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A Carbon Photograph, in the strict sense of the word, is an image in carbon produced by the action of light. The term, as commonly used by photographers, has, however, a wider application, and is employed to designate any sun picture produced in permanent pigments, whether consisting of carbon or not.

Almost all the methods which have been proposed for the production of such pictures depend upon one principle. They are based on the fact that light renders certain soluble bodies insoluble in the usual menstrua. This principle admits of varied application in producing pictures; but in the processes which have been brought to the highest practical perfection, some coloring matter—Indian ink or lampblack for instance—has been added to a colorless body like gelatine or gum, which, on being rendered insoluble by the action of light in parts, imprisons the coloring matter, and thus forms the dark parts of the image. A sheet of paper coated with such a substance mixed with a pigment, exposed to light under a negative, and then washed to remove all the soluble matter, will produce a picture, the blacks of which are formed by the insoluble substance and pigment, and the whites by the surface of the paper from which the colored coating has been washed away.

It will be seen, however, that in the production of an image by means of a photographic negative, in coloring matter so imprisoned, there is no provision for the rendering of gradation of tint. A layer of substance capable of being rendered insoluble by the action of light, if extended on a sheet of paper and exposed to light under a stencil plate, would be rendered insoluble wherever the luminous action penetrated the apertures in the plate. If the paper were then treated with a solvent of the substance with which it was coated, the coating would be removed from all portions protected from the action of light by the opaque parts of the plate, and a perfect transcript of the design would be formed on a white ground. If, instead of the stencil plate, a photographic negative be employed, the image in which is formed by varying gradations of opacity, the result is somewhat different. The layer of soluble matter is rendered insoluble wherever the light has penetrated sufficiently through the transparent parts of the negative; but where the more opaque parts of the negative, through which light has penetrated with much less intensity, protect the coated surface, a portion only of the coating is rendered insoluble, that portion being the surface in immediate contact with the negative. When the prepared paper is submitted to the action of a solvent, the thoroughly-exposed portions, being quite insoluble, are not removed, but those parts representing the lighter tones of the picture, having become insoluble on the upper surface only, the layer underneath is readily dissolved, and the whole film in such parts is thus removed by the solvent. An imperfect image, possessing only deep blacks and masses of white without gradation of half-tone, is the result.

This was the great difficulty of carbon printing in the early experiments which succeeded the discovery by M. Poiteven, in 1855, of the principles upon which it is based, although the cause was not at first fully understood. After two or three years of comparatively unsuccessful effort, it was discovered, that in order to succeed in producing gradations of half-tone in such pictures, it was necessary to wash away the unaltered and still soluble matter at the side of the film opposite to that exposed to light, in order to preserve intact every portion of the film which had been rendered insoluble, and so leave a film varying in thickness according to the depth to which light had penetrated; this depth being governed by the varying degrees of transparency of the different parts of the negative. This varying thickness of a colored translucent film upon a white ground, accordingly rendered the gradations of the picture, a thick layer representing deep blacks, and thinner layers, various gradations of half-tint.

This principle recognized, it became possible to produce carbon prints, which were true transcripts of a photographic negative; but a mode of rendering the principle practically useful was wanting. The first idea was to place the back of the prepared paper in contact with the negative, so that the light should, after traversing the paper, act upon that surface of the sensitive layer which was in contact with the paper, rendering it insoluble in varying degrees. The soluble portion was then washed away from the upper surface, leaving undisturbed the insoluble layer of various thicknesses, which forms the picture on the paper. This method was found to be comparatively impracticable from the long exposure it rendered necessary; the picture suffered also, and became coarse, from the granular texture of the paper through which the light had to pass. Various methods were tried, the details of which appear in a subsequent chapter; but none were found efficient until Mr. Swan discovered the method of preparing a sensitive film, which, having been exposed to light in direct contact with the negative, could be transferred to another support, so as to permit the easy washing away of the unaltered material at the side opposite to that which had been exposed to light. This method was found not only more simple in practice, but more excellent in result than any method previously attempted. It gave pictures of exquisite delicacy and force, rendering perfectly every gradation in the negative with a degree of beauty which had rarely been obtained even by the usual methods of silver printing.

It is unnecessary to enter into a consideration here of the possibility of producing permanent pictures by the usual photographic processes of silver printing. The stigma of instability has been hitherto the chief drawback to the beautiful results of photography. In no silver printing process has immunity from fading been obtained; and although many photographs have been preserved unchanged for years, others, produced under apparently the same conditions, have become, during the same period, faded and worthless; and uncertainty of permanence, if not certainty of fading, remains a characteristic of all silver prints.

In Swan's carbon process the image is produced in the pigments of the painter, and whatever of permanence may be predicated of a water-color drawing, may be affirmed of the photographs produced in the same materials. If carbon be regarded as the most stable coloring matter which can be employed, lampblack or other similar preparations of carbon can be used in the production of the print. Sepia, bistre, Indian-ink, or any combination of these or other pigments, by which a pleasing monochrome may be produced and permanency attained, is equally available in this method of printing.

Thus, a more certain control over the tone and quality of the picture is obtained, whilst absolute stability is secured. The surface of pictures produced by this process is devoid of the gloss, which is regarded by many as vulgar, in prints on albumenized paper; the lights being absolutely flat or dead, the shadows only presenting, in some cases, a slightly glossy surface. It will be thus seen that the pictures gain not less in artistic qualities of color and texture than in permanency.

Mr. Adolph Braun, Dornach (Haut Rhin) France, has already applied the carbon process successfully in a direction heretofore considered almost impossible to reach. He has made negatives of thousands of the drawings of the old masters in the art museums of Europe, and reproduced them in the colors of the originals, which embrace almost every shade and color.

This is an immense reach for photography, and will greatly lead towards the elevation of the art, and the cultivation of artistic taste. Mr. Braun has purchased Mr. Swan's patent for France and Belgium, and is the largest worker in carbon printing in the world.

It is not necessary, however, to enter into more extended comment here on the beauty of the results, as the reader will form his own opinion from an examination of the print which accompanies these pages.

Before concluding these introductory remarks, it may be desirable to give a brief summary of the grounds for stating that permanency is secured in the method of printing to be described. They are these: the image is formed of carbon or other known permanent pigment; the vehicle or menstruum in which the pigment is held is gelatine, rendered quite insoluble by combination with the oxide of chromium; the amount of gelatine present in the image, and uniting it with the paper on which it rests, is not greater than that on the surface of a sheet of well-sized writing-paper, so that there is as little danger of the vehicle cracking or decomposing, as there is of the pigment fading. Absolute imperishability cannot probably be predicated of any picture formed of pigments and paper; but, as much permanency may be anticipated for these prints as is found to pertain to an Indian-ink drawing, to which experience permits us to award a duration of at least several centuries.