The photogravure or heliogravure is an old etching process for reproducing photographic models, probably the noblest of all processes of photomechanical reproduction. A grain of aquatint and a photosensitive layer of gelatin apply successively on a copper plate, that lets the etch solution penetrate in proportion to the light received during exposure.
The plate is pickled in baths of ferric chloride of different concentrations to produce delicate gray tones without using a frame.
With photogravure it is possible to combine the realism of photography and aesthetics of printmaking.
History
The origin of the photogravure is closely linked to the beginnings of photography. Where in the first part of the 19th century, people were looking across Europe for basic principles of photography, the ground for photogravure was also being prepared.
Early attempts to develop a method of reproduction close to the photogravure were made by the Frenchman Joseph Niepce in the 1820s who applied a photosensitive layer (made of a solution of asphalt in oil) on tin plates to expose them, etch them and print them.
A process for etching and printing plates was published by Grove in 1841.
The Englishman Talbot developed the method with a gelatin based photosensitive silver, a grain of traditional aquatint and an etching in baths of different concentrations (patented in 1852).
An important influence for the invention of photoengraving owes to Mungo Punton who in 1839 published the results of his research on photosensitive dichromate solutions and organic substances (such as gelatin).
Pretsch (Vienna) and Poitevin (Paris) were the first to make use, almost simultaneously in 1854, of dichromate and gelatin to produce heliographic plates.
In 1879, the Austrian Karl Klic finally combined the Talbot's method with that of the Englishman Swann, who introduced the pigmented paper (paper covered with a layer of gelatin) in the photo-transfer technique, and with that, has become official inventor of photogravure.
Technique
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The starting point for making a photogravure is a photographic negative, from which on makes a positive for a desired size in a darkroom.
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The pigmented paper (also: carbon paper), paper covered with a gelatinous layer is sensitized in a bath of potassium dichromate (very toxic!). After drying, the paper is insolated under the positive (with a gray scale).
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After exposure to UV light, the exposed paper is soaked in a bath of cold water to swell the gel layer, then immediately stuck to the copper plate. It has been previously prepared with a grain of aquatint (asphalt dust) in the traditional manner. Then the gelatin continues to soften in a hot water bath (40 ° C) to detach the paper first and then reveal the image display. The gelatin was more or less hardened with ultraviolet light (according to the gray tones of the model), it remains soluble where it was not exposed. In this way, we can strip the gelatin until obtaining a negative image that represents a layer of gelatin, whose thickness is proportional to the densities of the positive.
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After drying, the plate is to be etched in successive baths of ferric chloride of different concentrations starting with the most concentrated bath. More the etching solution is diluted, more gelatin swells and dissolves, liberating the copper. The perchloride then gradually penetrates the gel layer to produce shades of gray values proportional to the densities of the positive. The last and most diluted etching solution brings lighter shades. The etching progression can be controlled using the gray scale.
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After etching completed, remove the gelatine leftovers and the asphalt grain. The plate is then ready for intaglio printing.
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