Gelatin hardened where exposed to light. Washed with cold water, dried. Gelatin served as resist for etching solution. Etching solution penetrated the soft unexposed gelatin and etched underlying metal. All printing processes benefited.
Printers learn how to reproduce the fine grain and tonal variations of photographic print.
Talbot principle of the halftone screen. Black cloth between a light source and a photosensitive steel plate. He had imaged the plate with fine dots of varying size. This method is used to reproduce photographic images in all printing processes. Auguste Godchaux received a patent for a reel-fed rotary gravure perfector press in 1860, press still in use in 1940.