(c) WORKING-UP 465. General Notes. By the term " working up " is meant the various methods of correcting negatives by hand in such a way as to modify the relative printing values of certain areas of the image which have a simple outline. This may be done in order to remedy any irregularity arising from faulty working in making the nega tive ; or it may be to improve the rendering, for example, by reducing the contrast, which is sometimes great, between sky and foreground in a landscape or between the lights and shadows of an interior photograph ; or to obtain the details of the image of white clothing, or to subdue the too pronounced details of a back ground on a portrait negative or on the negative of a commercial subject.
In all the methods of working-up involving the application, with a brush, of an intensifier, a reducer, or a dye, it is an advantage to use liquids which are sufficiently dilute and to carry out the operation by successive applications of the same reagent. In this way the appearance of sharply-defined edges to the area treated may be avoided ; such sharp edges, if they do occur, frequently do not coincide exactly with the outline which is intended to be followed. The edges of partial, successive treatments overlap one another to some extent, and give a graded effect which is much less obvious.
These operations are best carried out on a retouching desk, and if they have to be done as a matter of commercial routine the use of folding desks is to be avoided. These are by no means steady and do not give sufficient support to the forearm. The amateur may construct a luminous table by placing a strong piece of glass on two boxes. The under side of the glass must be covered with white tracing paper and may be illuminated by means of light diffused from an inclined sheet of white cardboard. The light may be supplied to the latter from a window or lamp which is screened by a blind from the view of the operator.
466. Local Intensification and Reduction with a Brush. In order to allow of control, it is necessary to choose methods which make use only of a single solution and which do not produce more than temporary coloration of the gelatine ; thus the mercuric iodide intensifier and Farmer's reducer, 1 or one of their variants, are suitable.
The operation is preferably carried out on the wet negative, which has been well wiped. The negative is mounted horizontally and illurnin ated from below (for reducing a sky, it may, however, be held almost vertically, with the image of the sky at the bottom). In order to facilitate the adherence of the liquid to the gelatine and to prevent its excessive diffusion on to neighbouring regions, its viscosity may be increased by dissolving sugar or by substituting glycerine or glycol for a part of the water in the mixture. In the case of reduction, the nega
tive must be rinsed between each application.
467. Local Tinting of the Gelatine. Working up may be done by the additive method of tinting those parts of the negative which are too clear, or by the subtractive method of uniformly tinting the gelatine and then de colorizing it in those parts of the negative which are too deep. Which method is preferable depends on the relative areas of the parts which are required to be tinted and to be left plain.
For additive tinting one chooses an aqueous solution of a dye which is absorbed by the gelatine but is not fixed by it, and which may therefore be completely removed by washing in cases where it is desired to obtain the negative again in its original condition. Of the substances complying with these conditions are notably new coccinc, giving a poppy-red colour (F. Schmidt, 1913), and tartrazine, a lemon-yellow dye. The first trial applications should be made very carefully, only experience being able to teach the effect of a given intensity of colouring, red or yellow.
At the start a comparatively concentrated stock solution of one or other of the dyes is prepared, and from this stock solution are made up two solutions for use ; one of these is so dilute that a single application by means of a brush on a transparent part of the gelatine produces only a very weak coloration which can hardly be seen ; the other is about five times more concentrated. The colour is applied with a. sable brush No. 2 or No. 3 for very small sur faces, No. 4 to No. 8 for large surfaces, according to the size of the negatives.
The brush, soaked in the diluted solution and well wiped, is moved over the parts to be tinted without breaking contact between the brush and the gelatine until all the liquid has been ab sorbed. The brush is re-charged and wiped, and the process continued, returning as often as necessary to the same parts, and, if necessary, allowing the gelatine to dry when it becomes saturated with water. It is only by the super position of a great number of very thin layers that perfect uniformity is obtained, and slight errors of outline are made negligible. The more concentrated solution is only used when a certain amount of skill has been acquired, and even then only to obtain very intense tints ; in any case it is as well to prepare the film by at least one treatment with the diluted solution.