Heliogravure (héliogravure au grain, photogravure, photoaquatint, photogravür) is the most sophisticated among the photo-mechanical processes, socalled because to obtain a print there is a ‘photo’-graphic part transferring the image, plus a ‘mechanical’ part, owing to a hand pulled press to print the image on the paper.
The print matrix is made of a copper plate prepared, grained and etched in the same way as in the technique of aquatint. The only difference is that the transfer of the image onto the plate occurs photographically instead that manually.
To obtain the printing matrix we will start from any negative film or modern digital file and produce a positive film in a darkroom, or throughout an ink-jet printer, or from a photolitho service. The film must fit the process for density and tonal range and be of the same size of the final print.
Through the film we impress with a UV lamp a gelatinized, rather porous, colored — and sensitized at use — paper, called ‘carbon tissue‘ (pigment tissue, pigment paper) .
The ‘carbon tissue’ after exposure is dampened and placed face down onto the surface of the polished copper plate so it can transfer a colored jelly image with different thikness to reproduce the negative of the original lights and shadows.
The dry plate is settled in a dust-box for acquatint to be covered with an uniform surface of bitumen powder, then put in a oven to melt the rosin grains and make the resist for the etching.
The etching (or ‘biting’, with ferric chloride solutions) permit to obtain the rich intaglio and is driven visually, peeping at the biting of a step-wedge densities — drowned side to the plate — and writing the progression on a form for constant check.
At least 4 baths at different concentration of etching salt are requested to pierce the thicknesses of the gelatine layer.