In the early 20th century, many superb prints were produced using photogravure printing. Photogravure printing was invented in 1879. The basic process is to photographically transfer an image to a metal plate, etch the image on the plate, then print from it.
Photogravure and gravure prints have warm blacks and an amazing range of subtle gray shades. Often sepia ink was used, giving the print an even warmer appearance.
It is quite easy to identify a photogravure print. Look at the print with a good magnifying glass, and you will see a characteristic honeycomb appearance. This is caused by the grid used in the printing process. The image also appears soft and the dark areas seem pitted. Occasionally a mixture of photogravure and half-tone occurs in the same prints, so there may be a dot pattern detected in some parts of the image, though this is not common and usually found in gravures from the 1920s and 1930s before the widespread use of offset lithography.