Prepare Positive Transparency   Sensitize Resist   Expose Resist

Adhere Exposed Resist to Copperplate   Develop Resist   Etch Copperplate

Original prints are made by transferring ink to paper from three different types of plates: <1> relief, with the ink above the plate surface (woodcuts), <2> planographic, with the ink on the plate surface (lithographs and silk-screens), and <3> intaglio with the ink under the plate surface (including engraving, drypoint, etching, aquatint, mezzotint, and photogravure). Each medium has its own signature look.

In a gravure print (also known as photogravure, héliogravure, or gravure à l'aquatinte), the tones and variations in light and shadow, and the textures of various surfaces, are the distinguishing features. Under magnification, the aquatint grain of a photogravure prints is visible. The gravure printmaking technique I use is based on the inventions of W H F Talbot and Nicéphore Nièpce in the 1830s. The history of this technique and its relation to printmaking in Europe is sketched in my Inklings essay. Following is a description of how to make photogravure prints.

How to Make Photogravure Prints

Prepare Positive Transparency

1. From a well-exposed photographic negative, make a transparency on graphic arts film. Enlarge or contact-print to the size of the intended final image.
2. Develop the graphic arts film in any standard film developer, or with developer mixed according to this formula which allows contrast to be varied.
3. If a densitometer is available, expose and develop the graphic-arts film so that there is a density range of at least 1.5, for example a highlight density of 0.2 and a shadow density of 1.7. (Density is a numerical expression of opacity, on a scale of zero to three, of how much of the light is blocked.)
4. Remove dust spots and any distracting highlights from the positive transparency.
5. Mask the transparency with red lithographers' tape to create a 'safe edge' about 15 mm wide around the image area. Press the tape down from the center outward so that there are no creases or air bubbles. Pierce any air bubbles with a pin.

Sensitize Resist ('Carbon Tissue')

1. From carbon tissue supplied in 100-meter rolls, place weights on unrolled end and cut to size, making sure the gelatin coating does not crack.
2. Using cotton or synthetic gloves when handling the carbon tissue, cut pieces that are 1 ~ 2 cm larger than the transparency and 1-2 cm smaller in all dimensions than the copperplate.
3. Storeflat between stiff boards,the cut sheets of unsensitized carbon tissue that will not be used immediately. Secure with clips, wrap in foil or some other vapor barrier, and store it in the refrigerator if possible. Unsensitized carbon tissue can be stored this way for up to two years.
4. Under an amber safelight, mix the sensitizer according to this formula and pour it into a tray which is used only for this purpose.
5. With rubber gloves on, place a cut sheet of the resist face up in the tray, noting the time. Hold the corners and edges down until it is fully immersed and lies flat (about one minute).
6. Pick up the carbon tissue by one corner and turn it face down, sliding it smoothly into the tray.
7. After 3.5 minutes have elapsed since first immersing the carbon tissue in the sensitizer solution, pick up the resist by one corner and let it drip onto a piece of clean plexiglass.
8. Holding two diagonally opposite corners, first set the center of the sensitized resist face down on the plexiglass, then set the corners down so that no air is trapped between the surfaces.
9. Squeegee from the center outward with sufficient pressure to remove excess sensitizer and adhere the resist to the plexiglass, but not so hard as to weaken or distort the delicate gelatin.
10. Dry the sensitized resist evenly by directing the air current from a fan over (not onto) the back of the resist. After about two hours (more if humidity is high), gently peel the sensitized carbon tissue off the plexiglass, making sure the surface does not crack.

Expose the Resist

1. Handle the sensitized resist only with cotton or synthetic gloves. Use an amber safelight.
2. Test UV exposure with step wedges (transparencies with a series of known densities) to indicate the proper exposure for a given UV light source. The test procedure is the same as the procedure for an actual transparency as described below. The exact time of ultraviolet exposure depends on the spectrum, intensity, and distance of each light source from the resist surface.
3. Adjust UV exposure as needed for aging of the carbon tissue, density of the transparency, sensitizer concentration, the order in which the resist has been soaked in sensitizer, and whether a light-toned or dark-toned gravure print is desired.
4. Position the masked transparency over the sensitized resist in the vacuum frame, taking care that the entire image area is covered.
5. If a screen is used as aquatint grain, expose the screen 1.5X the image exposure, to ensure that the 'lands' of the plate will be more prominent than the brightest highlights of the image.
6. If an aquatint grain is used, expose the transparency only.

Ultraviolet light (UV) sets off a chemical reaction that hardens or crystallizes the gelatin. When exposed in contact with a transparency (Figure 1), the shadows block most of the ultraviolet(UV) light, leaving most of the gel soluble. Under the transparency's highlights which admit more UV, however, the gel hardens more. Metal-halide light sources give more consistent results and are safer to use than carbon-arc lamps.

Figure 1. Exposing the Resist

 The aquatint grain creates microscopic 'lands' on the copperplate which remain unetched. Particles of asphaltum are allowed to rain down on the copperplate and are then fused over high heat until they flow into an amoeba-like shape, as shown enlarged in Figure 2.

Figure 2. Aquatint grain (from Johan de Zoete, A Manual of Photogravure)

The unique look of photogravure depends on the variable depth of the etching, the ability of the copperplate to transfer far more ink to the shadows than to the highlights of the print, and to register subtle gradation of tone. What enables the plate to be etched to various depths is the linear sensitivity of dichromated gelatin to ultraviolet light. Unlike conventional silver-based photographic materials, dichromated gelatin is sensitive to every nuance of light intensity in equal measure. This enables it to register highlight and shadow detail not found in conventional photographs. The gelatin resist is partially permeable: It allows the etchant to seep through depending on how thick or thin the gel is. Where the gel is thin, etching is deep, and where thick the etching is shallow. The deeply etched parts of the plate hold more ink and form the dark parts of the gravure print, the lightly etched parts less ink, creating the highlights.

