The technique I describe, teach and practise is the original 19th century Carl Klic / Fox Talbot photogravure process, utilizing a dustgrain aquatint.

Photogravure is a true continuous tone photographic etching process. The tonalities are created by an ink layer, gradually varying in depth, with a very fine aquatint (unetched islands of approximately equal size ) to hold the ink. This is achieved by etching the plate gradually from the deepest shadows to the brightest highlights. This way the photogravure is capable of producing a much wider range of tones than any other photographic process, from a deep velvety black to sensitive, bright highlights. Paul Strand considered photogravure perfectly suited for his work because of the technique's capacity to achieve the almost infinite tonal values which lie beyond the human hand. The finished plate is printed, as an etching, on a high quality paper. The ink is a stiff oil based etching ink of any tone or color. The result is an archival continuous tone photographic print on a paper of the artist's choice with the plate mark embossed, giving the final print an almost three-dimensional quality. However, the technical difficulties of this process can seem equally infinite at times. Ansel Adams once said that photogravure was a beautiful technique, but he would not recommend that anyone do it.

In contrast, most photographic printing techniques use halftones to simulate values with smaller or larger dots of the same tonality. They are used in the traditional halftone screens in newspapers, catalogues, magazines, postcards and art books. Modern halftones can also be created by the accumulation of equal size small dots as used in the stochastic screens of inkjet printers. The image appears lighter or darker depending on how much white paper shows or how much area is covered by ink.

Terminology

In the last decade photogravure has had an enormous comeback and has enjoyed increasing popularity. An internet search will come up with dozens of results. Upon closer look however, the term photogravure describes a multitude of techniques. It starts with the fact that in latin languages (Italian, French etc.) photogravure is the word for photoetching in English - referring to a halftone, not continuous tone image. In German, French and Italian, the word for photogravure is heliogravure (sun gravure).

Recently the development of various new photographic polymers (Image-on; Solar plates...) has given rise to polymer photogravure techniques, frequently just called "photogravure". In most cases they only approximate a true continuous tone with the use of the fine halftone dots given by the newer generation inkjet printers. Technically those are still just photoetching processes. The true continuous polymer photogravure techniques use a continuous tone film positive in the making of the plate and an exposed aquatint screen. The latitude of values in these prints however seems greatly limited. The plates are soft polymer plastic, which can not be reworked, nor hold up to a larger edition without damage. I expect more new developments in this field in the future, which could make certain applications of photogravure easier without great loss, but doubt they will replace the traditional photogravure anytime soon.

The term continuous tone has started to blur in the last decade as well. Traditionally continuous tone refers to a gradual change in values from black to white, much like an analog sound recording would record the actual shape of a sound wave. More and more people nowadays refer to anything that appears to most people's naked eye as continuous tone as such, even if it is only an extremely fine halftone. A look through a magnifying glass will in most cases clear the situation. Just as a digital recording is always only an approximation of an analog recording, the digital revolution has still a ways to go before believably imitating a true continuous tone.

Summary of the Process

A paper-backed gelatin is sensitized with a Potassium Dichromate solution and dried.
A full-size continuous tone positive is exposed to the gelatin. The light sensitive gelatin hardens under the influence of ultraviolet light gradually, in proportion of the amount of the light it receives:

Penetration of the light through the film positive into the gelatin.

The resulting hardening of the gelatin. More exposure will harden the gelatin deeper.

The gelatin is adhered with water to a clean, polished copper plate:

Exposed gelatin adhered to a copper plate.

Development is done in warm water. The warm water dissolves all the unexposed, unhardened gelatin and releases the backing paper (the melting point of the exposed gelatin has increased under the influence of light). The remaining gelatin leaves a negative relief of the image on the plate and will act as the etching resist:

Developed gelatin resist on a copper plate.

A rosin dust grain is applied and melted onto the dried gelatin (other aquatint methods using asphaltum or a mezzotint screen are possible as well):

Aquatinted photogravure plate ready to be etched.

Water from a Ferric Chloride solution has to soften the layer of gelatin before the copper plate can start to etch. The amount of water in the solution controls the speed at which the gelatin breaks down, and with it the final contrast of the image. More water will soften the gelatin more rapidly. During this process the thinnest areas in the gelatin (the shadow areas) break down first and start to etch.

Plate during etch.

Etching continues until the thickest layer of gelatin finally breaks down and the lightest highlights start to etch. The shadows have etched deep into the plate by the time the light areas etch to a light tone, allowing a wide range of gradations:

Plate etched to completion.

Once the plate has been etched to completion it is washed off and cleaned; the image is preserved and stable:

Etched photogravure plate.

The plate is inked up, and the4 surface wiped clean using a tarlatan and the hand. The aquatint has left unetched dots of copper which give the necessary tooth to the plate to hold the ink:

Inked up photogravure plate.

The ink is transfered in an etching press to heavy printing paper that has been softened with water.