Etch: Photopolymer Techniques
Friedhard Kiekeben’s Intaglio Manual, 1994 – 2025
Line art | Exposing the plate | Making up developer | Developing the plate | Etching and stripping the plate | Aquatinting | Using rasterized positive | Intaglio Type tonal work | Etching
With photopolymer etch techniques, the UV sensitive film allows the transfer of a positive image onto a plate and functions as a mordant resist during the actual etching process. As this method is a proper intaglio etching technique i.e. it involves the eroding of metal plates, it can be combined with virtually all other acrylic based etching methods. The possibilities for carrying out creative work on the transferred imagery after exposure of the plate are greater than with the non-etch techniques because any reprographic elements of an etched plate can be overlaid and combined with aquatint, open bite, drypoint etc. The etch methods provide artists with a greater range of artistic tools through the availability of multiple processes; while the non-etch methods very much depend on creative mark making at the stage where the positive is generated.
Line art
The best kind of positive to use for the photopolymer etch processes is line art. This reprographic term does not necessarily mean that only linear images can be used, but it denotes any kind of matrix that consists of clearly defined areas with totally opaque marks (no mid-tones). When using this way of working, you should be able to describe marks or areas of the image in simple YES or NO terms: either it is a mark opaque enough to block the UV light or it is a translucent area that will let the light through during exposure of the film. Correspondingly, during the etch process, the plate should have areas that are either covered by a photopolymer resist that stops the mordant from biting, or blank metal areas where a groove can be eroded. Positives that lack this YES or NO; BLACK or WHITE definition are less suitable for reprographic etch techniques. Maximum contrast and opacity make for the best positives.
METHOD
Make a positive as follows:
The photocopier is a simple, effective piece of equipment that can be used to generate positives. Ideally, use clear heat resistant acetate that has been designed for this use. Copies can be made from pretty much anything – original artwork, magazines, drawings, maps etc. Laser or ink jet printers can be used in a similar fashion to make positives from digital images. The best inkjet printers suitable for making positives have a ‘double matt black’ facility, which enhances the density of the black. Most Epson and Canon Photo printers offer a double matt cartridge mode (check each model).
With any of this equipment, the aim is to produce sharp images from an opaque ink deposit. If copies are faint or lacking in definition, detail may be lost during exposure. Larger marks on photocopies and computer prints can fail to be fully opaque and so in many cases positives benefit from some manual editing in these areas with a marker pen or Indian ink. Editing can also be carried out for creative reasons – copies can be painted and drawn on, creating an amalgam of reprographic and autographic marks; or indeed some areas of the positive can be scratched into or removed.
A useful trick if a photocopier or laser printer fails to produce a dense deposit is to sandwich two faint copies together to make one opaque image.Line positives of the highest quality can also be made by copying an original onto litho film, a process which in the past required access to reprographic facilities and a copy camera. However, today, high quality reproductive line art is likely to be generated by means of digital imaging i.e. where an original is scanned and then output at a high resolution onto film by an image setter or laser printer.
Positives can of course be made entirely by manual means by drawing straight onto acetate or true grain with opaque pens, inks, masking fluid, crayons etc.
Exposing the plate
When all creative work on the positive is complete the plate can be exposed using a UV exposure unit. A vacuum frame guarantees perfect contact between a plate that has been laminated with photopolymer film and the positive.
NOTE: If an exposure unit is not available, an improvised exposure can be carried out by sandwiching the plate and
positive between two heavy panes of glass and exposing for a few minutes in sunlight. At this stage a decision has to be made whether or not to remove the second layer of Mylar before exposure. Because this clear layer acts as an extra barrier between the positive and the photopolymer emulsion it can cause a very slight reduction in definition. However, if the Mylar is removed, be careful that the film does not adhere to the positive.
METHOD
Expose photopolymer plates as follows:
1. Place the positive on top of the plate, making sure it is reversed. If there is text on the positive, it should now read backwards. Reversing at this stage will ensure that the image will read correctly after it has been printed. In many cases, the positive can be registered on top of the plate by eye, but if accuracy is required or if printing multiple plates, use a registration system. Good registration can be achieved by incorporating registration marks representing the plate perimeter on the positive itself – these can be easily drawn with a waterproof marker pen or masked out with strips of tape. These marks can then be aligned to the plate on the vacuum bed. The positive can also be prevented from slipping by attaching it to the back of the plate with pieces of clear sellotape.
2. Now close the vacuum frame and activate the suction pump.
3. Any available UV protection such as lightproof doors or curtains must be in place before exposure can begin. As conditions vary, some testing should be carried out to determine the correct exposure times. Large professional exposure facilities measure the amount of UV actually emitted in light units and allow printmakers to replicate exposure conditions with great accuracy. However, an exposure calculated with a simple timing device can also be relatively predictable if the printmaker keeps to a set procedure. For example, remember to let mercury vapour lamps cool down before using them again in order to exactly replicate the condition of the first exposure. With line art, the exposure process tends to be more forgiving than with delicate half tone, and usually a satisfactory exposure time is not hard to determine. There are, however, two things to remember:
(i) Under exposure may not harden the photopolymer film sufficiently and delicate areas
may be damaged by the developer as a consequence.
