8. Charles-Guillaume Petit : Halftone Engraving

Charles-Guillaume Petit is the inventor of one of the most important printing techniques, that of transforming the halftones of a photograph into a system of regularly spaced points or cross-hatching that produce printed surfaces of varying dimensions. Petit called the system 'halftone, engraving' (similigravure). It is one of the first techniques to produce halftone shading without departing from traditional printing methods.



The set of points, or screen, was made from an asphalt produced image (Figure above). Petit explains: "Using a knob mounted on my engraving machine, I incised zigzag lines of alternating points into a copper plate and I pulled an engraved print from this master plate. The points were black on a white paper background. I photographed it reducing it about a third. With the negative I made a positive image using asphalt on copper plates later to be engraved (a and b). The plates bore a matrix of evenly spaced points (b) and could serve many purposes. All that needed to be done when an order came was to cover them with bichromate gelatine, to make a negative of a given subject (c) and to engrave using iron perchloride which filtered through the gelatine layer as is done in helioengraving (e).

The larger white areas were bitten into first, thinning the specks of asphalt into pinpoints. The pale halftones were done next and so on up to the darker halftones". For these last halftones, stopping the action of the chemical in time left the size of each point unchanged. As chemical action was prolonged, the circumference of each point was decreased to a pinpoint when the halftones were pale. As for white, the points disappear. The process took about three hours.

The photochronographs of Jules Marey, a precursor of cinema, are one of the first examples of images produced by Charles G. Petit's process. These were published in the Comptes-Rendus de I'Académie des Sciences on 3 November 1890. After having exhibited his photographs, Marey declared: "What remains to be done is for people to make faithful reproductions of them and to print a number of copies. The consistent progress made by photoprinting leads us to hope that its use will enable physiologists to show the phases of movement with complete fidelity. The figure opposite, made by Mr. Charles G. Petit from a photochronographic view is an example of what can be obtained using his method".

Petit's halftone print reproduction of Nicéphore Niépce's La Table Servie is another example. This appears in the text of the symposium during which A. Davanne unveiled the existence of this picture.