Some notes from the book by Dan Welden and Pauline Muir.
Digital Images and Photogravure
Digital images are printed on to transparencies as halftone images before transferring to a solar plate. The clarity and definition of a halftone image is called the resolution. The resolution is measured by the number of dpi or lpi. In commercial printing the resolution is usually described in terms of lpi. E.g. newspaper images are printed at 85-100 lpi, while high quality magazines usually have a screen ruling of 150-175 lpi.
Making a relief digital plate is very similar to other relief plates, but requires extra care in development and printing. Because of the very fine dot structure of the plate you have to wash away the residue very thoroughly without dislodging any small printing elements. When printing, apply ink in very thin layers with a good quality medium roller in order to avoid filling in the tiny cells in the polymer. Plates made with film with a screen ruling greater than 110 lpi become increasingly difficult to print since the plate filles with ink often after only one or two impressions. Manual presses and rollers are not sensitive enough for such fine plates.
For an intaglio print you require a positive halftone film. For the best results always print out images on a printer at 600 dpi or at the highest resolution available. You can also rework the image by scratching and sanding the toner, by drawing, and by adding solvents. Making and printing an intaglio digital plate is the same as for other intaglio plates. You can use either the single exposure or double exposure technique for making plates, but the double exposure technique will minimise the dot structure for images printed froma personal printer. When printing, experiment with different inks, the consistency of the inl, and the dampness of the paper, as for any intaglio plate. In general, digital intaglio images print darker than the original screen image and you can compensate for this by reducing the darkness of the original by up to 10%, but you may need to do a number of tests to reach the desired effect.
If you are planning to make mulit-plate colour images, you have to consider the screen angle. The screen angle is the angle at which the grid of dots on a conventional halftone film is printed. For a single b&w image the grid is aligned at 45 deg to the horizontal. If you are making a two-plate print the second transparency must have a different screen angle to avoid generating moire patterns that superimpose and interfere with the final printed image. For example, for a two-plate print, the grid of dots of the film representing one colour is printed at a 45 deg angle, while for the second colour, the grid is printed at a 15 deg angle.
Photopolymer gravure
Trad photogravure attributed to Viennese printer Karl Klic in 1878.
A continuous-tone film positive is usually more opaque than a digital or copy machine transparency and allows the film to withstand a longer exposure time to UV light. It also eliminates undercutting, the problem of UV light diffusing under the opaque film, which results in a lighter image and/or softer images, and appears in prints when using 'thinner' or less opaque transparencies.
Solar plates, by their very nature, tend to create an image with a higher contrast than trad copperplate gravure. You can manipulate the development of the film positive to maximise the tonal range. This will minimise a high contrast image on the solar plate.
Another consideration is the density (how opaque or transparent) of your film positive which determines the exposure time of a solarplate: the less opaque the positive, the less time required. The density can be measured with a transmission densitometer, and film positives need to be in the range of .55, to achieve the greatest details in highlights, and 1.40, to retain shadow detail.
Make a solarplate using the double exposure technique, and do test exposures for each film positive, since each positive will have a varying degree of opacity. For example, test exposure times can vary by as little as ten seconds and make a difference to the highlights and shadow regions of the image. (not clear if this under the sun or using UV box)
Proofing and editioning a photopolymer gravure plate is a more delicate operation than printing other intaglio solarplates. To capture maximum detail and tone only use printing papers with a fine texture and smooth surface. Soak sheets for a minimum of 25 minutes for even hydration and flexibility, and print when the paper is fairly damp. Experiment with the opacity of the ink by adding transparent base, and add #1 lithographic varnish to loosen the ink. When working with tarlatan and newsprint, always wipe from light regions towards the darkest areas. If necessary, hand-wipe to define details and brighten the image. For printing use three blankets, but you may find with an additional sizing catcher and greater pressure you can capture the finest detail.
