Engraved copper printing plate and positive print on paper: Les Moissonneurs dans les marais pontins (Harvesters in the Pontine Marshes) by Louis Léopold Robert

In 1843 Hippolyte Fizeau registered a patent for a photographic engraving process enabling the printing of a daguerreotype without retouching. The daguerreotype is treated with an acid that acts on the dark parts of the image without affecting the light areas. The second procedure is the further deepening of the dark areas with linseed oil, as in intaglio printmaking. The plate is then electromechanically gilded. With its prominent parts now protected by the gold coating, the surface is again treated with acid. Now transformed into an engraved plate, the daguerreotype is delivered to the printer, who can then print a large quantity of copies on paper, comparable in quality to the results obtained by aquatint printmaking. Fizeau presented his process to the Académie des Sciences, emphasising the quality of the prints, which nevertheless depends on the quality of the original daguerreotype.