From a recent email…. "I am studying photogravures and don’t understand the difference between photogravure and photo-etching. Can you clarify this for me?"
Embarrassed not knowing the answer, I turned to Jon Goodman, who replied….
Photogravure is an intaglio printing process where a continuous tone image (photograph) is etched into a copper plate by means of a gelatin resist and an aquatint or screen substitute. The gelatin resist controls the etching in a manner that creates a true continuous tone rendering of the image being etched. It is a continuous tone ink printing process. There is no conversion of the “grayscale” into “half-tone” dots. “Photo-etching” as the word is commonly used is an intaglio process where line or tone is created through what is essentially a black or white “half-tone” process. The etching process either etches the plate or not, there is very little (no) variability in the tone due to the uniformity of the depth of etch. Gray tones are either created by converting them to “half-tone” or by etching the plate multiple times for varying amounts of time to create different depths in the plate.
The gelatin resist used in photogravure is essentially a “Carbon Print” that has been transferred onto a copper plate instead of a piece of paper. It is the act of the transfer that allows the gelatin to control the etching in a continuous manner. Since the exposed “face” of the gelatin is in contact with the copper plate the hot water development allows the gelatin to adhere to the copper in thickness that is in proportion to the amount of exposure received. If a gelatin (or other) was simply coated onto the copper and then exposed (as in photo-etching) and developed (no transfer) it would be virtually impossible to render a long continuous scale of tones.