<3> Cross-linking of UV-exposed Resist

Molecules of gelatin activated by UV light bind to one another in long chains through a mechanism known as cross-linking (Figure 3). Cross-linking makes the resist more or less insoluble depending on how far it extends through the gelatin. The amount of cross-linking, remarkably, tracks the amount of UV precisely, even at the extremes of light and darkness. While ordinary photographic films and papers miss the near-whites and near-blacks because of their uneven sensitivity, UV-sensitive materials register the finest gradations of tone faithfully.

IV. Adhere Exposed Resist to Copperplate

1. Remove any scratches from copperplate by scraping and burnishing with plate oil.
2. Polish copperplate with metal polish.
3. De-grease copperplate with fresh sodium hydroxide solution, followed by a fresh acetic acid / salt solution, wiping each time with a clean rag.
4. Dry copperplate quickly so that there is no tarnishing, and place it conveniently to hand.
5. Soak exposed resist in distilled water for 1.5 minutes.
6. Immerse the clean copperplate in 27° C. distilled water.
7. Quickly adhere one edge of the exposed resist to the copperplate, pull both out of the warm water, and squeegee the resist from the center outward until it is firmly adhered to the copperplate, making sure no air is trapped between the two surfaces.
8. Remove excess liquid from the back of the adhered resist, and let dry ('cure') under pressure for two hours.

V. Develop the Resist

1. Immerse the plate in warm (27° C.) water, gradually increasing the temperature to 41° C. until the backing sheet is loose enough to peel off without dislodging the resist.
2. Peel off backing sheet slowly.
3. Agitate plate gently in 41° C. water for about five minutes until all the unexposed resist is washed off. The 15-mm 'safe edge' prevents the edge of the image area of the patterned resist from washing away.
4. While the copperplate with the now-patterned resist is still immersed, cool the water gradually by adding cold water; then remove the copperplate and place it in a tray with a 50% alcohol / 50% water solution for 30 seconds.
5. Remove the plate from the alcohol / water solution, hold it vertically, and immediately rotate it while drying the edges with a paper towel or rag. Do not allow water to flow back onto the image area. Keep rotating the plate and drying the edges for 10 or 15 minutes.
6. Set the plate in a vertical position, and rotate it 90 degrees every 15 minutes for one or two hours, to prevent uneven drying of the resist.
7. With a 10X or 15X loupe, inspect the plate for flaws such as bubbles, blisters, under-exposure, over-exposure, resist breaks, mottling, poor adhesion, etc. Unless flaws appear easy to correct after etching the plate, scrub off the patterned resist and start over.
8. If the inspection shows a well-modulated range of tones so that both shadows and highlights can be etched while retaining full detail, and there are few or no flaws in the patterned resist, let it dry in a dust-free room for about 15 to 20 hours.

Figure 4. Patterned Resist on Plate

The dried resist looks like a laterally reversed negative (Figure 4), light in the shadows (where the resist is thin) and dark in the highlights (where the resist is thick). It is actually a cast of the image that shapes the action of the etchant in the next step. To prepare the plate for etching, mask the image area and the back of the plate with packaging tape. The print made from this plate is Hokokuji, showing a bamboo garden in Kamakura.

Figure 5 shows an enlarged cross-section of the copperplate ready for etching, with aquatint grain or screen pattern below the permeable gelatin resist. A random-pattern screen may be used instead of the aquatint grain to create unetched 'lands' on the plate.

<5> Pre-etch copperplate

VI. Etch the Copperplate

1. Mask the image area of the patterned resist on the copperplate with packaging tape, and apply stopout to any breaks in the resist. Where the tape meets at the corners, smooth it down so that no etchant can get through.
2. Prepare a series of ferric chloride solutions ranging from 45-Baumé to 37-Baumé (a measure of concentration) at 20° C. (68° F.). Dilute the solutions, usually supplied as 45-Baumé, with distilled water, allowing time for the water and ferric chloride to mix. Pour the etchant into four or five trays arranged from most to least concentrated.
3. Place the positive transparency where it is visible, and identify the darkest areas, midtones, and highlights, in order to follow the progress of the etching.
4. Wearing rubber gloves and alab apron, immerse the copperplate in the most concentrated (highest Baumé) solution of ferric chloride and note the starting time.
5. Move the plate to the next, more dilute solutions in sequence, watching for etching to begin. Etching begins when the ferric chloride seeps through the resist and touches copper, causing a flow of dark precipitate. Note the time when this shadow etching begins. Typically shadow etching begins after five to 15 minutes.
6. If no new areas of the plate are being penetrated, move the plate to the next, more dilute solution to start midtone etching, and note the time when that begins.
7. Try to pace the etching so that the shadows etch for at least 10 minutes, preferably 15 minutes, while the brightest highlights etch for two minutes or less, with the total etching time between 20 and 30 minutes.
8. Let the highlights etch only briefly in the most dilute solution of ferric chloride. If the resist was over-exposed enough to slow highlight penetration, try breathing on the plate; the moisture in the breath helps the etchant to get through stubborn resists.
9. End the etch by immersing the plate in cold water and moving it around to remove all the etchant.
10. Scrub off the resist and aquatint grain (if any), and clean the plate with alternating baths of sodium hydroxide (NaOH, a strong base) and a solution of acetic acid mixed with salt.
11. Inspect for etching flaws with a 10X or 15X loupe.

Figure 6 shows in enlarged cross-section how some parts of the plate are etched much more deeply than others.

Figure 6. Etched Copperplate

Information provided by: http://www.kamprint.com

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