(ii) Over exposure may lead to the UV light penetrating the opaque marks causing unwanted hardening in some areas of the image. The risk of this happening is greater on positives that are not of the highest quality, where opaque black marks are in fact grey as a result of insufficient toner deposit from a photocopier or laser printer. Another factor to be considered when calculating exposure time is the thickness of the material onto which the positive has be reproduced. For instance, if lith film has been used it may require an exposure more than twice as long as a thin sheet of clear acetate.
Making up developer
Unlike many photographic processes the concentration of the developer should always be kept constant rather than being regarded as a variable. A solution that is too strong would damage and dissolve the hardened areas of the plate. A commonly used ratio of soda crystals (sodium carbonate) to water is 10 grams of crystals to 1 litre of water which results in a 1% sodium carbonate solution. Always make up ample amounts of solution sufficient for the size of plate to be developed. For example, for a densely covered plate of about A4 size, at least two litres of solution should be used to ensure the solution remains active until the plate is fully developed.
As the soda crystals can irritate the skin, wear rubber gloves and a dust mask when handling them. Despite the relativelylow corrosiveness of the sodium carbonate solution, the use of goggles is also recommended as even mild alkaline solutions can damage the eyes if it comes into direct contact.
MATERIALS
Products and equipment needed to make up the developer:
soda crystals
- gloves
- goggles
- tray
- jug
- spoon etc. for stirring
- kitchen scales
- hot and cold running water
METHOD
Make up the developer as follows:
1. Measure the required amount of soda crystals (10g per litre) into a jug.
2. Add a small amount of hot water to the crystals and stir until all solid particles have been dissolved.
3. Now top up with cold water to the amount required. Stir again.
4. Pour the developing solution into a tray or storage container
Developing the plate
In photopolymer printmaking the term developing denotes a process quite different from photographic developing. Here, it is quite a tactile process where whole layers of plastic material are actually washed away and dissolved.
METHOD
Develop the plate as follows:
1. After exposing your plate switch off the exposure unit and the vacuum bed and remove the plate.
2. At this stage, the remaining layer of clear Mylar should be removed. Push your finger along the edge of the plate until a corner of the film comes away, then simple peel off the film.
3. Ideally the developing solution will have been made up beforehand and is now ready for
use in a tray. If a lot of photopolymer work is to be done it is practical to keep a ready supply on tap in large plastic containers or a barrel.
4. Immerse the plate in the tray. The plate needs to remain in the solution for 3 to 5 minutes.
Brush and agitate to aid the process. A good way to aid the removal of the film from the unhardened areas is to continually stroke all areas of the plate with your hand. Wear gloves for protection. Alternatively a soft sponge or brush can be used but be careful not to use any abrasive implements as these may damage the polymer marks which are still delicate and vulnerable in the wet state.
5. A well exposed photopolymer plate will soon show bare metal shining through in all areas that are black on the positive. At the same time the developing solution becomes cloudy and will acquire a milky blue tinge, indicating that significant amounts of photopolymer have been dissolved. Even after 3-5 minutes when the plate seems fully developed, traces of this milky blue liquid may still remain on the plate and without further treatment would stop mordant from biting. To prevent this, all spent developer should now be discarded.
6. Gently wash the plate surface with a further quantity of dilute developer then thoroughly rinse under cold running water. A drop of washing-up liquid can be added to the final rinse to ensure drying without stains. Excess water can now be blotted off.
7. Now spritz the plate with a 50%-50%water-vinegar mixture and let this work for 30 seconds. Rinse again and blot the plate with paper towels.
8. Dry the plate quickly in a hot cabinet or with a fan.
Etching and stripping the plate
Before the plate can be etched any imperfections in the etch resistant photopolymer surface need to be touched up with stop-out varnish or covered with pieces of adhesive tape to prevent foul biting. Often this is necessary along the edges of the film which can show some cracking.The plate can now be etched as a conventional line bite for any required length of time. Due to the extreme resistance of hardened photopolymer film there is little chance of foul biting and even the deepest etches can be carried out with great accuracy and minimal lateral biting. Problems may occur if insufficient quantities of developer have been used; if the plate is under developed or if the plate has been insufficiently rinsed. In these cases it is not always necessary to remake the plate; often a post developing stage in a stronger than usual developing solution removes polymer residues from the metal surface; and the plate can be re- etched.
After etching the photopolymer film can be removed in the stripping solution for acrylic based etching to reclaim the plate for further use. Stripping can also be carried out in a concentrated developer solution – about 1 part soda crystals to 4 parts water. However you may wish to leave the film on after biting so that a proof can be taken to assess the depth of bite before reimmersion for further biting (in this case the plate needs a good degreasing after printing). The hardened photopolymer film can also be left on if you are looking for a very pale plate tone which is typical for intaglio prints taken from smooth plastic surfaces.
