Although a Line photogravure had been produced as early as 1827, it
was not until the 1870s that the method was perfected.
Tone Photogravures came into commercial use in the 1880s.
The process was similar to aquatinting involving the exposure of a positive
transparency image in contact with a sheet of light-sensitized gelatin.
The gelatin hardened according to the amount of exposure the various
highlights and toned areas received.
When the sheet was laid onto a very fine grained ground and immersed
in acid the gelatin acted as a variable acid resist enabling the acid
to exactly reproduce the tones of the original image on to the copper
When printed the image had the rich texture of an aquatint but with
the added detail of a photograph.