Gum bichromate
The rubber pressure, even Gummibichromatverfahren, is a so-called precious printing process.
History
The application of the rubber printing process culminated in particular in art photography at the turn of the 20th century. As the inventor of the process, both of Louis- Alphonse Poitevin Frenchman for the year 1855 as well as the Englishman John Pouncy apply. Pouncy, the process was patented in 1858.
As an inestimable artistic value, the resulting 1900 rubber prints of French photographer Robert Demachy and Constant Puyo as well as those of the Brothers Hofmeister and the group of the Vienna clover apply ( Heinrich Kühn, Hans Watzek and Hugo Henneberg ) from the German-speaking countries.
A basic distinction is between single and multi-layer or combined rubber pressure. While the first-mentioned consists of only one print layer and a few tonal reproduces, several layers can be copied each other by multiple rubber pressure, which allows a wider range of tonal values . A further development of the multilayer rubber pressure is the three-color method according to Hans Watzek.
Technique of rubber pressure
Although potassium or ammonium dichromate in the pure state are not sensitive to light, they are sensitized in conjunction with colloids for photochemical reactions. Colloids are glue-like substances such as gum arabic, isinglass or gelatin. Light irradiation results in the chromates an instantaneous responsiveness with the colloids, which are then tanned depending on exposure to light, or harden and lose their water solubility. The rubber printing method uses this property. It is an emulsion of chromate salts, gum arabic, and color pigments prepared, which is applied as a thin layer on a paper. The coated paper is exposed to light through a negative, color separation negative in the contact process, after which the exposed parts tan and are insoluble in water. The rubber layer is then washed out. The hardened by the light points remain. The color pigments are bound to the exposed areas so that an image is revealed.
Method
The flexographic printing process the negative is placed in contact on the light-sensitive paper, and exposed to light, and therefore a negative should first be selected, which can then be increased at will. Through the contact method corresponds to the final image the size of the negative. Until it comes to exposure, the following steps must be followed:
Selecting paper
The paper serves as a substrate for the photosensitive emulsion rubber and should therefore not be too smooth, otherwise the layer may slip. Since it is exposed to water baths to strong watercolor and handmade paper are particularly well suited for it. Before the paper can be used, it should be shrunk so that the fit does not change during multiple printing. To shrink the paper is placed in a hot water bath 50 to 60 ° C and taken out after about 15 to 30 minutes, and dried. The colored rubber emulsion must not penetrate into the paper fibers, which is why a well- sized paper should be used. If multiple printing is planned, the sizing is usually not sufficient, so a resizing is necessary. For this purpose, 25 g of gelatin was dissolved in 500 ml of water and applied evenly with a bristle brush on the paper and then dried. The choice of paper is crucial for the grit and character of the image. In order to achieve specific artistic effects, watercolor or expensive handmade papers were used deliberately.
The color
The color is the chrome rubber emulsion mixed later added, should be water soluble and not oily. Theodore Hofmeister, an eminent German art photographer, inter alia, to tempera and watercolor paints and writes further: " Every artist strives to ensure that its images remain with regard to the color effect immutable and after centuries still the same face showing as at the time of the completion of the image. The straight we still admire in the pictures of the old masters. "Among the photographic process especially the rubber pressure can meet these demands. The color pigments are bound firmly in the hardened layer after exposure, not flake off and get - depending on the light fastness of the color - very long their luminosity. Furthermore, the addition of color can be seen as a fundamental change in photography. In the 1850s and around the turn of the century, the photographs were black and white, the simple rubber pressure, however, has produced monochrome images, especially browns were preferred. The multiple pressure makes it possible to combine different colors. To create a special mood, color stylizations were most desired - absolutely naturalistic playback is not possible with this method.
