Photogravure is an intaglio printmaking process in which a copper plate coated with a light-sensitive gelatin tissue is exposed to light which is shined through a film negative (just as is done when printing a photograph.  The light degrades the gelatin tissue.  The plate is then washed with acid.  The acid etches the copper where and to the extent that the light impacted the gelatin.  It reproduces very well the deep and continuous pools of gray and black in the photograph.