: The design is reproduced multiply onto a glass plate, usually (but not necessarily) with a step & repeat camera.
This plate, termed the "Multipositive", is used to print a carbon, a sheet of gelatin sensitized with dichrornate & backed with paper.
An image is not visible, but the areas more strongly exposed are proportionately hardened.
The carbon is then applied to a metal (copper) printing cylinder, & allowed to st& long enough to develop adhesion. The cylinder is washed with water, which not only removes the paper backing, but dissolves the unhardened parts of the gelatin as well.
The cylinder is then etched with ferric chloride solution, which attacks the metal in proportion to the degree that it is not protected by hardened gelatin. Afterwards, when all of the gelatin is removed, a printing surface with the design in recess remains; it is usually given a light chromium plating before use.
In other meaning, Photogravure is a kind of recess printing, in that the ink is contained in recesses in the printing plate.
Unlike Line Engraved recess printing, where the recesses are produced mechanically, the photogravure plate has its recesses produced by etching. The steps involved in this process are:
1) Finalisation of the accepted essay.
2) Normal negative photograph of the approved design.
3) Multipositive of 100 stamp images.
4) Carbon print of the multipositive on thin gelatin tissue, for transfer to the printing plate.
5) Chemical etching of the plate, producing recesses in the form of positive images (i.e. the deeper recesses represent areas to be dark in tone).
6) The large format stamps result from processes starting with a larger multipositive plate.
* The foregoing explanation leaves out one essential step: Screening
Screening is necessary to break up the large solid & continuous tone areas into small dots so that the printing surface will be able to hold ink in the proper places.
There are two types of screening: Grid
(Mesh), & Granular
In the "Grid" screening, the carbon is exposed to a plate bearing an image of a grid of fine lines to imprint the grid on the carbon before the "multipositive" is imprinted.
The eventual result is that the metal cylinder is etched with a pattern of tiny pits, or cells, the depth of which is in proportion to the amount of light transmitted at each point.
The light areas of the stamp design have shallow cells, which thus retain very little printing ink, & the dark areas have deeper cells.
If the screen is fine the cells will be small & numerous & an appearance of continuous tone is conveyed in the printed image.
A "Mesh" (Coarse) screen can be seen easily with the naked eye, a very fine screen may be difficult to detect even with a magnifying glass. The dots that make up the image are always evenly spaced & of the same size, except insofar as they may tend to merge in the darker areas because of ink spread.
A Corn-Grain screen consists of irregularly scattered dots of irregular size & shape, & is produced by dusting a surface with a powder, usually resin or bitumen.
This may be done on a glass plate, which can then be used in the same way as a grid screen, but the earlier procedure. The grains of resin thus protected parts of the cylinder from etching.
The printing result is the same in each case, colored areas of the design have a mottled, Granular appearance, which if it is fine gives softness to the image.
The Half-Tone (Coarse) process also makes use of a screen to break up the image into a mass of dots. However, the screen is the reverse of the grid screen & consists of a pattern of square dots.
Instead of cells, the printing surface consists of dots that are high (i.e. at surface level) in the case of Half-Tone relief printing, & essentially so in the case of lithography.
The dots vary in size with the intensity of the light coming through the screen, dark areas of the stamp design having larger dots (which may even run together).
The result in the printed stamp is a simulation of continuous tone, as with "Photogravure", but the nature of the dots is different, & the effect not so delicate, for the dots all receive the same amount of inking.
Half-Tone may be used in the preparation of plates for relief printing, lithography, & offset-lithography.
Hope it helps