THE GUM-BICHROMATE PROCESS.
When a photographer has had the courage to go through some technical article on the working of a new printing process, he feels the joyous conviction that on following closely the author's instructions he will be able to produce a good print by the process as described ; at any rate he knows beforehand what qualities this standard print ought to have. I do not see the way to produce this happy confidence in the present case, for although I think I am capable of giving a definition of what a good albumen or gelatino-chloride print ought to be, by comparison with a bad one, I am quite unable to define the good gum-bichromate print, and no instructions of mine will enable even an ardent worker to produce what a true artist will call a good gum-bichromate print. All that I can do is to enable him to produce a sheet of coated paper, the coating of which will dissolve and disappear in the places where light has not caused insolubility, while in other parts this coating will remain more or less adherent, according to the quantity of light admitted.
This is the definition of the process in a nut-shell : What we want is a film—a colored coating which, after exposure to light under a negative, will give us a surface quite soluble in some parts, partly soluble in others, totally insoluble in the rest. If we succeed in manufacturing this sheet of coated paper we shall have in our hands the foundation of a good gum bichromate print—it rests with us to make a picture out of this, or a daub, or simply a common-place, faithful, photographic print.
For in the gum-bichromate process we cannot work with the same standards as in other processes. For example, in the silver printing process, under-exposure gives a very bad print of a dirty color without strength or contrast. With gum-bichromate, considerable under-exposure means total absence of image. One cannot sav that the print is a bad one, for in this case there is no print whatever, all the coating will disappear in the developing. Similar with great over-exposure ; in this case the paper remains just as it was before printing—a dull surface of black, brown or red. But whenever an image has been developed and has remained on the paper the resulting print may be good, for it can be made into a picture, and a fine picture, too, if the man who works at it is an artist and knows what he is about.
My meaning is that if the color used for coating is well chosen and sufficiently deep, its nature will not be affected subsequently as it is by toning or developing from white to black in other printing processes, so most of the conditions which cause failure in silver chloride, bromide or in platinum papers are not to be taken into account with this special method, for color and depth of color have both been chosen before printing and development. In other words, as we work from dark to light, the condi tions of ordinary printing are naturally reversed. What we aim at is to keep the color that w e have already applied instead of striving to build it up.
The materials are simple : Gum Arabic, Bichromate of Potash, and the cheapest of all pigments—earths. The gum need not be pure white gum, the ordinary red Gum Arabic works just as well as the purer samples. I have tried both and found no difference whatever in the results. Bichro mate of Potash can be had of any druggist. Any sort of paper is suitable provided it is sized ; all drawing and water•color papers are good, so are most writing papers, but highly glazed paper is most difficult to coat properly, though it works well when this difficulty has been surmounted.
Powdered colors may be used successfull•—they are cheaper than moist tubes, but they are not ground so fine. To make the three conven tional tints of monochrome work, the following colors are necessary : Lampblack, umber and burnt umber, sienna and burnt sienna, Vandyke brown, bistre, yellow and brown ocre, red ocre, brown red, Venetian red, indigo. These colors are permanent. They can be mixed in different proportions according to the taste of the worker. No colors should be used pure, the result is generally displeasing, A saturated solution of Bichromate of Potash (to',:') is made with hot water, and another of gum aragic and cold water which has to be worked up to a density marking IS to 20 degrees by the saccharometer or densitometer. The Bichromate solution keeps indefinitely. The gum solution turns acid after a few days and seems to give better results in this state. If it thickens by evaporation or gets thinner by fermentation, a small quantity of water or a denser solution must be added until it registers a new IS to 20 degrees. Ten cubic centimetres of this gum solution are mixed with pigment and four cubic centimetres of Bichromate are added to the gum and pigment. It must be understood that these measures are purely approximate. According to the depth of color or to the nature of the pigment, more or less gum or more or less Dichromate will have to be added. The photographer will be guided by the behavior of the sensitive mixture, which must allow of rapid and even coating. For example, if he uses Venetian red as pigment the resulting mixture will be thin compared to a mixture prepared with Vandyke brown, for the coloring power of the first pigment is greatly superior to that cf the latter, consequently there will be much more pigment in bulk in the second case than in the first and a few drops of Bichromate and one or two drops of thin gum will have to be added. In reality every proportion is constantly varying in this process, the only important and unvarying factor should be the thickness of the sensitive mixture, for smooth and even coating is a necessity, and only a certain thickness will allow of proper coating. This is a matter of experience to be acquired by a few preliminary trials, though I have worked at the process for several years and it is rare indeed when I do not find it necessary to thicken or dilute the sensitive mixture before beginning to coat.