3. Charles Nègre's Heliographic Engraving Technique (1820-1880)

The techniques employed by Niépce and Niépce de SaintVictor made a picture from a positive original (an image projected in a camera obscura, engraving on paper or positive photographic proof on glass). That used by Charles Nègre had the advantage of arriving at the same result but directly from a negative like those obtained from a camera obscura. There was no need to print a positive countertype.



In Nègre's method (Figure above), the first operations - preparation of the varnish, spreading it on the plate, contact exposure under the negative, developing with a solvent - are identical to those used by Niépce (a). The image produced by the asphalt on a metal base is positive. The areas where the metal is bare are those that are not to be engraved. This is where the originality of Nègre's methods enters in. He used the insulating properties of asphalt varnish to cover the bare sections with a fine layer of a metal like gold, less oxidizable than the base often made of steel or zinc (b). The method used was electroplating.

The metal plate holding the image produced by the asphalt acted as one of the electrodes of a battery and was plunged in a solution of metal salts that were to be deposited on it. When the gold deposit was made, the asphalt layer was eliminated (c). On the base, the areas that were not to be engraved became visible because of the layer of gold that protected them. When the plate was subsequently plunged in an acid, the gold was not affected but the unprotected areas were gradually engraved (d). Nègre thus obtained an engraving that could be used for printing.

In addition, the gold salts gradually penetrated the varnish, which was prepared in the manner of Niepce de Saint-Victor. This resulted in the formation of a thin gold network that, after etching by acid, produced a grain on the surface of the image and facilitated the fixing of ink and provided excellent adhesion during printing.

Thanks to this method, Charles Nègre was able to produce some of the most beautiful heliographic engravings in the history of photomechanical techniques. For example, the view made in 1854 of the Cathedral of Chartres, Central Window of the Royal Portal. The dimensions of this plate are 725 by 480 mm. The extraordinary quality of this print makes us admire Nègre's technique of uniformly spreading the asphalt on a plate of such dimensions and his mastery of revealing, of electroplating, of engraving and of printing.





Cathédrale de Chartres,
by Ch. Nègre.
725 x 480 mm.

Nègre exhibited his "heliographic engravings" for the first time in public during the Universal Exposition of 1855. He received the first class medal with a diploma drawn by Ingres, whose student he was. Nègre then received a large number of orders for heliographic engravings, from the architect-inchief of the restoration of Chartres cathedral, from the painter Corot, from the prince Napoleon Joseph Bonaparte and from Nadar.