Photogravure on metal using a photoresist
Introduction
The principles of photogravure have been discussed in a previous chapter. We shall use a so called 'photoresist', i.e. a UV sensitive coat that can be sprayed on metal from a canister. This enables one to reproduce rasterized pictures or line graphs photorealistically in B/W on silver or other metal. In addition one can use black and white lines and areas, which would be more appropriate to enamellers. Use nitric acid or ferric nitrate to etch on silver, and ferric chloride to etch on copper. The procedure for rasterizing (screening) photographs, and of transforming images to black and white, have been previously discussed. Before working with silver it is suggested that you practice using copper first.
List of materials
- Computer
- Imaging software: CorelDraw® or Adobe® Illustrator®, Adobe® Photoshop®
- Epson Stylus Color 800 printer
- Océ InkJet Overhead sheets IJ130 (Art. 99896154)
- UV exposure unit or UV lamp
- Photoresist (Positiv 20 Photocopying Paint)
- Developer (Seno 4007 Universalentwickler)
- Rubber gloves, 1 litre bottle for developer solution
- Etching fluid (ferric chloride, FeCl3, solution in ½ litre or smaller bottle; or nitric acid/ferric nitrate)
- Copper or silver plaque
- Large plastic tray, small tray for developing and etching, a pair of tongs
- Detergent, ammonia
- Asphaltum varnish or beeswax or other varnish used in etching, acetone.
Required skills
It helps if you know how to handle computers and imaging software.
Applying the photoresist
- Select a silver (or copper) plaque of suitable dimensions.
- Carefully remove any scratches from the metal surface, polish it and clean it thoroughly. The latter can be achieved by washing in warm water to which a detergent and some ammonia has been added. Dry the metal with a clean towel, and from then on do not touch the surface with your hands. Copper should first be sanded with for example Scotch Brite and a liquid cleaner. If you pour water on the surface and it forms a continuous film even at the edges of the metal, then the metal has been properly degreased. Licking the surface with your tongue before you start enamelling is also an excellent way of degreasing a small object.
- The work place should be well ventilated and free of dust: if dust settles on the photoresist this will almost certainly lead to imperfections in the final result. Put the silver or copper plaque e.g. on top of a newspaper on a table. Shake the canister gently before use, and apply a thin but continuous film by spraying from a distance of about 20 cm (8"). Do not ever carry this job out near fire (do not smoke) or open flame, even in a well-ventilated space, because the material is highly flammable. Carry the work out in subdued light.
- Store the object at room temperature in the dark, again in a dust free environment so that it dries in about 24 h. It is much quicker to dry it in an oven at 70-80 °C (160-180 °F) in about 15 min.
Exposure
Expose the photoresist to UV light via a negative on very transparent material. We usually use computer generated images printed on some special overhead sheet. The generation of these sheets has been dealt with in a previous chapter.
The photoresist that has been exposed to UV light dissolves during the development process. Portions of the silver or copper object that need to be depressed should therefore be entirely transparent in the negative, and portions that should not be etched out need to be completely black on the overhead film.
- Put the silver or copper plaque with the photoresist facing up, and the negative facing down on top of it, in the exposure unit, and fix the whole firmly under clean, transparent glass. Check carefully that there is good contact between the photo resist and the negative.
- The exposure time must be determined experimentally. It depends on the amount of light emitted by the UV bulb, the distance between the lamp and photo resist, the thickness of the photo resist, the thickness of the overlying glass, etc. In our unit 3-4 min. exposure time worked out well. Do a trial run the first time: cover 1/3 of the negative with black paper, expose for 2 minutes; then move the paper to the next third and expose for another 2 minutes. Finally remove the paper and expose for 3 minutes. You now have a photo resist exposed for 3, 5 and 7 minutes.
Development
Use rubber gloves when handling developer (and fluids for etching). Take note of the safety precautions printed on the packing material or labels, avoid any skin contact and surely contact with the eyes. Keep the chemicals and solutions out of the reach of children.
- Prepare 1 litre of developing solution by adding the contents of 1 bag of developer chemicals to 1 litre of water at 50 °C (90 deg Fahrenheit.) while stirring. You can prepare your own developer by adding 7 g of caustic soda to 1 litre of water.
- Store the fluid in airtight bottles or containers.
- Work in subdued light. Put the silver or copper in a developing tray (protect the table by putting this tray in a much larger one) and add enough developer to fully submerge the metal. Regularly tilt the tray to spread the fluid. Stop the development when the photoresist exposed to UV light has completely dissolved.
In the case of overexposure development takes place very rapidly and the unexposed parts dissolve within 1-2 minutes. You may wish to dilute the developing solution if you want the whole process to take 5 minutes, so that you can quietly monitor progress and intervene, if necessary.
A pair of tongs, such as used by photographers, is very useful if you want to lift the metal out of the solution and rinse it off with water under the tap for inspecting progress.
- When finished wash it off with running water to completely remove any developer.
- Return the developer solution to a storage bottle, or to a container for disposal.
- Carefully rinse the gloves and the tray under running water.
Etching
First prepare a working solution. Always wear rubber gloves. Take note of the safety precautions. Work in a well ventilated space, because the fumes generated during the etching process are highly toxic.
Copper can be etched with nitric acid or with ferric chloride. Nitric acid tends to etch relatively fast; the disadvantage is that it etches in all directions, so that it also eats under the contours. Ferric chloride eats more vertically into the copper, keeping the contours cleaner. Use nitric acid to etch silver. If you do not want to work with these strong chemicals, apply galvanic (=electrolytic) etching.
- For copper prepare a solution of 1 part ferric chloride to 4 parts water. Store this safely in a carefully resealed bottle. Add ferric chloride to the water, not the other way around. For silver add one part nitric acid to 2 parts water (in that order: always add acid to water!); use a greater dilution if you want the process to proceed more slowly. With 'slower bites', such as for copper 1 part nitric acid to 4-7 parts of water, the whole process takes much longer, but the edges will be more clear-cut.
- Cover all parts that should not be exposed to the etching fluid with nail polish, beeswax, asphaltum varnish or other varnish. Apply fluid varnish with a brush, put granules or rub wax on the metal and let it adhere by gently warming.
- Put the metal in a small tray (which in turn you put in a larger one) and pour etching fluid over it so that it is completely submerged. Do wear rubber gloves.
- Tilt the tray regularly to mix and spread the fluid. Small (highly toxic, do not inhale the fumes) gas bubbles are formed during etching that locally retard the etching process. Remove them regularly with a feather. At regular intervals remove the metal plaque from the fluid with a pair of tongs, wash it off under running water, and inspect with your nail's edge whether the metal has been etched sufficiently deep; etching takes 2-4 hr, but considerably longer with 'slow bites'.
- When finished rinse the metal generously under running water, to remove all traces of etching fluid.
- Return the etching fluid to a bottle and seal it off, or dispose of it as chemical waste.
- Remove the asphaltum varnish with spirit of turpentine, the nail polisher with acetone, the wax by heating, and remove the photoresist with acetone.
- Thoroughly rinse the gloves and tray under running water.