The chrome rubber emulsion
The light-sensitive coating is composed of an aqueous solution of gum arabic, potassium - or ammonium dichromate and an addition of color. It is applied with a brush in dim lamplight. Exposure to light causes then at the chromates an instantaneous responsiveness with the colloids, which are tanned depending on exposure to light, or harden and lose their water solubility. With a 30% ammonium dichromate solution can be prepared. This is sensitive to light than the maximum of 10% potassium dichromate solution, the latter being commonly used. For gum solution gum arabic is dissolved in water, - this may take several hours - usually a 40% solution is used. The rubber pressure is not a silver halide process. In practice can be mixed with the dissolved rubber and color in different ways, the chromate solution. It is important, ultimately, that the photosensitive layer is applied as evenly as possible and not too thick. " By the year 1897, the paper was prepared for the rubber pressure such that a 40 percent solution of gum is mixed with a pigment and a 10-percent solution of potassium bichromate and is thinly applied to the paper. " When the newer method, the James Packham developed method designated " first after which the paper is impregnated with a chromate and then painted with a color of the rubber. The chromate acts through its water- repelling force as a doubled sizing of the paper, so that the rubber color does not penetrate so much in the paper fiber. " Finally, it should be noted that the chromate content determines the hardness of the image, so that smoother images with a high chromate and harder images are generated by a lower chromate. If the gum arabic is mixed with the potassium dichromate solution in a 1:1 ratio, it is called co pressure. The midrange pressure is obtained by the ratio of 1:2, the lights finally pressure by the ratio 1:3. The lights pressure " should cover everything translucent and allow only the highest lights the paper quite free. " Since each printing process only one tone can be copied, various pressures are usually combined to achieve the desired result.
The exposure
If the paper was coated with the emulsion and dried, it is light sensitive and can be directly exposed under a negative in the contact process. In the correct exposure time is the main focus of the process, where different factors play a role: as sensitivity and thickness of the spread, density of the negative, use of color, exposure to artificial light and with sunlight in the winter or in the summer. The exposure time is played out in the order of about ten minutes and is basically the shorter the thinner the layer, and the less color and rubber can be used. Through a thick spread and a short exposure a coarse grain can be produced, conversely, a fine-grained picture by a thin layer and a long exposure occur. In addition, the color choice is critical. " Experience has shown that need green and blue hues shorter - about a third - copy time, like black and yellow or even brown, the need to copy the longest. " So the picture is not over-or under -exposed must be the exact exposure time are taken. This is often a photometer as an aid. To achieve a particular blur the paper can be exposed from the rear side, as shown in Theodor Hofmeister.
The development
After the template was exposed under a negative, the exposed parts are hardened and become insoluble in water. To get a finished picture, the unexposed and thus water-soluble layer must be washed out. " You put the exposed arc, shift down in cold [ ... ] water, pour in about 10 minutes " from the discolored water. The aim of the bath, it is to separate the color of the unexposed areas, as well as contained in the photosensitive layer from the support chromates. After that, new water is added, and allowed to " develop the arc undisturbed by looks up temporarily. After 40 minutes, the painting is finished. "During the development in the water can be influenced in various ways on the resulting image. To obtain a coarse grain size, the water can be added among others sawdust. It can also be used with a water jet, a so-called push processing. It is selectively more color is removed by the beam, so that for example, the lights can be highlighted. The rubber printing is completed after the image has dried. When combined rubber pressure, a pressure will be performed again.
It should be noted that the rubber pressure is basically a simple process, but requires a lot of skill for a good result.
The advantages of the process
Compared to other photographic processes of the rubber pressure and the associated processing can be carried out in subdued or even full daylight. Since the chemical reaction of the bichromate to the very slow vonstattengeht colloids in the wet state. Should also be noted that the chromates are completely washed out, so that the final image has an unlimited shelf life and will not darken. Another reason for the stability is that the color, and thus the image " is fixed by an insoluble substance directly on the paper surface. " This is in contrast to pigment printing, wherein the image to another surface is transferred, and it is therefore possible that the paint peels off over time. The possibility of a final impression to prepare new and copy new, to achieve an improved result, are major advantages of the rubber pressure. It can be modified grain and colors are combined. For example, the pasting of clouds in a landscape in two operations a popular procedure for combination printing. Basically, offers every step - from the choice of the paper until exposure - a way to intervene in the resulting image. Ultimately, this is the decisive advantage of the rubber pressure, which is why the use of particularly in the artistic field of application was and is.
