PHOTO-ENGRAVING. One of the photomechanical processes, the generic term being applied chiefly to the making of etched metal printing plates or blocks (zinc, copper, brass, etc.), the design being in relief for typographical (letter-press) printing. Parts of the process are also used in the photo-lithographic and rotogravure processes for the production of plates and cylinders.
Photography was not originally used in their production, the image on metal being obtained either by manual design or by lithographic transfer. The introduction of the photographic nega tive permitted the reduction or enlargement of the original de sign for reproduction.
"Nature Autotypy," an early reproduction process, furnished copies of natural objects which, while imperfect reproductions in the sense of our present standards, were still good enough to con vey the image in multiple editions. Leonardo da Vinci describes the method in his "Codex Atlanticus" (1490-1519): "This paper must be coated with lampblack mixed with sweet oil. The color of the leaves is thinned by applying white lead dissolved in oil, as printers do with typefaces and then it is printed as usual. The leaves (i.e. the impression) will appear dark in the depression and light in the raised parts. . . ." Several botanical publica tions of the late 57th and of the 18th Centuries were illustrated by this method and are still preserved.
Niepce's early attempts to etch photographic intaglio im ages on metal sensitized with asphaltum initiated but did not hasten progress in the photomechanical processes. As early as 1841 daguerreotypes were etched in galvanic baths for printing on gravure presses. Donne was the first to show proofs of etched daguerreotypes; Fizeau, Claudet, Berres in Vienna and Grove in England also etched them. Paul Pretsch in London etched them in 1856. He was the originator of the later spray or blast system employed today in etching and was also the first to discover the swelled gelatin process of which many splendid examples are still in existence. The negatives for his work were usually made by
the photographer Roger Fenton, using the wet collodion process; the printing plates were electrotypes moulded from the hardened gelatin reliefs.
As early as 1862 Pretsch exhibited at the London Exhibition half-tone plates for letterpress printing. Poitevin, independent of Pretsch's work, pursued the same idea and produced relief electrotype printing plates from chromated gelatin originals. George Scamoni in Russia experimented and succeeded inde pendently in the same process. Swan and Woodbury in England produced the Woodburytype (1863).
In the development of reproduction processes, it was soon realized that the prime necessity for faithful reproduction con sisted in finding a means for breaking up the continuous tones of photographs or drawings, reproducing in effect the quarter, half, and three-quarter tones of the subject. Graining and dusting of the printing surfaces, long employed, did not furnish the answer. Talbot recommended as early as 1852 in his gravure process the use of a gauze netting for breaking up the continuous tones into separate printing elements.
As in most inventions, the credit for the invention or discovery of the half tone process of photo-engraving cannot be awarded to any one man.
A one-line glass screen in which parallel lines were scratched on an opaque background is described in a French patent of De cember 14, 1857, by Berchtold. Burnett published a method of single- and cross-line screens for photography. Screens are men tioned in a patent of 1865 by the brothers Bullock. Egloffstein's patent of the same year can hardly be cited as his lines were engraved on steel and his process met with no success. Swan in 1866 also used line screens and so did Leggo in the United States in 1871, Jaffe in Vienna in 1877, Stephen H. Horgan in 1880. Meisenbach (1882) used a single-line screen which he turned at 9o° after half the exposure was made.