Photopolymer is a very interesting material: A thick plastic sheet which is partly soluble in water. Exposure to UV light causes the polymer plastic to harden and become insoluble.
It’s used to make printing plates for letterpress and other ink printing processes.
A negative is contact-exposed onto the polymer. Clear areas are exposed to UV and harden, black (on the neg) areas receive no light and stay soft.
‘Development’ consists of rinsing the material in cold water. The soft unexposed areas dissolve, leaving the harder exposed parts in relief. The difference is small (0.1mm or so) but it's enough to make a very detailed printing plate.
Testing:
The material is Toyobo KF152. It's relatively expensive for full sheets but the nice people at Lyme Bay Press (https://main.lymebaypress.co.uk) sell packs of assorted offcuts for around £27. This gives an ample supply of pieces for testing and making small plates. It can be cut with strong scissors or a craft knife.
* (There is also a KF95 - a thinner material said to be just as effective, if not better on thin papers. I chose the 152 purely because it was in stock at the time!)
As it’s UV sensitive it’s supplied in a black plastic bag but can be handled in low tungsten lighting - much like cyanotype or argyrotype materials in fact. It’s an amber colour, with a slightly odd (but not offensive) smell and can be cut with strong scissors or a craft knife. There’s a protective film on the emulsion side and a harder plastic backing.
The manufacturers recommend using a Stouffer gauge (a graduated step wedge negative) and it does work but in practice I think photographers will find it easier to make an actual test negative on the printer and use that.
Lith negatives
Negatives for this process can be made with ‘lith’ or ’line film’ which are very high contrast materials with essentially a black and a white, but no mid tones at all. This is the traditional method but digital negs are easier to make for most people! I’ll describe the process in detail elsewhere. I used a laser printer and cheap transparency material.
It can be confusing to work out the full process (dense areas of the neg stop the light so the material will remain soft and dissolve, reducing height and so not retaining printing ink. clear areas allow light through to harden the polymer and producing an inked area) so for my very first tests I made a dual image with both positive and negative versions!
For the record, you need to make a NEGATIVE (tones inverted) image on the transparency material.
Simple exposure tests.
For an experienced darkroom printer this is a curiously counterintuitive process. Slight underexposure results in more apparent relief between shadow and highlights but overexposure makes a plate which looks 'thin' and without contrast. These test pieces are tiny little images only about 15- 20mm high as this scale is easiest to set up and test in a printing press or as a hand stamp. Large (up to A3) images can be made but apart from the expense they are trickier to get right with press setup etc.
It’s not really possible to gauge which plate would print best by eye (though I’m sure experienced photopolymer printers can do it) so I made a range of test pieces: This photo of Algernon in both positive and negative at various exposures, plus this set of nine identical letter Ps which I exposed as test strips: 100 -900 seconds and then 900 - 1700 seconds. This stuff needs a LOT of exposure!
Post-exposure.
Once the photopolymer has been exposed it’s rinsed in cool (less than 23 deg.C) water for 2-5 minutes. A soft brush is recommended to help remove the soluble polymer (I used an eyeliner brush which has very fine bristles like a paintbrush but with more stiffness so there’s a gentle scrubbing effect). Too much rinsing can damage the surface so rinse just long enough until the milky-looking and slimy-feeling soft stuff is gone. The plate is then dried thoroughly and then re-exposed for 1.5 to 2 times the original exposure. This hardens the remaining polymer material completely, making it durable and ready for printing.
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| Exposed plate before washing out. A faint image is visible already. |
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| Washing in running water dissolves the unexposed areas and 'develops' the plate. |
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| Washed out to reveal the relief pattern of the image. This must be dried, then re-exposed to ensure it's fully hardened. |
Adventures in letterpress.
I’m lucky to have inherited my grandfather’s 1930s ADANA 6”x4” printing press. These little machines were made to print with metal type onto paper in the true letterpress method. It’s an awful long time since I did any traditional printing like this so I’m re-learning it all but enjoying the process. The advantage of a ‘proper’ press is that everything from the amount of ink applied to the pressure is very controllable. It is possible to use this stuff in a simple screw press or even as a hand stamp but the results will be more variable.
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| Adana 6"x4" letterpress machine. A wonderfully mechanical device in the clutter of the workshop |
Type height.
British printing presses use a standard type height of 0.918 inch (23.32mm). Any printing block needs to be exactly this thickness to work properly. The KF152 photopolymer is 1.52mm thick so it needs mounting to a block (usually of wood) to raise it ‘type high’. This height is crucial: Printers will paste thin pieces of paper to the back of blocks to adjust the height until it’s perfect. The ink rollers are set to deliver precisely the right amount based on this size: any lower and the print will be pale und under-inked, higher and too much is applied, leading to rough, blurry edges and loss of detail. I took some time with my block for the test pieces to get it right.
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| All type should be exactly the same height. 0.918 inch |
The block is put into a frame (called a ‘chase’) along with any other type needed for the design and then packed out with ‘furniture’: pieces of metal or wood before locking it in place with expanding blocks called ‘quoins’. Once everything is assembled on a flat surface it’s called a ‘forme’.
The forme goes into the press bed, the rollers transfer ink from the disk at the top to the printing surface and then it’s ready for the paper. Everything is very accurately controlled so results are consistent. All this means I could test the differently exposed blocks side by side for comparison.
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| Photopolymer plate installed on wooden base and inked up. |
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| The best print from the test plates described above. |
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| -and the test strips. Exposure times go from 100 seconds (top left and invisible!) to 1700 seconds (bottom right) |
1200 seconds (20 minutes!) exposure seems to give the best detail with neither fine lines too faint or darker areas spreading out. This is quite a long time by traditional alternative process standards - I reckon on about 6 -12 minutes being plenty for cyanotypes with this unit but it's fine. I could use another more intense UV source but it's easy to use my standard exposure box.
These are my very first forays into this technique so I know they aren't very good. Once I've got more experience, and learned more I'll write another article. If you have used it already, please get in touch and share your expertise!
As promised I've written another article on the neg making process. Making a pure black and white image with no mid tones is an interesting exercise in itself. Find it here:
http://oldphotoprocess.blogspot.com/2021/12/making-lith-negatives-for-photopolymer.html
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