The importance of the rubber pressure for the art photography movement in 1900
The photograph presented for the Pictorialists is a form of art-making, finished with a stand-alone photographic means. The technical perfection of these funds was the example of Heinrich Kühn, a major Pictorialists, a life goal. In order to achieve " the greatest possible freedom of art work ," she improved camera, recording material and copying processes. The most important printing process of this movement was the rubber pressure, particularly since this technique allows the formation of the image to intervene. This freedom is reflected for example in the use of the color down that " regardless of the brown and blue tones of silver, iron, and platinum processes " makes the photographer. The photograph, which was often seen in art discourse as a purely technical / mechanical medium, got the rubber pressure to another state. Although the negative yielded a mechanical camera, but the finished image has always been unique. Decisive for the resulting image is the rubber pressure of the artists and their actions - that is, for example, already influenced in the choice of the paper, the character of the final image. In addition, relatively large rubber prints were made , which were compared only by their format paintings. Especially this comparison were intent on fine art photographers. The editor of the photographic weekly paper wrote in 1906 that "there [ ... ] strange [ is ] how to play the character of different art techniques with this method, such as the pastel, watercolor or oil painting, and like any practitioners of its special character brings it expresses. The pictures of Henneberg have, for example, the character of a watercolor that of the Watzek an oil painting. "
The history of the rubber pressure
In 1852 Henry Fox Talbot noted that the exposed emulsion of potassium dichromate and gelatin lost its water solubility and swelling capacity. Louis- Alphonse Poitevin developed from 1855 pigment and rubber pressure. However, the images were reversed and halftones could not be brought for display. Abbé Laborde and J. C. Burnett resolved this issue a little later. Finally, John Pouncy was the process in 1858 in England patented and presented his findings publicly. Thereafter, the procedure was partially used for copying landscape, portrait and architectural images, but the rubber pressure soon fell into oblivion. Only in the 1890s he was again employed. "The English photographer and co-founder of the Linked Ring, Alfred Maskell had the procedure in 1893 and picked up again stimulated Robert Demachy from the Photo- Club de Paris to experiment with rubber printing. " Other sources put the Frenchman A. Rouillé Ladevèze in the foreground, 1894 had issued rubber prints in the Salon de Paris, which Demachy incited to present a year later his first results in the Photo- Club de Paris. An eye-witness, John Gaedicke, editor of the weekly paper photographic seem to connect the details: "The English Art Photographer Alfred Maskell presented in London in 1893 from gum prints. The first, published a method for the production of pigment images without transmission using rubber as a binder was Rouillé - Ladevèze, who in his brochure: so sepia photo et Sanguine - Photo ( Paris 1894, Gauthier -Villars et fils ), the process stated, as it is usually applied today. "Anyway, started with the pictorialist photographers the success story of the rubber pressure. The rediscovery of the rubber printing and its widespread use around the turn of the century can be associated with a particular interest in the decorative effect of the visual arts, as they represented at the time of Art Nouveau and the various secessionist movements, in conjunction. At the same time the amateurs could easily take off with her rubber printing by professional photographers who avoided the time-consuming process. After Paris excited " in London and Brussels Salon in 1895 [ ... ] some rubber prints of Demachy [ ... ] attention because of their artistic treatment. The end of 1895 were seen by Demachy " at the exhibition of the Vienna Camera Club five rubber prints.
In German-speaking of the rubber pressure of Vienna common starting. "While Theodor and Oscar Hofmeister 1896 showed only carbon and pigment prints, they found 1897 exclusively of rubber prints. They were fascinated by this precious printing method that Heinrich Kühn had first presented in 1896 in Hamburg. " ( The certificate issued by Kühn image is entitled" Dawn ". )
Hans Watzek developed in the winter of 1896 the three-color flexographic printing, in order to present him on New Year's Day 1897 can. In the same year Kühn's first experiments were shown in color in Hamburg, where they were in turn emulated by the brothers Theodor and Oskar Hofmeister immediately, in 1898 followed by the Berlin-based photographer with multi-colored images. Together with the British photographer Alfred Maskell wrote Robert Demachy a description of the method, which in 1897 in English and 1898 under the title Le Procédé à la gomme bichromatée ou photo aquatinte it was was published in French. Then, for example, the American photographer Alfred Stieglitz became aware of the rubber printing process. With art photography motion photography has found its place in the museum space, so in 1902 exclusively rubber prints were shown at the annual exhibition of the Vienna Secession. The possibilities to intervene in the process, and will foster active as an artist a picture, make the benefits of the rubber pressure dar. After Theodore Hofmeister, there is no pressure in the rubber chance, because everything is in the hands it artist. Of course, this opinion was controversial and the rubber pressure was strongly criticized especially for its blur. At the latest with the New Look of the 1920s, the rubber pressure had little meaning. In the United States, the procedure was to the 1940/50er years still to be found the most. In recent years, the rubber pressure experienced as a counter-movement to digital photography a renaissance. However, the heyday of the rubber pressure was around the turn of the century.
Hazard Statement
Ammonium dichromate and potassium dichromate are toxic and carcinogenic