1819-1824 > INVENTION OF PHOTOENGRAVING   • 1816-1818  • 1825-1829
	After the Gaïacum resin , Niépce used another resin but mineral this time : asphalt or Judea bitumen . He demonstrated that under light action this resin became non-soluble with his usual solvents . From 1822 on he succeeded at reproducing drawings put in contact with bitumen coated bases (glass plates , calcareous stones then copper or tin plates ). Then he used the aqua fortis process to etch with acid the images he made and printed them on paper . This process will remain for a while the base of photoengraving used to print photos and graphical documents
	> Principle and technique
	In order to reproduce drawings, around 1822-1823, Niépce conceived what we now call the contact print. He explains clearly how he applied varnish to the verso of an etching to make the paper translucid, and once dry he applied this etching directly in contact with the copper or tin plate coated with bitumen varnish . He exposed the lot in full daylight during three to four hours, then he rinced the plate in lavender oil diluted with white kerosene . The bitumen that had been protected from the effect of light under the lines of the drawing then dissolved and let appear the raw metal . On the other hand the light transmitted through the translucid paper had made the bitumen non-soluble and remained on the plate after the lavender oil rinse . The bitumen image was the drawing’s negative : the back is colored in the dark bitumen brown and the lines are represented by the raw metal . Then Niépce imagined a process that would allow to get the drawing etched in the metal . It was a well known and simple principle as it was the aqua fortis one . The plate carrying the Judea bitumen image is dipped in an acid bath that bites the metal where it was not protected, meaning the places corresponding to the lines of the drawing . Because the bitumen varnish is acid resistant, this one can penetrate down to the metal . Once the lines are etched in the plate, the inventor eliminated the bitumen varnish from the metal base to keep only the drawing etched on it . The first successes of this method can be dated to 1822 as far as contact reproductions are concerned , because this year he made a copy of Pope Pius VII portrait on a glass plate . This was not yet acid etched engraving . The earliest attempts of etching in 1823 are not on metal but on lithographic stones . A Dijon printer produced paper prints from those stones . So Niépce got the proof that his process allowed after contact reproduction to multiply an original through printing . In 1825 , Niépce etched his images on copper, then on tin from 1826. The acid process is perfectly appropriate to lines drawing reproduction where the gradations are represented by hatchings . In the case of images with continuous hues ,these one are reproduced by various thicknesses of bitumen that acid etching cannot render because the acid solution cannot permeate the varnish . Niépce understood this and he worked a lot to reproduce etchings . Many museums throughout the world preserve metal plates etched by the inventor with his process . The Niépce museum owns ten of those metal plates on which Nicéphore reproduced an engraving . Other Niépce etched metal plates are preserved at “La Societe française de Photographie", at “ The Royal Photographic Society “ or in Janine Niépce’collection. After all his repeated failures to etch continuous tones images obtained with a camera obscura , progressively Nicéphore gave up etching to stop around july 1827.