Photogravure, from Alan Smith's book "Etching a guide to traditional techniques"
Photogravure is the only form of photo-etching that uses a continuous tone film positive. The positive is exposed onto a light- sensitive film of gelatin. This gelatin is called 'carbon tissue' and is made light-sensitive using a photosensitizing solution. The photosensitizing solution is made with distilled water and any of the following: ammonium bichromate, ammonium dichro- mate, potassium bichromate or potassium dichromate. All four are interchangeable. They are also all equally harmful. Do not breathe in the dust. Do not allow dust to enter eyes. Avoid skin contact. Wear gloves, goggles and dust mask when handling the powder. Wear gloves and goggles when handling the liquid.
Make the solution and sensitize the carbon tissue in a darkroom under yellow or red safe-light. An aquatint is used so that the different tones in the positive will print as greys and blacks. The gelatin layer is thinnest where the film positive is darkest and thicker in the lighter areas. The copper plate is etched in a strong solution of ferric chloride until the black areas have part- etched. The plate is then put into a second bath of ferric chloride, which is made weaker than the first by the addition of water. As gelatin is water-soluble, the thinner areas are dissolved allowing them to be etched. The plate is then put into another bath of ferric chloride that is weaker again. This dissolves the medium thickness of gelatin and starts to etch mid-grey areas of the image. Finally the plate is put into the fourth and weakest bath of ferric chloride to etch the very lightest tones. Photogravure has been used since the 1850s.
Method to make a photosensitizing solution
To make 1 litre (pint) of photosensitizing solution:
950ml (18 fluid oz) Distilled water
100g (2oz) Ammonium dichromate.
Put the water into a measuring jug. Gently add the ammonium dichromate, stirring all the time. For best results the solution should now be filtered. Place a coffee filter paper in a funnel and put the funnel into a correctly labelled bottle. Pour the solution into the bottle through the filter paper in the funnel. The solution is now ready to use.
Sensitizing the Pigment Paper (carbon tissue)
In a darkroom under yellow or red safe-light pour the ammonium dichromate solution into a clean photographic dish. Put a piece of carbon tissue into the solution and gently rock the dish for 3 minutes. Carefully lift the wet tissue out of the solution by the two corners of a short side, and lay it face down onto a clean piece of perspex. Use a squeegee to get rid of any air bubbles and excess solution. If there is excess solution on the edges of the perspex, blot it dry using newsprint. Leave the tissue on the perspex to totally dry in the dark. This should take about 4 hours. This can be speeded up to about 1 hour by using a warm fan. When the tissue is dry it will easily peel off the perspex from a corner.
Before using the tissue, store it for 24 hours in total darkness. A photography film bag is good for this. If stored at a low temperature it will keep for up to a week. Pour the ammonium dichromate solution back into its correctly labelled bottle through a clean filter paper. If stored in cool, dark conditions it will keep for up to a week. One litre of this solution is enough to sensitize several sheets of A4 carbon tissue.
Exposure to a Ultra Violet light source
To expose the sensitized carbon tissue, place it face up in the exposing frame and then place the continuous tone film positive on top of it emulsion down. Expose the tissue. Exposure times will vary depending on the density of your positive. Common exposure times using a 2kW lamp at a distance of 600mm (24in) vary between 1 and 3 minutes.
Attachment of the exposed carbon tissue to the copper plate
Now take your filed, polished and degreased copper plate and put it face up in a shallow bath of room temperature (between 16 and 20°C) distilled water. Remove it from the bath and place on a flat surface so that it has a thin film of distilled water on its surface. Dip the exposed carbon tissue, face down, into the same water. Making sure that there are no air bubbles on the front of the tissue, place it gelatin-side down onto the wet plate. If you find it too difficult to handle the wet tissue without it folding in on itself, you can leave the copper plate in the water bath and use it as a support to lift the tissue. Hold the tissue on the plate by the two corners of the plate that you take out of the water first. Use a squeegee to get rid of any air bubbles and excess water, then blot the plate dry with newsprint, front and back. Leave to stand for 5 minutes.
Peeling off the backing paper and removing the soft, unexposed gelatine.
Place the plate into a bath of water about 45°C (115°F). After about 2 minutes peel off the paper backing, gently and slowly. Once the paper backing is off, gently rock the bath so that the warm water is moving evenly over the plate. Carry on doing this until you can see that the gelatin has come away from what will be the black areas of your plate. This should take between 6 and 10 minutes. Move the plate to a tray of cold water and agitate, by rocking the bath, for between 1 and 2 minutes. This will stop the gelatin from developing any further. Take the plate and put it into a solution of methylated spirit with a little distilled water in it (about nine parts meths to one part water). Leave to soak for about 5 minutes, without agitating. The methylated spirit helps to dispel water from inside the gelatin. This makes the gelatin harder and more acid-proof.
Applying a rosin dust aquatint
Leave the plate to dry. This is best speeded up using a hairdryer. Once the plate is totally dry, apply a rosin aquatint. Use an aquatint box. A small to medium dot-size aquatint works best, as the grain on a large-dot aquatint will interfere with the information that is on the positive. You should be trying to cover the plate evenly all over with about a 50 per cent dusting of rosin, as with any standard aquatint. Allow the plate to cool, and leave it uncovered for between 1 and 3 hours. Humidity in the atmosphere will be drawn into the gelatin and replace the moisture that was evaporated when the aquatint was fused on. It is best to leave the plate in the acid room in which it is going to be etched.
Preparing etching baths
Take four acid trays and pour ferric chloride into each of them. The first tray should contain 45° Baume ferric chloride. The second tray should contain 43° Baume ferric chloride. The third tray should contain 41°, and the fourth 39° Baume ferric chloride. Baume is a unit used to measure specific gravity. All four baths should be the same temperature as each other. It is best to mix the different strengths of ferric at least one day before use.
In general, photogravures need to be etched for a total of between 8 and 25 minutes. Etching times depend on the temperature, the dot size of your aquatint and to a lesser extent on the area of the plate that is being etched. The higher the temperature, the faster the etch. The smaller the aquatint dot the less time it needs in the ferric, the larger the dot the longer it needs.
Paint out the edges of the plate outside of the gelatin with stop-out varnish. Stop out any gelatin borders outside of the image area. Let the stop out dry without heating the plate. Put your plate in the 45° Baume bath. Within the first minute or two, you should see the uncovered areas of your plate getting darker as the copper etches. If it does not start to get darker fairly quickly, there is probably a thin layer of gelatin over all the plate. In this case, put the plate in one of the weaker solutions until signs of etching can be seen. As soon as you see any etching, return the plate to the 45° Baume bath. At no time during the etching of a photogravure do you rinse the plate in water between the different strength baths. When you think that the areas that are etching would print a dark/mid-grey, transfer the plate to the 43° Baume bath. When you think that the new areas that are etching would print a mid-grey, move the plate to the 41° Baume bath. The plate should need progressively less time in each bath. When you can see that new areas have started to etch, and would print as a light grey, move the plate to the 39° Baume bath. When your lightest tones have etched, remove the plate from the ferric chloride, and rinse it well under running water. Remove the stop out, then degrease the plate with whiting and ammonia to remove the gelatin.
Print the plate.