In the early 20th century, some superb artworks were produced using photogravure printing, including the fascinating magnified photographs of plants by Karl Blossfeldt.
Photogravure printing was invented in 1879 and 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 charming range of subtle shades of gray.
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, as seen below.
... and in this magnified close up of a photogravure
The images above are a photogravure by Karl Blossfeldt, and a portion of it magnified fifty times. Notice the characteristic honeycomb appearance.
Photogravure is rather a complex process with several variations, but the essence of it is as follows:
This is done by coating a plate with light sensitive gelatin and then exposing it to the light from a photographic image (imagine shining a slide on it). The gelatin hardens most where most light falls, and remains softest where no light falls. So, if the gelatin is exposed to a negative photographic image, the darkest areas of the image will remain softest. These correspond with the lightest areas in the original photo.
This grid remains flat and un-etched in the next step while the square openings can be etched down to different depths according to the hardness of the gelatin.
The acid eats away the softest gelatin fastest, so it eats deepest into the metal plate where the gelatin was softest i.e. where it was exposed to least light. So, if the image was a negative, the least light comes from the darkest areas. In the actual photo, these would correspond to the lightest areas.
Most ink goes into the deepest etched pockets, less into the shallowest. The plate is then wiped to remove surplus ink.
The areas with most ink print darkest. In later years, this step was automated with machines, but in the early days it was done by hand.
If you are interested in photogravure prints, we highly recommend the ones by Karl Blossfeldt.
Nowadays, the process is undergoing something of a revival as computer and laser technology has driven down the cost of producing the metal printing plates. In fact, these machines have opened up many interesting, innovative uses for the process of photogravure, or photo etching as it sometimes called these days.