the plate in the bath also cause thinness or weakness of the film. This class of negative is the easiest to make, because it allows the most latitude for careless, heedless, slovenly ma-nipulator. In such cases resort is had to intensification, but in it, also, trouble occurs,unless care be excercised. Formula for Intensifiers are given at at page 108. Here only the troubles of it arc treated.
187. All who have the patience to make portraits of children a specialty, know the value of an intensifier close at hand. In such extreme cases it is excusable, but as a habit it is uncalled for. In making copies, too, where sufficient printing density cannot otherwise be had, resort is had will be surprised at the difference between a negative thus dried and one that is allowed to dry spontaneously. Negatives, after washing, should be flowed while wet with a solution of gum-arabic in water. This prevents the hard varnish from changing them."
It often happens with an unde r - exposed negative that it is desirable to intensify only certain portions of it, as, for example, a portrait in which the hands and face would be completely spoiled before the other portion obtained sufficient density. With careful manipulation, the following dodge will be found useful: After development, stand the negative on end to drain, and when half dry - i.e., when it has lost its watery appearance - hold it over a tiny gas jet, allowing the flame to come and the drying to commence in the centre of the face. Have the water ready to hand, and, when the small dry patch has extended to the edge of the face, dash the water over the negative The intensifying may now be proceeded with, and much of the detail in the features will be saved which would otherwise have been lost - R. V Harman.
The chief defects that arise through intensifying are those which may also occur in development. Fog and a red deposit are chiefly to be anticipated. The former may occur before fixing if the pictures be over - exposed; the latter, both before and after fixing, by the addition of too much free nitrate of silver to the intensifier; or again, after fixing, by the imperfect washing of the film before the intensifler is applied. The red stain will often yield to treatment with a solution of acetic acid and water (half to half). It should be noted that the larger the amount of silver added the more rapid will be the intensification; but the half-tones will not be brought up proportionally to the high-lights. The smaller the quantity of silver used, the greater will be the comparative force given to them, and the longer time it will take. Thus, a negative lacking in contrast may be corrected by using an intensifier with large, and one too rich in contrast with small, doses of silver. When the mixture of pyrogallic acid and silver becomes turbid, it must be rejected, and a fresh solution poured on. Sometimes a bluish precipitate will form in the shadows; when this takes place, it is an indication that the intensifier is not sufficiently acid. Intensification with iron is equally as rapid as with pyrogallic acid. When using the former, however, it is necessary to use about twice as much silver. With landscapes and portraits, intensifying is comparatively easy work. With reproductions of drawing, plans, or manuscript, it requires more time and great care. The fine lines become easily veiled or the plate unequal. - J. L. Gihon.