Tonal work in the etch technique
Aquatinting
Any kind of tonal work i.e. the greys and blacks of photographs or drawings, have to be broken into some kind of dot structure in order to translate them into etched intaglio marks on the metal plate. The need to do this also applies to opaque black areas on the positive that are broader than a line of around 1 mm. If these areas are exposed and etched without additional treatment they will print as a shallow open bite patch rather than as a black. The conventional way of filling these areas with tone is by applying an aquatint to the plate before etching it. The printmaker has all the options of the acrylic spray aquatint system at their disposal;
- overall or partial application of a dot structure different dot sizes and density
- a range of tones depending on the length of bite
A reprographic AQUATINT can easily be applied by exposing the photopolymer plate to a fairly coarse random dot screen before exposing it a second time. This works best with positives that show strong contrasts. If the positive has a broader tonal range, only part of those tones will register on the metal surface and another more suitable half tone method should be used.
Using rasterized positive
As photo etching is a reprographic technique, the dot structure or raster required for tonal work can already be incorporated in the reprographic stages of making the plate, prior to etching. The traditional, and now largely obsolete way to do this in workshops and in industry was by using professional screens that turn a continuous tone image into one broken up into a dot or half tone structure. Today, the copy camera and gelatine based dot screens have largely been replaced by digitally made screens.
The standard resolution of screens for traditional photo etching was limited to about 85-100 lines per inch which equates to the quality of a picture reproduced in a newspaper. Their use is recommended on normal thickness photopolymer film if the artist wants the dots to be clearly visible for artistic reasons, or if very long etches of photographic imagery is desired.
Since a rasterized positive is free from any true grey tones but consists only of black dots it can be exposed and developed in the same fashion as a line positive. A very straightforward way of making rasterized positives for photopolymer work is to use photocopies of images from magazines or newspapers which already have a dot structure. Tonal images can also be digitally scanned and given a dot structure using a computer program such as Adobe Photoshop and then printed out onto clear film.
Intaglio Type tonal work
In the past the medium of photo-etching used to be limited to this low definition etch technique unless the highly demanding process of photogravure was used. The most straightforward method of carrying out tonal photo-intaglio work today is the non-etch technique devised by Keith Howard. This can not only convey all the detail and delicacy of an ink wash or a photograph, but even enhances its pictorial depth and richness. The main advantage is that high definition reprographic material can be combined with any conventional intaglio technique such as open bite or line etch, and if a steel faced copper plate is used it is possible to produce a large edition from a single plate. With the Intaglio type process, the dot size is no longer limited to 100 dpi to make an impression on the film.
METHOD
The simple Intaglio Type method is as follows:
1. Thin the film down to a fraction of its original thickness by placing a ready laminated photopolymer plate in a bath of developer.
2. After removing the protective Mylar, allow the film to dissolve undisturbed for 2-10 minutes, depending on the thickness of the film. The length of time needs to be tested out for different types of film. Use 8/9ths of your standard development time to thin the film. If for example your standard development time is 9 minutes, the film will be thinned correctly in 8 minutes.3. Now place the plate in a bath of water to stop the dissolution process. Brush any excess dissolved film away with a soft sponge.
4. Carefully blot the plate and leave it to dry fully in a drying cupboard.
The very thinly coated plate produced this way is sensitive to half tone reproduction as with the gelatine emulsion of conventional photogravure. It now allows the use of reprographic halftone positives with a very high resolution for making etched plates – in this instance, halftone screens of up to 500dpi/lpi can be used. The tones produced on a printed etching now appear to be continuous and the actual dot structure that constitutes them is only visible under a magnifying glass.
Etching
A quick immersion of the dried plate in the mordant will cause oxidisation which reveals whether or not all the detail of the positive has registered on the plate. If some areas have not been fully revealed the plate can be reimmersed in the developing solution to open them up. The plate can be etched in the usual manner and the tonal range, detail and depth of the plate very much depends on the duration of the bite. Unlike the non-etch methods, this gives the artist greater control over making a half tone intaglio plate because certain areas of the plate can be emphasised or held back by etching and stopping-out in stages. This then produces an intaglio structure of varying depths similar to a stepped aquatint. This process can be further enhanced by including additional reimmersions in developer. As a result, photopolymer film can function as a pseudo permeable resist, comparable to traditional photogravure – but thorough testing and fine-tuning of all the variables is necessary to yield perfect results.
Here is a methodology which on one hand allows for the use of dot screens of up to 500dpi; whilst on the other opening up
the option of permeable etching. If the former is used, a double exposure is advisable – first to the half tone screen, then to
the continuous tone positive. If the latter is favoured, a single exposure to the continuous tone positive may suffice, especially if this has some grain in itself. In both cases the plate should be post baked before etching begins. Due to the thinness of the photopolymer resist the etch process itself can produce the tonal variation by gradually breaking through layers of resist, depending on their thickness, thus creating intaglio marks of varying depths. This technique requires some experimentation with the variable aspects i.e. thinning, exposure, and biting times – but can produce superb tonal reproduction on the intaglio print.
For more information on photopolymer techniques click on the following links:
Intaglio Type
Aquatint Screen