EVERY photographer knows that the ordinary plate is practically color-blind, that is to say, it reproduces the violets and blues more or less like white, and the greens, yellows and reds more or less dark or black. As in color photography we have to reproduce reds, greens and blues as of equal value, the plates used must be sensitive to these colors; therefore, we must use the so-called panchromatic plate. Any plate can be color-sensitized by bathing for a short time in certain dye solutions. While this is not a difficult operation and the resultant plates actually have a higher color-sensitiveness than commercial plates, which are made by the addition of the dye to the emulsion just before coating, it is not a process that can be recommended for the average worker. In the first place, the necessary dyes are very costly, and must be purchased in commercial quantities sufficient for a great many plates, although actually only a very small quantity is used; secondly, efficient drying arrangements must be installed; thirdly, control of results is extremely difficult, in consequence of the many factors involved; fourthly, bathed plates have not such a long life as commercial plates; lastly, sensitizing of plates can not be carried on successfully in a room that has been used for developing and fixing. The author's advice to beginners in color work is to adopt commercial panchromatic plates; quite enough troubles and failures will be met with in the ordinary manipulations without adding extra worries about the sensitive media.
If it is desired to experiment in this field, the following instructions will be helpful. In the first place, it is not advisable to select the fastest plates, as they have, as a rule, a greater tendency to fog than slower ones, and this tendency is always increased by bathing, particularly if the plates are to be kept some time. It is not advisable to bathe less than a dozen plates at a time. A medium plate, normally clean-working, should be chosen. Before entering upon the actual bathing process, it may be as well to point out that as the plates are rendered sensitive to red it is not advisable to use red light for the darkroom illumination. It can be used, but as one can see far better with a green light of equal power it is preferable to use one of those described later on (See P. 41). But the plates should not be unduly exposed even to one of these, and with a little experience it is astonishing how easy it becomes to work in total darkness, just switching on the green light as required.
While grooved troughs are the most convenient for bathing a dozen plates, they are very wasteful of solution, and so flat dishes should be used. If one sufficiently large to hold four plates at once be employed, it is almost as easy as using a trough. An interval timer, clock or watch, is an absolute essential. A grooved trough is preferable for washing. There is one extremely important detail, on which the author lays great stress, and that is that the dish used for sensitizing should have been used for no other photographic operation and should be kept for this sole purpose, particularly if of earthenware, as this soon acquires surface cracks into which the developing and fixing solutions enter, so that minute traces of these are extracted by the dye solutions and all sorts of curious markings and fog may be the result. Glass vessels are preferable to all others and they should be of the deep variety. In no case must metal dishes be used for sensitizing, as there seems to be some peculiar action that results in the formation of a characteristic coarsegrained fog, which increases somewhat rapidly with time.
As regards the drying apparatus, a proper drying cupboard with a constant supply of heated air is naturally the most convenient, but, failing this, the simplest arrangement is a light-tight, and as far as possible airtight, box. This should be of generous size; 24 x 12 x 12 inches is not too large for a couple of dozen 1/4 plates. The author used for a long time an ordinary metal deed box with draught-excluding rubber tape round the lid. In the center of the box should be placed a block of wood, which should be of a goodly size, for instance, for the above size box 9x6 inches, and of such a height as will permit a flat dish to be placed on top and yet allow the lid to be properly shut, with about an inch to spare above the top of the dish. This dish should be filled with desiccated calcium chloride, which can be obtained from any chemical house. It should not be in too large lumps nor in powder; about the size of small walnuts or chestnuts is right.
The plates should be arranged round the sides of the box with the sensitive surface facing the center; they must not be placed with this side towards the walls of the box or unequal drying will ensue with unequal sensitiveness and more or less fog. It is advisable to place on the bottom of the box two or three layers of clean stout blotting paper, which prevents the plates from slipping.
A far more convenient arrangement is the use of an electric dryer such as is used by hairdressers; this can be attached to any electric light socket, and if an endless box be used to confine the air currents, it will be found possible to dry a dozen plates in less than half an hour. This is a great advantage, as it may be accepted as an axiom that the more rapidly bathed plates are dried, the greater their keeping power and the greater their freedom from fog, with increased color-sensitiveness. The box should be about two feet long and sufficiently large to accomodate two wooden drying racks, placed one behind the other, but too much space must not be allowed around the ends of the racks. The dryer should be placed at such a distance from the end of the box that the temperature half way down the box should be from 33° to 35° C. (90° to 95° F.).